Of The Attraction Of Cohesion

I've had to do a bit of reading for this post due to the subject matter veering rather too close to a sizeable gap in my scientific education; namely chemistry. I studied virtually no chemistry after the age of 14 for reasons that seemed sensible at the time; those being, that I didn't get it or enjoy it. This has occasionally been a bit of a regret, never more so than in the first year of studying physics at university; I wish someone had told me that a basic knowledge of chemistry was pretty much essential before I started taking certain courses.

Anyway, I have acquainted myself with the modern concept of cohesion and other forces of attraction in order to pick apart these next two dialogues. Hopefully I won't mess it up too badly, please forgive me for a heavy reliance on Wikipedia.

CONVERSATION III

OF THE ATTRACTION OF COHESION.

  F. Well, my dear children, have you reflected upon what we last conversed about ? Do you comprehend the several instances which I enumerated as examples of the minute division of matter ?
  E. Indeed, the examples which you gave us very much excited my wonder and admiration, and yet, from the thinness of some leaf gold which I once had, I can readily credit all you have said on that part of the subject. But I know not how to conceive of such small animals as you described ; and I am still more at a loss how to imagine that animals so minute should possess all the properties of the larger ones, such as a heart, veins, blood, &c.
  F. I can, the next bright morning, by help of my solar microscope, show you, very distinctly, the circulation of the blood in a flea, which you may get from your little dog ; and with better glasses than those of which I am possessed, the same appearance might be seen in animals still smaller than the flea, perhaps even in those which are themselves invisible to the naked eye. But we shall converse more at large on this matter, when we come to consider the subject of optics, and the construction and uses of the solar microscope. At present we will turn our thoughts to that principle in nature, which philosophers have agreed to call gravity or attraction.
  C. If there be no more difficulties in philosophy than we met in our last lecture, I do not fear but that we shall, in general, be able to understand it. Are there not several kinds of attraction ?
  F. Yes, there are ; two of which it will be sufficient for our present purpose to describe : the one is the attraction of cohesion ; the other, that of gravitation. The attraction of cohesion is that power which keeps the parts of bodies together when they touch, and prevents them from separating, or which inclines the parts of bodies to unite, when they are placed sufficiently near to each other.
  C. Is it then by the attraction of cohesion that the parts of this table, or of the penknife, are kept together ?
  F. The instances which you have selected are accurate, but you might have said the same of every other solid substance in the room ; and it is in proportion to the different degrees of attraction with which different substances are affected, that some bodies are hard, others soft, tough, &c. A philosopher in Holland, almost a century ago, took great pains in ascertaining the different degrees of cohesion which belonged to various kinds of wood, metals, and many other substances. A short account of the experiments made by M. Musschenbroek, you will hereafter find in your own language, in Dr. Enfield's Institutes of Natural Philosophy.
  C. You once showed me that two leaden bullets, having a little scraped from the surfaces, would stick together with great force ; you called that, I believe, the attraction of cohesion ?
  F. I did : some philosophers, who have made this experiment with great attention and accuracy, assert, that if the flat surfaces, which are presented to one another, be but a quarter of an inch in diameter, scraped very smooth, and forcibly pressed together with a twist, a weight of a hundred pounds is frequently required to separate them.
  As it is by this kind of attraction that the parts of solid bodies are kept together so when any substance is separated or broken, it is only the attraction of cohesion that is overcome in that particular part.
  E. Then, when I had the misfortune this morning at breakfast to let my saucer slip from my hands, by which it was broken into several pieces, was it only the attraction of cohesion that was overcome by the parts of the saucer being separated by its fall on the ground ?
  F. Just so ; for whether you unluckily break the china or cut a stick with your knife, or melt lead over the fire, as your brother sometimes does, in order to make plummets ; these and a thousand other instances which are continually occurring, are but examples in which the cohesion is overcome by the fall, the knife, or the fire.
  E. The broken saucer being highly valued by mamma, she has taken the pains to join it again with white lead ; was this performed by means of the attraction of cohesion ?
  F. It was, my dear ; and hence you will easily learn that many operations in cookery are in fact nothing more than different methods of causing this attraction to take place. Thus flour, by itself, has little or nothing of this principle, but when mixed with milk, or other liquids, to a proper consistency, the parts cohere strongly, and this cohesion in many instances becomes still stronger by means of the heat applied to it in boiling or baking.
  C. You put me in mind of the fable of the man blowing hot and cold ; for, in the instance of lead, fire overcomes the attraction of cohesion ; and the same power, heat, when applied to puddings, bread, &c. causes their part to cohere more powerfully. How are we to understand this ?
  F. I will endeavour to remove your difficulty. Heat expands all bodies without exception, as you shall see before we have finished our lectures. Now the fire applied to metals, in order to melt them, causes such an expansion, that the particles are thrown out of the sphere, or reach, of each other's attraction ; whereas the heat communicated in the operation of cookery, is sufficient to expand the particles of flour, but is not enough to overcome the attraction of cohesion. Besides, your mamma will tell you, that the heat of boiling would frequently disunite the parts of which her puddings are composed, if she did not take the precaution of enclosing them in a cloth, leaving them just room enough to expand without the liberty of breaking to pieces ; and the moment they are taken from the water, they lose their superabundant heat, and become solid.
  E. When Ann the cook makes broth for little brother, it is the heat then which overcomes the attraction which the particles of meat have for each other, for I have seen her pour off the broth, and the meat is all in rags. But will not the heat overcome the attraction which the parts of the bone have for each other ?
  F. The heat of boiling water will never effect this, but a machine was invented several years ago, by Mr. Papin, for that purpose. It is called Papin's Digester, and is used in taverns, and in many large families, for the purpose of dissolving bones as completely as a lesser degree of heat will liquefy jelly. On some future day I will show you an engraving of this machine, and explain its different parts, which are extremely simple. *

