THEIR PHYSICAL INDEPENDENCE. 67 



tation, possesses an abundant internal evidence of truth. In our times extensive and 

 important applications have been made of it, so that it now includes an explanation of 

 all the phenomena of reflexion, refraction, polarization, double refraction, interference. 

 As yet. it has furnished no cL-ar account of the phenomena of absorption, the very phe- 

 nomena which are at the basis of all physiological and chemical facts in their relations 

 to luminous agency. As we shall presently prove, the decomposition of carbonic acid 

 and the function of digestion in plants depend on the absorption of light. Nor is 

 there yet included in it any representation of the various phenomena of heat In the 

 course of years, these things will probably be added, and one great generalization em- 

 brace all the phenomena of the imponderable agents. But the purposes of science re- 

 quire, until that event takes place, that we should continue to speak of the various 

 imponderable principles as though they were different agents, and treat them as though 

 they were separate existences. 



244. The first decisive evidence brought forward to establish the physical independ- 

 ence of the heat and light of the sunbeam, as has been stated (165), was derived from 

 the experiments of Sir W. HERSCHEL, the illustrious astronomer. He foun ! that, when 

 a beam of light is dispersed by a prism, and the resulting spectrum examined, by placing 

 thermometers in its different coloured spaces, the most luminous rays are not the hot- / 

 test, the maximum temperature occurring, not in the yellow, but in the red ray ; and even 



out of the red ray, and where the eye could detect no light whatever, heat was present, 

 for the thermometer there rose rapidly. Starting from these experiments, MELLONI added 

 farther proof by showing that transparency for light is not necessarily transparency for 

 heat ; that there are certain media more or less opaque to one of these imponderables, 

 and more or less transparent to the other, and that, by appropriate combinations, media 

 can be obtained which will allow light to pass them with very little diminution of its 

 intensity, but which stop the heat almost entirely. 



245. In the dark rays which come from a vessel of hot water, we have radiant heat 

 without light ; in the moonbeams we have, on the contrary, light without heat. 



246. By a series of experiments of a similar kind, the physical independence of the 

 tithonic rays has also been established. During the last century, it was observed by 

 ScriEt LE, that these rays occur abundantly beyond the violet extremity of the spectrum, 

 where the eye can discern no trace of light, an observation essentially of the same kind 

 as that made by Sir W. HERSCHEL for radiant heat In consequence of this discovery, 

 the occurrence of invisible rays was at once assumed, and, without any inquiry as to 

 their nature, their existence became an admitted fact in science. 



247. It does not appear, however, that any clear views were entertained as to the 

 precise character of these dark rays. Writers on optics spoke of them indifferently 

 under the name of violet rays, chemical rays, deoxydizing rays, and invisible rays. Of 

 all the benefits which can be conferred on an infant science, those arising from estab- 

 lishing clear, bold, prominent, decisive views of its fundamental agencies and their ac- 

 tions are by far the most valuable, for they fasten the attention forcibly. In all pro- 

 gressive sciences, each epoch of evolution, for sciences advance forward by starts, is 

 traceable to the announcement of some clear and vivid idea. 



