NATIONAL ACADEMY OF SCIENCES. 



and in the violet regions remained unaffected. An attempt was 

 then successfully made to effect the decomposition of carbonic-acid 

 by the green parts of plants also placed in the solar spectrum. Water 

 freed from air by boiling was saturated with carbonic acid gas, and 

 in it was immersed grass whose surfaces had been carefully freed of 

 air. Seven tubes thus prepared were placed each in one of the seven 

 colors of the spectrum and carefully observed. In a few minutes 

 after the beginning of the experiment the tubes on which the orange, 

 the green, and the yellow rays fell began to give off minute bubbles 

 of gas, and in an hour and a half sufficient was collected for accurate 

 measurement. Forty-three volumes of gas were produced in the 

 yellow-green, twenty -four and three-fourth volumes in the red-orange, 

 four and one-tenth in the green-blue, one in the blue, and none in 

 the other colors. The memoir in which these results were first pub- 

 lished was read to the American Philosophical Society on the occa- 

 sion of its centennial anniversary in 1843. 



But it was mainly by means of spectrum investigations that Dr. 

 Draper studied the components of radiant energy. In the earlier 

 stages of his work he, in common with his contemporaries, regarded 

 heat and light as imponderable agents, entirely distinct from one 

 another, though coexisting in solar light. In his later memoirs* 

 however, having accepted the essential unity of radiant energy, he 

 discusses with great ability, in his memoir of 1872, on the distribu- 

 tion of the chemical force in the spectrum, the dependence of the 

 result upon the nature of the surface upon which the light falls. In 

 this memoir he shows : First, that so far from chemical influences 

 being restricted to the more refrangible rays every part of the spec- 

 trum, visible and invisible, can produce chemical changes and can 

 modify the molecular arrangement of bodies ; and, second, that the 

 rays effective in producing chemical or molecular changes in any 

 special substance are determined solely by the absorptive power of 

 that substance. Thus silver salts, for example, blacken most 

 readily in the more refrangible regions. But even these have their 

 peculiarities. An iodide of silver plate long exposed to the spectrum ? 

 with absolute exclusion of extraneous light, is normally darkened 

 in the more refrangible and receives a white stain in the less re- 

 frangible regions. If, however, the plate receive weak diffused 

 light during exposure it will be found on developing that every ray 

 that the prism can transmit, from below the extreme red to beyond 

 the]extreme violet, has been active. Indeed, it was by this very process 



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