* See Pneumatics, Conversation XVIII.

A slight side note before we get onto the matter of cohesion; I'm extremely glad to see that Emma is very sensibly being suspicious of the idea of animalcula having complex innards. I believe Father misses the point a little by offering to show the workings of a flea when some of the creatures he was talking about were orders of magnitude smaller. I am a little alarmed at how straight forward Charles thinks this all is; he seems to be taking an awful lot of it on faith. Having complemented her on her critical thinking, Emma does seem to be a little clumsy with her mother's precious crockery.

Our understanding of why things stick to each other is, unsurprisingly, far more refined than that exhibited in this dialogue. This mainly stems from our understanding of matter having constituent parts; atoms and molecules. These bits exert various forces on each other; when they attract each other strongly enough they stick together and form the stuff that is matter. At a larger scale these forces can apply to the surfaces of lumps of matter when they are close to other material.

The property that we currently label cohesion is specifically that of similar molecules or particles sticking to each other. A similar property is adhesion, which is the property of dissimilar particles sticking together. As I have understood it, these forces generally apply to fluids (i.e. liquids and gases). In solids, the constituent parts are tightly bound by a different range of forces.

In fact, there is only one of the examples given in this conversation which exhibits what we would call cohesion today; that is the melting of metals. In its solid form, lead atoms are held together by metallic bonds; once the lead has melted the atoms exhibit a cohesive force and form a puddle. Unfortunately for us, Father ascribes greater cohesion to the solid state of the lead. The attractive force between the atoms is undeniably greater in this solid form. However, it is not actually cohesion until the tight bonds fail due to the energy provided by the heat.

There is a mention of the esteemed 18th century Dutch philosopher Musschenbroek, he seems quite extraordinary. I am confident that he is Prof. Dr. A.L.M. et Med. Petrus van Musschenbroek; using the Latin name Petrus in the place of his given name Pieter, as was the habit of men of philosophy at the time. He held at least four professorial chairs and was a member of at least four major Europeean science societies. He worked alongside Farenheit in Germany and is credited as the inventor of the Leyden Jar (which we shall explore in more detail later in the book). I deeply respect a man who, in all seriousness, published a paper on poking a stick into butter. Dr. Enfield’s Institutes of Natural Philosophy is available from Google as a free eBook and it looks like an excellent work.

"Heat expands all bodies without exception" is a statement which struck me as wrong and attracted my attention. While it is true that there are few exceptions to thermal expansion, there is a notable one that I learnt at a very early age; this is water. I became familiar with the fact that ice floats on water (and is therefore less dense) so long ago that it seems strange to me that a man of science would not be aware of it. Of course, artificial refrigeration has not been around forever. In fact, it was not a commercial reality until the middle of the 19th century; about the time that this edition of the Dialogues was originally purchased. When the book was originally written, ice was only available in cold places or in very few laboratories.

I think it's safe to say that Mamma's puddings were probably held together by some form of adhesion. Sticky flour and water mixtures are made of many different types of particles. At the risk of going off topic, puddings at this time would have been packed with fruits and nuts; very similar to modern Christmas/plum pudding. With the chief difference that we have moved to steaming as opposed to boiling them.

I'm intrigued by Papin's Digester; it sounds like it would fit very well in a mad professor's laboratory. However, we shall have to wait and see its exact workings, the volume on pneumatics is a long way off yet.

There's more on cohesion next time. I'll be on slightly more familiar ground with subjects such as surface tension, menisci and capillary action.

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Of Matter: Of the Divisibility of Matter

In this instalment Father suffers from a major episode of the expositions. I think he may have some anecdotes which he has a great urge to share with the children. This is precisely the sort of conversation during which I imagine Charles and Emma getting extremely bored and tickling each other.

You should probably prepare yourself for mathematics and some apparently bad science.

CONVERSATION II.

