CONVERSATION III

Of the Attraction of Cohesion.

FATHER. Well my children, have y ou reflected upon what we last conversed aboat? Do you comprehend the several instan ces which I enumerated as examples of the minute division of matter?

Emma. Indeed the examples which you gave us very much excited my wonder and admiration, and yet from the thinnes: 3 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 mi ute, should possess all the properties of the larger ones, such as a heart, veins, blood, &c.

Father. I can the next bright morning, by the help of my solar microscope, s how

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.

Charles. If there be no more difficulties in philosophy than we met with in our last lecture, I do not fear but that we shall, in general, be able to understand it. Are there not, papa, several kinds of gravity?

Father. 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 C

touch, and prevents them from separating, or which inclines the parts of bodies to unite, when they are placed sufficiently near to each other.

Charles. Is it then by the attraction of cohesion that the parts of this table, or of the pen-knife, are kept together?

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 Insti tutes of Natural Philosophy.

Charles. You once showed me that two deaden 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 un

luckily 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 dif ferent 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.

Charles. You put me in mind papa, of