480 THE POPULAR SCIENCE MONTHLY. 



Quite different was it with a method the origin of which, we be- 

 lieve, 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 prop- 

 erties, and look into its peculiar individuality, its specific chemical 

 quality. The principle which is the basis of the new method of spec- 

 trum 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 result- 

 ing from the analysis of the beam of rays emitted by any body will 

 vary according to the chemical nature of that body. Spectum analy- 

 sis 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 sepa- 

 rates the conception of the body, viewed as to its general properties, 

 from the notion of it as individualized in such a manner as to consti- 

 tute 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 examina- 

 tion 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 de- 

 pendent for the principal characteristic on the length of the cord, and 

 for the secondary phenomena of volume, tone, etc., on other circum- 

 stances 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 decom- 

 position, 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 



