692 THE POPULAR SCIENCE MONTHLY. 



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 airreement 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 com- 

 mand 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 mole- 

 cules of gases, and measured their velocity with a high degree of pre- 

 cision. 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 num- 

 ber 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 accu- 

 rately 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 conject- 

 ured that a million-million-million-million hydrogen molecules Avould 

 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 re- 

 siding in the forces by which the molecules of matter are bound to- 

 gether. 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 accom- 

 plished by heat. The force required is enormous ; but, since the deter- 

 mination, 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- 