OF MATTER. - OF THE DIVISIBILITY OF MATTER

  F. Do you understand what philosophers mean when they make use of the word matter ?
  E. Are not all things which we see and feel composed of matter?
  F. Every thing which is the object of our senses is composed of matter differently modified or arranged. But in a philosophical sense matter is defined to be an extended, solid, inactive and moveable substance.
  C. If by extension is meant length, breadth, and thickness, matter, undoubtedly, is an extended substance. Its solidity is manifest by the resistance it makes to the touch.
  E. And the other properties nobody will deny, for all material objects are of themselves without motion ; and yet it may be readily conceived, that, by application of a proper force, there is no body which cannot be moved. But I remember, papa, that you told us something strange about the divisibility of matter, which you said might be continued without end.
  F. I did, some time back, mention this curious and interesting subject, and this is a very fit time for me to explain it.
  C. Can matter indeed be infinitely divided; for I suppose that this is what is meant by a division without end ?
  F. Difficult as this may first appear, yet I think it very capable of proof. Can you conceive of a particle of matter so small as not to have an upper and under surface ?
  C. Certainly every portion of matter, however minute, must have two surfaces at least, and then I see that it follows of course that it it divisible ; that is, the upper and lover surfaces may be separated.
  F. Your conclusion is just ; and though there may be particles of matter too small for us actually to divide, yet this arises from the imperfection of our instruments ; they must nevertheless, in their nature, be divisible.
  E. But you were to give us some remarkable instances of the minute division of matter.
  F. A few years ago a lady spun a single pound of wool into a thread 168,000 yards long. And Mr. Boyle mentions that two grains and a half of silk were spun into a thread 300 yards in length. If a pound of silver, which, you know, contains 5,760 grains, and a single grain of gold, be melted together, the gold will be equally diffused through the whole silver ; insomuch, that if one grain of mass be dissolved in a liquid called aqua fortis, the gold will fall to the bottom. By this experiment, it is evident that a grain may be divided into 5,761 visible parts ; for only the 5,761st part of the gold is contained in a single grain of the mass.
  The goldbeaters, whom you have seen at work in the shops in Long-acre, can spread a grain of gold into a leaf containing 50 square inches, and this leaf may be readily divided into 500,000 parts, each of which is visible to the naked eye: and by the help of a microscope, which magnifies the area of surface of a body 100 times, the 100th part of each of these becomes visible ; that is, the 50-millionth part of a grain of gold will be visible, or a single grain of that metal may be divided into 50 millions of visible parts. But the gold which covers the silver wire used in making what is called gold lace, is spread of a much larger surface, yet it preserves, even if examined by microscope, a uniform appearance. It has been calculated that one grain of gold, under these circumstances, would cover a surface of nearly thirty square yards.
  The natural divisions of matter are still more surprising. In odoriferous bodies, such as camphor, musk and assafœtida, a wonderful subtilty of parts is perceived ; for, though they are perpetually filling a considerable space with odoriferous particles, yet these bodies lose but a very small part of their weight in a great length of time.
  Again, it is said by those who have examined the subject with the best glasses, and whose accuracy may be relied on, that there are more animals in the milt of a single cod-fish, than there are men on the whole earth, and that a single grain of sand is larger than four millions of these animals. Now if it be admitted that these little animals are possessed of organized parts (such as a heart, stomach, muscles, veins, arteries, &c.) and that they are possessed of a complete system of circulating fluids, similar to what is found in larger animals, we seem to approach to an idea of the infinite divisibility of matter. It has indeed been calculated, that a particle of blood of one of these animalcula is as much smaller than a globe one-tenth of an inch in diameter, as that globe is smaller than the whole earth. Nevertheless, if these particles be compared with the particles of light, it is probable that they would be found to exceed them in bulk as mountains do single grains of sand.
  I might enumerate many other instances of the same kind, but these, I doubt not, will be sufficient to convince you into what very minute parts matter is capable of being divided.
  Captain Scoresby, in his Account of the Greenland Seas, state, that, in July, 1818, his vessel sailed for several leagues in water of a very uncommon appearance. The surface was variegated by large patches of a yellowish-green colour. It was found to be produced by animalcula, and microscopes were applies to their examination. In a single drop of the water, examined by a power of 28,224 (magnified superficies) there were 50 in number, on average, in each square of the micrometer glass of 1-340th of an inch in diameter ; and, as the drop occupied a circle on a plate of glass containing 529 of these squares, there must have been in this single drop of water, taken at random out of the sea, and in a place not the most discoloured, about 26,450 animalcula. How inconceivably minute must the vessels, organs, and fluids of these animals be ! A whale requires a sea to sport in : a hundred and fifty millions of these would have ample scope for their evolutions in a tumbler of water !

