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 IIIOF 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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