166 Physical and Chemical Properties of the' [Sept. 



employed terms so general as to be applicable, whether the 

 properties treated of belong to a substance really distinct and 

 combined with light, or result simply from original differences 

 which exist among the different molecules of the same principle, 

 which, according to differences in the size or the velocity, or 

 -in both united, become capable of producing chemical combi- 

 nations, vision, and heat. 



Without attempting to decide between two opinions, which 

 go both beyond the facts observed, we may at least weigh their 

 relative probabilities, and compare the number of hypotheses 

 necessary in each to represent the same number of facts. If 

 we wish to consider solar light as composed of three distinct 

 substances, one of which occasions light, another heat, and the 

 third chemical combinations; it will follow that each of these 

 substances is separable by the prism into an infinity of different 

 modifications, like light itself; since we find, by experiment, 

 that each of the three properties, chemical, colorific, and calo- 

 rific, is spread, though unequally, over a certain extent of the 

 spectrum. Hence we must suppose, on that hypothesis, that there 

 exist three spectrums one above another ; namely, a calorific, a 

 colorific, and a chemical spectrum. We must, likewise, admit 

 that each of the substances which compose the three spectrums, 

 and even each molecule of unequal refrangibility which consti- 

 tutes these substances, is endowed, like the molecules of visible 

 light, with the property of being polarized by reflection, and of 

 escaping from reflection in the same positions as the luminous 

 molecules, &c. 



Instead of this complication of ideas, let us conceive simply, 

 according to the phenomena, that light is composed of a collec- 

 tion of rays unequally refrangible, and of course unequally 

 attracted by bodies. This supposes original differences in their 

 size and velocity, or in their affinities. Why should those rays, 

 which differ already in so many things, produce upon thermo- 

 meters, or upon our organs, the same sensations of heat or 

 light ? Wty should they have the same energy to form or sepa- 

 rate combinations ? Would it not be quite natural that vision 

 should not operate on our eyes, except within certain limits of 

 refrangibility ; and that too little or too much refrangibility 

 should render it equally incapable of producing that effect. 

 Perhaps these rays may be visible to other eyesthan ours, perhaps 

 they are so to certain animals, which would account for certain 

 actions that appear to us marvellous. In a word, we may con- 

 ceive the calorific and chemical faculty to varv through the whole 

 length of the spectrum, at the same time with the refrangibility, 

 but according to different functions; so that the calorific faculty 

 i:> at its minimum at the violet end of the spectrum, and at its 

 maximum at the red end; while, on the other hand, the che, 