Where to start? I'll begin with a glossary of some unusual terms that appear in this dialogue:

• Grain - a measure of weight based on a grain of wheat; is still in use in the US. In modern terms it is a mass of exactly 64.79891 milligrams. Father mentions a pound consisting of 5,760 grains; this indicates that he is referring to Troy pounds. This is entirely understandable considering that Troy weights were part of the accepted weights system prior to the introduction of the British Imperial measures in 1824.
• Aqua Fortis - a fantastically alchemical sounding archaic name for Nitric acid.
• Gold lace - according to the Shorter Oxford English Dictionary this is "a braid formerly made of gold or silver wire, now of silk or thread with a thin wrapping of gold".
• Milt - has two meanings, the one I think that is being used here is fish semen/testicle. The other meaning is spleen, although I don't think there should be that many things moving about in one of them.
• Animalcula - microscopic animals, including what we now call protozoa.
• Magnified superficies - seems to mean an increase of the surface area, I found a reference from the Royal Society which equates "magnified 15 times in diameter" to "225 times in superficies"

The assertion of matter being infinitely divisible is quite preposterous from our current level of knowledge. Yet, from the facts available at the beginning of the 19th century, it makes a reasonable amount of sense. Experiments had been done and the results showed that matter appeared structurally identical at every level of division possible. There was no evidence that a limit to this division could be reached; the Rev. Joyce does admit that there was a practical limit to splitting materials due to the lack of precise enough tools to reach smaller parts. This is certainly a case of a theory based on the available evidence which is supported by general consensus. This is not bad science at all, this is how science should work.

The idea of matter being made of discrete indivisible bits had been around for a long time before this book was written. However there was no experimental evidence; this was later found and the theories were developed over the 19th century. From this work Dmitri Mendeleev developed the first periodic table. It wasn't until the very end of that century and into the 20th that scientists discovered that even atoms were divisible (into protons, neutrons and electrons). Later still, some of those subatomic particles were discovered to be divisible yet again (protons and neutrons into quarks). This is where our current understanding still sits; we have The Standard Model, which thousands of scientists around the world are still working to completely prove or disprove and refine. We should have some news regarding this during 2012 from the work being done by the LHC experiments at CERN.

I think it would be an interesting exercise to see how near to the atomic level they reached, so I shall do it in rough approximation. I suspect they were a very long way from it. There is a reasonable chance I shall mess the maths up or approximate too wildly; feel free to help me out in the comments if I do.

Mixing gold and silver together and then seeing if gold is present in a small portion of that mixture is an elegant way of testing the hypothesis. Unfortunately, the quantity of silver required to dilute the gold down to a single atom is mind-bogglingly huge. It turns out there are getting on for 200 million million million atoms of gold in a grain (1.98E18). Repeat the experiment with that many grains of silver instead of 5760 and you'll still end up with a single atom of gold; if you stir it thoroughly enough. It's safe to say that this is not possible to attempt as you would need almost 13 million million metric tons of silver!

In the example of the goldbeaters, 1 grain of gold was spread over an area of 50 square inches. I need to go metric to make sense of this, so that's 0.064798 grams spread to 322.58 square centimetres; that looks a lot simpler doesn't it? The density of gold is 19.30 grams per centimetre cubed, so a grain of gold has the volume of a cube with an edge of just under 1.5 millimetres (approximately 0.00336 centimetres cubed). If the leaf has been uniformly beaten, we can calculate that it's thickness is pretty close to 0.01 millimetres. That's exceptionally thin for any practical purpose. However, the size of an atom is around of 100 picometres (a picometre is truly tiny at 0.000000000001 metres). That means that the gold leaf is still around 100000 atoms thick. The gold lace makers did a great deal better; their leaf would have been around 130 atoms thick. To get an indivisibly thin leaf (i.e. with a thickness of just a single atom), you would have to spread it out over an area a hundred thousand times bigger; a single square sheet with sides 57 metres long should do it. Still, they were exceptionally close to the limit compared to my initial instincts.

I don't really want to think about fish semen but I have for the sake of this post. I have looked into the subject just enough to try to get an idea of whether there is any truth in the statement "there are more animals in the milt of a single cod-fish, than there are men on the whole earth". The population of the world sat somewhere around 1 billion at the time that the book was authored. Even though I couldn't find any hard figures for cod sperm counts, I did discover that male fish have enormous gonads when they're ready to spawn (10-20 percent of their body weight). Considering the figures that I found regarding sperm density in mammalian semen and that fish let all of it out in one go, I think I can say that the statement may well be true; it's definitely in the right ball park.

It's an interesting assumption though that all "animals" have the same level of internal complexity, no matter how small. I guess this is another case of the contemporary tools not allowing them to examine closely enough to see the very real differences that arise in nature at these minute scales.

The account of Captain Scoresby's encounter with a discoloured sea sounds very much like a plankton bloom; a massive collection of tiny ocean plants (phytoplankton) that thrive under certain conditions. Seeing as zooplankton (equally tiny ocean animals) feed on phytoplankton, I can imagine that a bloom would be full of wriggling animalcula. I think that probably, the good Captain was seeing a mixture of both microscopic animals and plants; observing that some of them were moving, he may well have assumed that they were all animals. It looks like the numbers they give are very high; modern studies have recorded blooms with 100 million phytoplankton per litre of sea water.

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Introduction

The first conversation of the book is a simple one that introduces us to the participants. It sets the scene as to the way things are going to be discussed in the rest of the volume and how the Author will instruct his students.

