and Molecules" before the British Association at Bradford in 1873, from which it is interesting to quote the following.
Each molecule 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 application of magnetic and electric fields. The apparent change of frequency of vibration with the motion of the source relative to the observer 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., hydrogen, 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.
- Maxwell used the term "molecule" where we now use the term "atom."