130 STUDIES IN THE DIFFRACTION SPECTRUM. [MEMOIR VII. 



ly admitted that there are heat rays which, like light 

 rays, have various refrangibilities, this conclusion was 

 quite accordant with Newton's results. Heat was con- 

 sidered as existing in the solar beam independent and 

 irrespective of light. In fact, the one might be easily 

 separated from the other. 



When, again, the Swedish chemist Scheele, investigat- 

 ing the chemical action of light, showed that there are 

 rays invisible to the eye, and of greater refraugibility 

 than the violet, which can produce the decomposition 

 of certain compounds of silver, these were considered to 

 be an additional element, and passed under the designa- 

 tion of chemical rays, deoxidizing rays, etc. Treated of 

 in the works of physics of those times as imponderable 

 bodies, there seemed to be no necessary limit to their 

 number. More than half a hundred ponderable sub- 

 stances were known. Why, then, should there not be 

 as many of these imponderable ones? This was the 

 view universally entertained at the time I began the 

 experimental study of radiations. For such as are con- 

 cerned in producing chemical changes I suggested a 

 special designation, which, however, did not find accept- 

 ance : the inappropriate and unmeaning appellation, ac- 

 tinic rays, was preferred. 



Meantime, however, the undulatory theory of light 

 had been steadily making its way. It was exhibiting 

 all the aspect of a great physical truth, in not only 

 rendering an explanation of known facts, but also in 

 predicting the occurrence of other facts previously un- 

 known. Persons who were in the front of the scientific 

 movement in this direction had thus their attention 

 forcibly drawn to a contemplation of the whole subject 

 from this new point of view. They very soon perceived 

 that from it bonds of interconnection between facts 

 hitherto supposed to be isolated might be discerned; 