MECHANICS

— —

CONVERSATION I.

INTRODUCTION

FATHER — CHARLES — EMMA

CHARLES. Father, you told sister Emma and me, that, after we had finished reading "Evenings at Home," you would explain to us some of the principles of natural philosophy : will you begin in the morning ?
  Father. Yes, I am quite at leisure ; and I shall, indeed, at all times take a delight in communicating to you the elements of useful knowledge ; and the more so in proportion to the desire which you have of collecting and storing those facts that may enable you to understand the operations of nature, as well as the works of ingenious artists. These, I trust, will lead you insensibly to admire the wisdom and goodness by means of which the whole system of the universe is constructed and supported.
  Emma. But can philosophy be comprehended by children so young as we are? I thought that it had been the business of men, and of old men too.
  F. Philosophy is a word which, in its original sense, signifies only a love or desire of wisdom ; and you will not allow that you and you brother are too young to wish for knowledge.
  E. So far from it, that the more knowledge I get the better I seem to like it ; and the number of new ideas which, with a little of your assistance, I have obtained the "Evenings at Home," and the great pleasure which I have received from the perusal of that work, will, I am sure, excite me to read it again and again.
  F. You will find very little, in the introductory parts of natural and experimental philosophy, that will require more of your attention than many parts of that work with which you were so delighted.
  C. But in some books of natural philosophy, which I have occasionally looked into, a number of new and uncommon words have perplexed me ; I have also seen references to figures, by means of large letters and small, the use of which I did not comprehend.
  F. It is frequently a dangerous practice for young minds to dip into subjects before they are prepared, by some previous knowledge, to enter upon them ; since it may create a distaste for the most interesting topics. Thus, those books which you now read with so much pleasure would not have afforded you the smallest entertainment a few years ago, when you must have spelt out almost every word in each page. The same sort of disgust will naturally be felt by persons who should attempt to read works of science before the leading terms are explained and understood. The word angle is continually recurring in subjects of this sort ; do you know what an angle is ?
  E. I do not think I do : will you explain what it means ?
  F. An angle is made by the opening of two straight * lines. In this figure there are two straight lines ab and cb meeting at point b, and the opening made by them is called an angle.
  C. Whether that opening be small or great, is it still called an angle ?
  F. It is ; your drawing compasses may familiarize to your mind the idea of an angle ; the lines in this figure will aptly represent the legs of the compasses, and the point b the joint upon which they move or turn. Now you may open the legs to any distance you please, even so far that they shall form one straight line ; in that position only they do not form an angle. In every other situation an angle is made by the opening of these legs, and the angle is said to be greater or less, as that opening is greater or less. An angle is another word for a corner.
  E. Are not some angles called right angles ?
  F. Angles are either right, acute or  obtuse. When the line ab meets another line cd in such a manner as to make the angles abd and abc equal to one another, then those angles are called right angles. And the line ab is said to be perpendicular to cd. Hence to be perpendicular to, or to make right angles with, a line, means one and the same thing.
  C. Does it signify how you call the letters of an angle ?
  F. It is usual to call every angle by three letters, and that at the angular point must always be the middle letter of the three. There are cases, however, where an angle may be denominated by a single letter ; thus the angle abc may be called simply the angle b, for there is no danger of mistake, because there is but a single angle at the point b.
  C. I understand this ; for if, in the second figure, I were to describe the angle by the letter b only, you would not know whether I meant the angle abc or abd.
  F. That is the precise reason why it is necessary, in most descriptions, to make use of three letters. An acute angle (Fig. 1, abc) is less than a right angle ; and and obtuse angle (Fig. 3, abc) is greater than a right angle.
  E. You see the reason now, Charles, why letters are placed against or by figures, which puzzled you before.
  C. I do ; they are intended to distinguish the separate parts of each, in order to render the description of them easier both to the author and the reader.
  E. What is the difference, papa, between an angle and a triangle ?
  F. An angle being made by the opening of two lines and as you know that two straight lines cannot enclose a space, so a triangle abc is a space bounded by three straight lines. It takes its name from the property of containing three angles. There are various sorts of triangles, but it is not necessary to enter upon these particulars, as I do not wish to burden your memories with more technical terms than we have occasion for.
  C. A triangle, then, is a space or figure containing three angles, and bounded by as many straight lines.
  F. Yes, that description will answer our present purpose.

---

* Straight lines, in works of science are usually denominated right lines.

Here are Father, Charles and Emma then. To me, Father starts off sounding a little pompous and patronising. This is probably because this has been set up as an intimate conversation between an adult and his children, yet it is quite obviously being played for the audience and he's over-acting. We are familiar with this sort of staged reality in the present day; producers of current semi-fictional pieces are much more adept at natural dialogue than this is. It flows more freely in my head if I imagine it to be a stage production where Father's exaggerated earnestness comes across more as exaggerated characterisation rather than clunky exposition. This also allows me to imagine Charles and Emma pulling funny faces at the audience when papa is being tiresome; which makes me smile.

