lecular weight of any substance, therefore, which can be brought into the gaseous condition, is found by simply determining experimentally the specific gravity of its vapor relatively to hydrogen.
In this way the physicist ascertains the molecular weights of all easily vaporizable bodies, and these are found to be in uniform and exact agreement with those which the chemist deduces from the law of combining proportions. The molecular hypothesis is thus brought to a crucial test; and two entirely independent lines of inquiry agree in giving it support of such a character as compels conviction. The law of gravitation and the undulatory theory of light do not command more cogent circumstantial evidence than this.
We have now briefly reviewed the fields from which the certain data of molecular science are gathered. We have weighed the molecules of gases, and measured their velocity with a high degree of precision. But there are other points, such as the relative size of the molecules of various substances, and the number of their collisions per second, about which something is known, though not accurately.
With regard to the absolute diameter of a molecule and their number in a given space, everything at present is only probable conjecture. Still, it may be interesting to state the views which are held on these questions by such investigators as Sir William Thomson and the late Professor Clerk-Maxwell; but we give these without attempting to indicate the character of the speculations on which their conclusions rest.
Summing up, then, both the known and unknown, we may say that the molecular weights and velocities of many substances are accurately known. It is also conjectured that collisions take place among the molecules of hydrogen at the rate of seventeen million-million million per second; and in oxygen they are less than half that number. The diameter of the hydrogen molecule may be such that two million of them in a row would measure a millimetre. Lastly, it is conjectured that a million-million-million-million hydrogen molecules would weigh about four grammes; while nineteen million-million-million would be contained in a cubic centimetre. Figures like these convey no meaning to the mind, and they are introduced here only to show the character and present state of the research.
A few concluding words must indicate the tremendous energy residing in the forces by which the molecules of matter are bound together. The molecules of water, for example, can not be separated from each other without changing the liquid into a gas, or, in other words, converting the water into steam; and this can only be accomplished by heat. The force required is enormous; but, since the determination, by Joule, of the mechanical equivalent of heat, we are able not only to measure this force, but also to express it in terms of our mechanical standard. It has been found that, in order to pull apart the molecules of one pound of water, it is necessary to exert a mechan-
