CONSTITUTION OF MATTER EUTHEEFOKD. 173 



is supposed to characterize a liquid can contemporaneously exist 

 with an ordered arrangement of some of the constituent molecules. 



LIGHT SPECTRA. 



We will now direct our attention to another type of phenomenon 

 which ultimately promises to throw much light on the detailed struc- 

 ture of the atom. When the light from an incandescent vapor or 

 gas is passed through a prism or reflected from a grating it is re- 

 solved and gives a characteristic spectrum consisting of a number 

 of bright lines. By suitable methods, the wave length of these 

 radiations can be determined with great accuracy. Each of these 

 lines represents a definite and characteristic mode of vibration of the 

 atom, and from the exceeding complexity of the spectra of many of 

 the heavy elements w^e are forced to conclude that an atom can vibrate 

 in a great variety of ways. When the meaning of the dark lines 

 in the solar spectrum was correctly interpreted we were enabled at 

 one stride to extend our methods of observation to the sun and the 

 farthest fixed stars. It was soon recognized that atoms of the same 

 element always vibrated the same way under all conditions. It was 

 found, for example, that hydrogen atoms in the earth vibrated in 

 exactly the same way as the same atoms in a distant star. The im- 

 portant bearing of this result on the structure of atoms was pointed 

 out by Clerk Maxwell, in his well-known address on Atoms and 

 Molecules, before the British Association, at Bradford, in 1873, 

 from which it is interesting to quote the following : 



In the heavens we discover by their light, and by their light alone, stars so 

 distant from each other that no material thing can ever have passed from one 

 to another; and yet this light, which is to us, the sole evidence of the existence 

 of these distant worlds, tells iis also that each of them is built up of molecules 

 of the same kinds as those which we find on earth. A molecule of hydrogen, 

 for example, whether in Sirius or in Arcturus, executes its vibrations in 

 precisely the same time. Each molecule,* therefore, throughout the universe 

 bears impressed upon it the stamp of a metric system as distinctly as does the 

 meter of the archives at Paris, or the double royal cubit of the temple of 

 Karnac. 



No theory of evolution can be formed to account for the similarity of 

 molecules, for evolution necessarily implies continuous change, and the molecule 

 is incapable of growth or decay, of generation or destruction. 



None of the processes of nature, since the time when nature began, have 

 produced the slightest difference in the properties of any molecule. We are 

 therefore unable to ascribe either the existence of the molecules or the identity 

 of their properties to any of the causes which we call natural. 



On the other hand, the exact equality of each molecule to all others of the 

 same kind gives it, as Sir John Herschel has well said, the essential character 

 of a manufactured article, and precludes the idea of its being eternal and self- 

 existent. 



1 Maxwell used the term " molecule " where we now use the term " atom." 