Given my previous misgivings regarding possible gender bias in the book, I'm pleased that it is Emma who brings up that philosophy is seen as "the business of men, and of old men too". Even though the response only addresses the age of students, I'd like to think that the fact a young girl asks the question shows a level of awareness and concern in the author concerning the education of women.

From this short exchange, Charles and Emma seem to be fairly indistinguishable from each other. It's possible that it was felt to be a "good thing" to have both a boy and a girl to appeal to the widest range of potential readers. What I'd really like is if they prove to have vastly different aptitudes; maybe Charles is a complete idiot at anything mathematical and Emma can't get her head round steam engines. Somehow I doubt it though. I imagine Father will turn out to be an exemplary teacher, while both Charles and Emma will be the swottiest of teacher's pets. Gold stars for everyone!

The book which the children had previously been enjoying, Evenings at Home, was a book of children's stories written in the 1790s which appears to have remained popular throughout the 19th century. It was authored by Anna Laetitia Barbauld and her brother Dr. John Aikin. Barbauld was a teacher and children's author at a time when female writers were exceedingly uncommon; her primers laid the ground for many educational volumes, these Scientific Dialogues would appear to be among their number. Evenings at Home is by no means the earliest example of children's literature, but the form was still very rare at this time.

Father looks to have done a good job explaining the concept of angles. I seem to remember my introduction to them being fairly similar. Having said that, I initially bridled at the assertion that 180 degrees was not an angle; to me angles have long since simply become a number between 0 and 360 (or more commonly 6.283). This is down to the extent of my learning and use of angles over the years, which far surpasses that of your average pre-teen.

The only observation I have regarding the triangles passage is to point out how extra-ordinarily short it is. It hardly seems worth having brought it up at all if all you're going to say is "this is a triangle, it has three angles". I think it says more about me than the author that I was looking forward to a discourse about the properties of a nice pointy isosceles triangle or maybe an elegant 3-4-5 right triangle.

The funniest thing in the dialogue appears during the attempt to explain notations on diagrammatic figures. Charles notes that he has repeatedly found technical notations confusing. Father presents Figure 1 to clear everything up. The only problem with this is that he's got it all upside-down. A slightly embarrassing start for the learned gentleman there!

The only bit of technical stuff here that I have never come across is straight lines being referred to as "right lines". In fact, a quick unscientific look at Wikipedia page reveals that the word right is not used in any sense on the page Line (geometry). [Not that you should trust Wikipedia without checking references.]


Next up, the divisibility of matter. I'm guessing we're not jumping straight into nuclear fission; that would be more than a little odd, not to mention massively anachronistic.

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Contents

So, here we are at the contents pages of this volume. Don't worry, we've almost got to the science. It's the first proper look at what delights the book has in store for us. There's a lot of transcribed text in this post so I'll try not to go on too much about anything. Besides, I wouldn't want to use up too much material for when we get around to the individual dialogues.

Let me have a look at this a chapter at a time and see what grabs my attention.

CONTENTS.

—

Conversation		Page
MECHANICS.
I.	Introduction	1
II.	Of Matter. Of the Divisibility of Matter	4
III.	Of the Attraction of Cohesion	7
IV.	Of the Attraction of Cohesion	11
V.	Of the Attraction of Gravitation	12
VI.	Of the Attraction of Gravitation	16
VII.	Of the Attraction of Gravitation	19
VIII.	Of the Attraction of Gravitation	22
IX.	Of the Centre of Gravity	26
X.	Of the Centre of Gravity	28
XI.	Of the Laws of Motion	31
XII.	Of the Laws of Motion	36
XIII.	Of the Laws of Motion	39
XIV.	Of the Mechanical Powers	42
XV.	Of the Lever	45
XVI.	Of the Lever	48
XVII.	Of the Wheel and Axis	52
XVIII.	Of the Pulley	56
XIX.	Of the Inclined Plane	59
XX.	Of the Wedge	61
XXI.	Of The Screw	63
XXII.	Of The Pendulum	67

Considering when the book was written, mechanics is an obvious place to start. We're relatively early on in the Industrial Revolution and there are fortunes to be made by people with an understanding of the basics behind the machines that will change the world. The dialogues concerning Cohesion and the Mechanical Powers sound intriguing. I'm not expecting too many surprises in this chapter though.

