Here's a second dialogue covering the subject of cohesion. This time we actually have some examples that exhibit what we would call cohesion! Father does manage to drift into the topic of foodstuffs again; I'm starting to wonder whether these paternal lessons are happening just before supper time.
This conversation is more conventionally "physicsy". This is something I feel a lot more comfortable with.
CONVERSATION IV.OF THE ATTRACTION OF COHESION.
F. I will now mention some other instances of this great law of nature. If two polished plates of marble, or brass, be put together, with a little oil between them to fill up the pores in their surfaces, they will cohere so powerfully as to require a very considerable to separate them.-Two globules of quicksilver, placed very near to each other, will run together and form one large drop.-Drops of water will do the same.-Two circular pieces of cork placed upon water at about an inch distant will run together.-Balance a piece of smooth board on the end of a scale beam ; then let it lie flat on water, and five or six times its own weight will be required to separate it from the water. If a small globule of quicksilver be laid on clean paper, and a piece of glass be brought into contact with it, the mercury will adhere to it, and be drawn away from the paper. But bring a larger globule into contact with the smaller one, and it will forsake the glass, and unite with the other quicksilver.
C. Is it not by means of the attraction of cohesion, that the little tea which is generally left at the bottom of the cup instantly ascends in the sugar when thrown into it ?
F. The ascent of water or other liquids in sugar, sponge, and all porous bodies, is a species of this attraction, and is called capillary * attraction : it is thus denominated from the property which tubes of a very small bore, scarcely larger than to admit a hair, have of causing water to stand above its level.
C. Is this property visible in no other tubes than those the bores of which are so exceedingly fine ?
F. Yes, it is very apparent in tubes whose diameters are one-tenth of an inch or more in length, but the smaller the bore, the higher the fluid rises ; for it ascends, in all instances, till the weight of the column of water in the tube balances, or is equal to, the attraction of the tube. By immersing tubes of different bores in a vessel of coloured water, you will see that the water rises as much higher in the smaller tube, than in the larger, as its bore is less than that of the larger. The water will rise a quarter of an inch, and there remain suspended in a tube, whose bore is about one-eighth of an inch in diameter.
This kind of attraction is well illustrated, by taking two pieces of glass, joined together at the side b c, and kept a little open at the opposite side a d, by a small piece of cork e. In this position immerse them in a dish of coloured water f g, and you will observe that the attraction of the glass at and near b c, will cause the fluid to ascend to b, whereas about the parts d, it scarcely rises above the level of the water in the vessel.
C. I see that a curve is formed by the water.
F. There is, and to this curve there are many curious properties belonging, as you will hereafter be able to investigate for yourself.
E. Is it not upon the principle of the attraction of cohesion, that carpenters glue their work together ?
F. It is upon this principle that carpenters and cabinetmakers make use of glue ; that braziers, tinmen, plumbers, &c. solder their metals ; and that smiths unite different bars of iron by means of heat. These, and a thousand other operations of which we are continually the witnesses, depend on the same principle as that which induced your mamma to use the white lead in mending her saucer. And you ought to be told, that though white lead is frequently used as a cement for broken china, glass, and earthenware, yet, if the vessels are to be brought again into use it is not a proper cement, being an active poison ; beside, one much stronger has been discovered, I believe, by a very able and ingenious philosopher, the late Dr. Ingenhouz ; at least I had it from him several years ago ; it consists simply of a mixture of quick-lime and Gloucester cheese, rendered soft by warm water, and worked up to a proper consistency.
E. What ! do such great philosophers, as I have heard you say Dr. Ingenhouz was, attend to such trifling things as these ?
F. He was a man deeply skilled in many branches of science l and I hope that you and your brother will one day make yourselves acquainted with many of his important discoveries. But no real philosopher will consider it beneath his attention to add to the conveniences of life.
C. This attraction of cohesion seems to pervade the whole of nature.
F. It does, but you will not forget that it acts only at very small distances. Some bodies indeed appear to possess a power the reverse of the attraction of cohesion.
E. What is that, papa ?
F. It is called repulsion. Thus water repels most bodies till they are wet. A small needle carefully placed on water will swim : flies walk upon it without wetting their feet : the drops of dew which appear in a morning on plants, particularly on cabbage plants, assume a globular form, from the mutual attraction between the particles of water ; and upon examination it will be found that the drops do not touch the leaves, for they will roll off in compact bodies, which could not be the case if there subsisted any degree of attraction between the water and the leaf.
If a small thin piece of iron be laid upon quicksilver, the repulsion between the different metals will cause the surface of the quicksilver near the iron to be depressed.
The repelling force of the particles of a fluid is but small ; therefore, if a fluid be divided it easily unites again. But if a glass or any hard substance be broken, the parts cannot be made to cohere without being first moistened because the repulsion is too great to admit of a re-union.
