Father. 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 the second edition of Dr. Enfield's Institutes of Natural Philosophy.

Charles. 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?

Father. 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.

Emma. Then, papa, 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?

Father. 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.

Emma. 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?

Father. It was, my dear; and hence you will easily learn that many operations in cookery are in fact nothing more than different methods of eausing 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.

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Charles. You put me in mind, papa, of the fable of the man blowing hot and cold: for in the instance of the lead, fire overcomes the attraction of cohesion; and the same power, heat, when applied to puddings, bread, &c. causes their parts to cohere more powerfully. How are we to understand this?

Father. 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 operations 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.

Emma. 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 bones have for each other?

Father. 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.*

Father. 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 force to separate them.-Two globules of quick

* See Vol. II. Conver. XL.

silver 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.

Charles. Did not you tell me that it was 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?

Father. 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 very small bore, scarcely larger than to admit a hair, have of causing water to stand above its level.

Charles. Is this property visible in no other tubes than those, the bores of which are so exceedingly fine?

* From capillus, the Latin word for hair.