Conversation		Page
ASTRONOMY
I.	Of the fixed Stars	70
II.	Of the fixed Stars	73
III.	Of the fixed Stars and Ecliptic	76
IV.	Of the Ephemeris	80
V.	Of the Solar System	85
VI.	Of the figure of the Earth	89
VII.	Of the diurnal Motion of the Earth	92
VIII.	Of Day and Night	97
IX.	Of the annual Motion of the Earth	100
X.	Of the Seasons	102
XI.	Of the Seasons	105
XII.	Of the Equation of Time	110
XIII.	Of Leap Year	114
XIV.	Of the Moon	116
XV.	Of Eclipses	120
XVI.	Of the Tides	124
XVII.	Of the Harvest Moon	128
XVIII.	Of Mercury	132
XIX.	Of Venus	134
XX.	Of Mars	137
XXI.	Of Jupiter	139
XXII.	Of Saturn	141
XXIII.	Of the Herschel Planet	143
XXIV.	Of Comets	146
XXV.	Of the Sun	147
XXVI.	Of the fixed Stars	148

Excellent! Astronomy is always entertaining. I like the use of the antiquated phrase "fixed stars", we now know them as "stars"; the balls of burning gas that illuminate the universe. However, in historical astronomy anything visible in the heavens was a star (with the exception I think of the Moon and ironically the Sun). This brings us onto the Ephemeris which is a table of data describing the position of objects in the sky (I had to look that one up). There looks to be a lot of good stuff on the solar system and the motion of it's components. Though, how it fits in with our current understanding remains to be seen.

Next up are conversations on the planets. One thing instantly caught my attention; what is the Herschel Planet? Well it's Uranus. It was discovered in the 1780s by Sir William Herschel; when prompted to name the planet, he suggested "Georgium Sidus" (translated as "George's Star") after the King of England; this did not go down at all well with the French (among others). The name Uranus was globally adopted in the mid 19th century and has served us well ever since. The absence of Neptune is understandable as it wasn't observed until 1846 and the author had been dead for some years by then. No Pluto either, so that's an improvement on my education in one respect.

Conversation		Page
HYDROSTATICS
I.	Introduction	153
II.	Of the Weight and Pressure of Fluids	157
III.	Of the Weight and Pressure of Fluids	162
IV.	Of the Lateral Pressure of Fluids	166
V.	Of the Hydrostatic Paradox	168
VI.	Of the Hydrostatic Bellows	173
VII.	Of the Pressure of Fluids against the Sides of Vessels	176
VIII.	Of the Motion of Fluids	179
IX.	Of the Motion of Fluids	183
X.	Of the Specific Gravity of Bodies	187
XI.	Of the Specific Gravity of Bodies	190
XII.	Of the Methods of finding the Specific Gravity of Bodies	193
XIII.	Of the Methods of finding the Specific Gravity of Bodies	197
XIV.	Of the Methods of finding the Specific Gravity of Bodies	201
XV.	Of the Methods of finding the Specific Gravity of Bodies	203
XVI.	Of the Hydrometer	208
XVII.	Of the Hydrometer and Swimming	211
XVIII.	Of the Syphon and Tantalus's Cup	214
XIX.	Of the Diver's Bell	218
XX.	Of the Diver's Bell	221
XXI.	Of Pumps	223
XXII.	Of the Forcing-pump — Fire-engine — Rope-pump — Chain-pump — and Water-press	226

My fluid dynamics is a bit (in reality, very) rusty. However, I'm fairly sure I have never learnt about the "Hydrostatic Paradox" or indeed "Tantalus's Cup". I'd like to think the Cup is a chalice adorned with images of frolicking satyrs filled with chocolate flavoured alcohol; I'm fairly sure I'll be disappointed about this. Other than those two, it all looks very sensible and relevant to the budding industrialist. I'm not entirely sure why he felt the need for quite so many chapters on specific gravity; maybe it was a lot more important a couple of centuries ago.

Conversation		Page
PNEUMATICS
I.	Of the Nature of Air	231
II.	Of the Air-pump	233
III.	Of the Torricellian Experiment	238
IV.	Of the Pressure of the Air	240
V.	Of the Pressure of the Air	243
VI.	Of the Weight of the Air	246
VII.	Of the Elasticity of the Air	250
VIII.	Of the Compression of the Air	254
IX.	Miscellaneous Experiments on the Air-pump	258
X.	Of the Air-gun and Sound	260
XI.	Of Sound	264
XII.	Of the Speaking Trumpet	268
XIII.	Of the Echo	270
XIV.	Of the Echo	274
XV.	Of the Winds	278
XVI.	Of the Steam-engine	283
XVII.	Of the Steam-engine	288
XVIII.	Of the Steam-engine and Papin's Digester	290
XIX.	Of the Barometer	293
XX.	Of the Barometer, and it's Application to the Measuring of Altitudes	297
XXI.	Of the Thermometer	300
XXII.	Of the Thermometer	303
XXIII.	Of the Pyrometer and Hygrometer	307
XXIV.	Of the Rain-gauge, and Rules for judging of the Weather	311

Now, I can completely understand that, to the Victorian, the importance of learning about steam engines and how pistons work was paramount. What's with a conversation about "the Speaking Trumpet" though? Especially considering that the science of sound looks to be quite well covered in the material.

I absolutely need to know about "the Torricellian Experiment" and "Papin's Digester". I'm not going to spoil it for myself by researching them yet.

I'm not convinced about the barometer being covered in this chapter, I would have thought it sat better in Hydrostatics. Also, "Rules for judging the Weather"? Really? This seems just a touch off topic.