The repelling force between water and oil is likewise so great, that it is almost impossible to mix them in such a manner that they shall not separate again.
If a ball of light wood be dipped into oil, and then put into water, the water will recede so as to form a small channel around the ball.
C. Why do cane, steel, and many other things, bear to be bent without breaking, and, when set at liberty again, recover their original form ?
F. That a piece of thin steel, or cane, recovers its usual form after being bent, is owing to a certain power, called elasticity, which may, perhaps, arise from the particles of those bodies, though disturbed, not being drawn out of each other's attraction ; therefore, as soon as the force upon them ceases to act, they restore themselves to their former position. But our half hour has expired ; I must leave you.
* From capillus, the Latin word for hair.
The first paragraph describes a number of examples that at least partially show cohesive forces at work.
Droplets of mercury and water are droplets specifically due to cohesion; the similar particles within the drop are sticking to each other and it is this which prevents them from succumbing to gravity and spreading to a monomolecular film all over the table and the floor. Once two similar droplets touch, it is cohesion that draws them together into a single larger droplet. However, until they touch there is insufficient cohesive force to join them; they will come to touch due to forces from the surrounding environment, such as rolling on an uneven surface or being blown by air currents. It is possible for the droplets to be attracted to each other if they are given an opposite electric charge.
An important effect of cohesion in a liquid is the phenomenon of surface tension, this is key to most of the examples in this dialogue. The molecules in a liquid all attract each other with this cohesive force. Seeing that, within the main body of a liquid, the molecules are equally close to each other, these attractions cancel each other out. However, a molecule at the surface of a volume of liquid has a net force acting on it pulling it into the body of the liquid. This results in the surface of liquids maintaining as smooth and as small a surface area as possible given all forces that are acting on it; those extra forces commonly include gravity and cohesive forces from contact with other surfaces (which may be that of other fluids).
Pieces of cork floating on water stick to each other by a combination of cohesion and adhesion. As previously described, the water molecules cohere to one another creating a surface tension. The water and the cork adhere to each other, which causes the water to be pulled up the side of the cork creating a meniscus. If two pieces of cork on the surface of a volume of water are near to each other then they are pulled together by the water's surface which attempts to minimise the water's surface area. I couldn't remember exactly how water and cork interacted so I did a quick experiment. I had never previously tried this with square pieces of cork; when the menisci of the two corks are overlapping you get an effect similar to a floating pair of weak magnets. I took a couple of photos, I hope you like them.
The force required to separate a smooth board from the surface of water has little to do with the weight of the board; rather, it is dependent on the surface area which is in contact with the water combined with the strength of the adhesive force between the particles of the two materials.
The example of transferring mercury from paper to glass and then to a larger globule of mercury shows the relative strengths of some of the cohesive and adhesive forces that apply to mercury. Mercury adheres to glass more so than it does to paper; the cohesive forces acting on mercury are greater than the adhesive ones between mercury and glass. This might be wrong; I don’t think that glass and mercury stick to each other very much at all. While I do possess some mercury in an ancient tilt switch, I shall not be breaking it open to have a play with the poisonous stuff.
When a narrow tube is put into a liquid, surface tension in combination with the attraction of adhesion cause capillary action. Where the cohesion of the liquid is less than the adhesion with the material of the tube, as is the case with water and glass, the liquid is pulled up the internal surface of the tube until the sum of all forces acting on the liquid equalise; the forces being cohesive, adhesive and gravitational. When the cohesion of a liquid is greater than the adhesion with the tube, as with mercury and glass, the liquid is pulled down the tube.
I'm not doubting that "Dr Ingenhouz" developed a cheese based glue. I think this is one of the most inelegant and back-handed instances of name dropping I've come across. Jan Ingenhousz was a Fellow of the Royal Society with a massive body of work in biology and chemistry, including papers that make him the discoverer of photosynthesis! In my opinion, it's frankly quite rude to refer to this man in print as a very able and ingenious philosopher with whom I once had a very interesting conversation about sticky cheese. The Reverend Joyce is going down in my estimation.
The examples of repulsion in the text are explicable with exactly the same mixture of cohesive and adhesive forces that I set out above. As in the case of the reverse capillary action of mercury which I mentioned, the important factor is that the cohesion within the liquid is greater than the adhesion of the liquid to the surface it seems to be repelled from.
The repulsion of oil and water is a fairly extreme case of the adhesive force between two liquids being low compared to their individual cohesive forces. They will not mix; but they can form a suspension of tiny droplets of oil within water which will eventually settle out into a layer of oil sitting on top of water. Molecules that display such dislike for water are known as hydrophobes, not to be confused with hydrophobia.
I don't think I need to go into elasticity at any length beyond the dictionary definition; an elastic object is one "that spontaneously resumes its normal bulk or shape after contraction, expansion, or distortion by an external force"
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