Quite different was it with a method the origin of which, we believe, goes back to the very birth of optics. This method is likewise founded on the actions of bodies on light, but, so rich and profound are the modifications with which it has to deal, that it has been able to pass over that in the matter which concerns only its general properties, and look into its peculiar individuality, its specific chemical quality. The principle which is the basis of the new method of spectrum analysis is as simple as general, and may be stated by saying that the elementary rays emitted by every kind of radiating gasiform matter depend upon and characterize the chemical species to which that matter belongs. Hence it follows that the spectral image resulting from the analysis of the beam of rays emitted by any body will vary according to the chemical nature of that body. Spectum analysis is founded upon the examination of spectra.
It must be added that the chemical nature of a body is not the exclusive element in the constitution of its spectrum, but that that may vary with the physical circumstances of the phenomenon, the temperature, the cause generating the radiation, etc.; but these are subordinate effects, which only add to the richness of the method, without detracting from its certainty and its value.
We have been able to leap over the enormous distance which separates the conception of the body, viewed as to its general properties, from the notion of it as individualized in such a manner as to constitute a chemical species by regarding light not only as a whole, but also in its elementary parts; by not only studying the whole beam and the general modifications that affect it, but by extending the examination to the elementary rays of which it is composed. The little mass of matter forming the chemical molecule, when it can vibrate freely, as in the gaseous state, emits a peculiar system of waves, a system which varies principally with the chemical species of the molecule, but which varies also, though rather secondarily, with the distance apart of the molecules and the nature and intensity of the forces that induce a vibratory movement in it. We might illustrate the nature of the system of luminous rays emitted by such a molecule by comparing it to the system of sounds given off by a vibrating cord, which is dependent for the principal characteristic on the length of the cord, and for the secondary phenomena of volume, tone, etc., on other circumstances accompanying the vibration.
It is proper to remark at this point that, when we analyze light in this way to examine it in its elements, we perform an operation entirely parallel to that of the chemist who separates the simple elements of a compound body. The elementary ray is a chemical species in light. It has all the characteristics of a species. It is incapable of decomposition, it has an individuality of its own, characterized by its wave-length, by the physiological effects it induces, whether acting alone or in association with other rays, and by the phenomena which it exhibits