Conversation		Page
OPTICS
I.	Light: the Smallness and Velocity of its Particles	316
II.	Rays of Light — Reflection and Refraction	320
III.	Refraction of Light	323
IV.	Refraction and Reflection of Light	327
V.	Different Kinds of Lenses	331
VI.	Parallel diverging and converging Rays	334
VII.	Images of Objects. — Scioptric Ball, &c.	338
VIII.	Nature and Advantages of Light	341
IX.	Colours	344
X.	Reflected Light and Plain Mirrors	347
XI.	Concave Mirrors	350
XII.	Concave Mirrors. — Experiments	353
XIII.	Concave and Convex Mirrors	355
XIV.	Optical Deceptions, Anamorphoses, &c.	358
XV.	Different Parts of the Eye	362
XVI.	Manner of Vision	365
XVII.	Spectacles, and their Uses	368
XVIII.	Rainbow	372
XIX.	Refracting Telescope	376
XX.	Reflecting Telescopes	380
XXI.	Microscope	382
XXII.	Camera Obscura, Magic Lanthorn, and Multiplying Glass	388

The optics in this chapter should be fairly close to what I learnt in school, the basics have been understood for a long time (with grateful thanks to Newton).

I was faintly surprised to see the use of the word "particle" in association with light. When I was first taught about the subject, I was taught to see light as a wave; only after years of study was the relatively recent (by which I mean "in the last century") wave-particle duality brought in and I started to think about particles of the stuff. I'm going to be interested to see how the behaviour of light is explained in this respect.

I like the sound of a "Scioptric Ball", I sincerely hope it's as exciting as it appears to be.

Having tried many times, I always struggle to explain rainbows. It is quite a tricky thing to put into easily understandable sentences; I'm looking forward to see how the Rev. Joyce has done it.

Conversation		Page
MAGNETISM
I.	The Magnet	392
II.	Magnetic Attraction and Repulsion	394
III.	Methods of making Magnets	397
IV.	Mariner's Compass	401

A short chapter on magnetism, it seems fairly simple to me. Knowing what we know now it should have been rolled into the next chapter though; Electromagnetism is where it's all at nowadays don't you know.

Conversation		Page
ELECTRICITY
I.	Early History of Electricity	405
II.	Electrical Attraction and Repulsion	407
III.	Electrical Machine	412
IV.	Electrical Machine	415
V.	Electrical Attraction and Repulsion	419
VI.	Electrical Attraction and Repulsion	424
VII.	The Leyden Phial	427
VIII.	Lane's Electrometer, and the Electrical Battery	431
IX.	Experiments with the Battery	435
X.	Miscellaneous Experiments	440
XI.	Electrophorus — Electrometer — Thunder-house, &c.	444
XII.	Atmospherical Electricity	446
XIII.	Of Atmospheric Electricity — of Falling Stars — Aurora Borealis — Waterspouts and Whirlwinds — Earthquakes	450
XIV.	Medical Electricity	455
XV.	Animal Electricity — of the Torpedo — of the Gymnotus Electricus — of the Silurus Electricus	458
XVI.	General Summary of Electricity, with Experiments	461

The chapter on electricity is the one I'm most looking forward to; it's the only chapter I read any amount of before deciding to start this project. Science at the time of writing the book had a limited understanding of electricity; this makes for some of the most entertaining wrong science. I guarantee that dialogue 14 on "Medical Electricity" will be absolutely brilliant.

Again, there are things that seem a little out of the scope of the subject; falling stars, waterspouts, whirlwinds and earthquakes. There may be some fascinating wrongness in those conversations.

There are even more wonderful sounding things that I have no knowledge of. I'll take a stab at what they might/should be:

• Electrophorus - I'll take a serious guess that this is something that gives off light when electricity is applied to it; like a light bulb.
• Thunder-house - I'd like to think this is an outhouse at a particularly bawdy freehouse.
• Gymnotus Electricus - was this Galactus' herald while the Silver Surfer was on an Alpine skiing holiday? (Although something tells me it's probably an electric motor or similar)
• Silurus Electricus - absolutely has to be a Doctor Who villain. Failing that, using my outstanding powers of etymology, it could well be a Welshman with his finger stuck in a power socket.

I'm going to say that the Leyden Phial is what we would now refer to as a Leyden Jar; an essential piece of equipment if you're going to try to blow things up with static electricity.

[I'm going to need to remember about these guesses when I finally get around to writing up the chapter.]

Conversation		Page
GALVANISM
I.	Of Galvanism; its Origin; Experiments - of the Decomposition of Water	465
II.	Galvanic Light and Shocks	468
III.	Galvanic Conductors - Circles - Tables - Experiments	472
IV.	Miscellaneous Experiments	477
GLOSSARY AND INDEX		481

Finally a little chapter that seems to be a bit of chemistry and biology. I'm expecting frog legs in here or I'll be sorely disappointed.

So that's it. The road ahead for this blog is laid out. I shall come back and edit this post to add links to all the entries as I go from now on.

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