DISTINCTION BETWEEN ATOMS AND MOLECULES.
See! I have dipped my pencil into the goblet, and brought up a drop of water. What force binds together the pencil and the drop? What holds the drop to other drops? Why is not this ice instead of water? If I shake the pencil in what direction does the drop fall? If the drop were larger than the world, which way would the world go? What other force is there in it which, according to Faraday, is equal to that in a flash of lightning? Here are, then, five great forces in a drop of water, yet none of them changes its nature. It is still H₂O.
Let us place this drop of water in the upright tube of an atomizer. Apply air. See, the drop has broken into thousands of particles. Now, suppose we could take one of these and place it in a flask. Apply heat, and we should separate the little particle into thousands of particles of steam, but each of these, and any lesser division of these, would still be H₂O. The minutest division of the water possible would be called a molecule, yet it would still be water, and composed of two parts of hydrogen and one of oxygen. The old ocean itself contains the same. The Cracow beds of salt are made of chlorine and sodium, and the minutest dust from one of its crystals would be found to contain the same elements, and in the same proportion, both by weight and measure. Molecules, then, combined, make only masses of the same nature. But molecules are composed of atoms, and whenever atoms of two or more different substances are combined they always form something different from either of these. The force that unites them is called chemical affinity, or sometimes the chemical force. For example, tenacious iron unites with a gas and forms a brittle, red substance, rust. Chlorine is a poisonous gas, and sodium will burn on water, both deadly; united they give us salt; absolutely essential to life. Hydrogen is the best substance in the world to burn, and oxygen the best supporter of combustion. When united they form water, which is universally employed to extinguish fire. Blue vitriol is blue, as the name implies, yet it is composed of four elements, two of which, H and O, are colorless, copper, which is red, and sulphur, which is yellow. Sulphur has little or no odor, and hydrogen has none, but when united they form a gas which has the odor of spoiled eggs. White sugar is nothing but black charcoal and water. It will thus be seen that here is a source of new things in nature. Whatever chemical affinity touches is changed.
And so we have found another force in our drop of water taken from the goblet, more wonderful than any yet named, a mighty, transforming energy which has but one worthy rival in the work of creating new things, the vital principle, and even that must yield at last to this all conquering power. If our goblet was large, and held a pound of water, (about a pint,) we should find that to pull the molecules apart, that is, make it into steam, would require a force which would raise four tons to the height of one hundred feet. But more wonderful still, to separate the pound of water into two chemical constituents would require, according to Prof. Cooke, an energy which would raise 5,314,200 pounds one foot. Our pint of water would then occupy 1800 times its present volume.
Let us now give a striking and beautiful illustration of chemical affinity. We will throw into this tumbler a piece of potassium (symbol K) half as large as a pea. This interesting metal was discovered by Davy in 1807. Its affinity for O is very great. As soon as it falls upon the water it abstracts oxygen and forms potassium oxide (potash), while the hydrogen and a small amount of volatilized K escape and are burned with a brilliant violet flame, on account of the heat evolved by the energetic chemical action.
POTASSIUM BURNING BY COMBINING WITH THE OXYGEN OF WATER.
IMPORTANT DATES.
The composition of water was discovered about one hundred years ago. Cavendish found hydrogen in 1776, and Priestly discovered oxygen in 1774, August 1st, a date which some one says “may almost be accepted as the birthday of modern chemistry.”