H2 THE POPULAR SCIENCE MONTHLY 



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 us 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 1 therefore throughout the universe bears impressed upon it 

 the stamp of a metric system as distinctly as does the metre 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. 



While there is no doubt that an atom of an element in the earth or 

 in a star vibrates in identical fashion under the same physical conditions, 

 it is now known that the frequency of vibration of an element is not 

 the exact constant that was at first supposed. It is altered to a slight 

 extent by motion of the source, by change of pressure, and by the appli- 

 cation of magnetic and electric fields. The apparent change of fre- 

 quency of vibration with the motion of the source relative to the ob- 

 server has proved an invaluable method for studying the motion of 

 stars in the line of sight, while the displacement of the lines of hydrogen 

 in the sun has in the hands of Professor Hale and his assistants proved 

 of great power in throwing light on some of the physical conditions 

 that exist in that distant body. It has been found that there is order 

 and system in the great complex of modes of vibration of an atom, and 

 that many of the lines can be arranged in definite series whose rates of 

 vibration are connected by simple and definite laws. It is only within 

 the last year or two that we have been able to form some idea of the 

 origin of these spectra and the meaning of a spectral series. The fact 

 that the lightest and presumably the simplest atom known, viz., hydro- 

 gen, gives a very complicated light spectrum was at first, and quite 

 naturally, believed to indicate that the hydrogen atom must be a very 

 complex structure. We shall see later, however, that the hydrogen 

 atom is believed to have an exceedingly simple structure, and that the 

 complexity of the spectrum is to be ascribed rather to a complexity in 

 the laws of radiation. 



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



