SKETCH OF DR. J. W. DRAPER. 363 



is that of the decomposition of carbonic acid by the leaves of plants, 

 under the influence of sunshine. On this the whole vegetable world 

 depends for its growth, and the whole animal world, directly or indi- 

 rectly, for its food. The decomposition in question is essentially a 

 deoxidation, and up to about 1840 it was generally supposed to be 

 due to the violet rays of the spectrum, which, in accordance with the 

 views held at that time, were regarded as producing deoxidizing 

 actions, and were consequently known as deoxidizing rays. But 

 this was altogether an assumption unsupported by experimental 

 proof. Prof. Draper saw that there was but one method for the abso- 

 lute solution of the problem, and that was by causing the decomposi- 

 tion to take place in the spectrum itself. In this delicate and beautiful 

 experiment he succeeded, and found that the decomposition was 

 brought about by the yellow rays, at a maximum by those in the 

 vicinity of the Fraunhofer fixed line d, and that the violet rays 

 might be considered as altogether inoperative. The memoir contain- 

 ing this result was first read before the American Philosophical So- 

 ciety, in Philadelphia, and immediately republished in London, Paris, 

 and Berlin. It excited general interest among chemists. Even up 

 to the present year it has furnished to the German experimenters the 

 basis of a very interesting discussion in photo-chemistry. 



In 1842 Dr. Draper discovered that not only might the Fraunhofer 

 fixed lines in the spectrum be photographed, but that there exists a 

 vast number of others beyond the violet, which up to that time had 

 been unknown. He also found three great lines less refrangible than 

 the red, in a region altogether invisible to the eye. Of these new 

 lines, which more than doubled in number those of Fraunhofer, he 

 published engravings. He also invented an instrument for measur- 

 ing the chemical force of light — the chlor-hydrogen photometer. This 

 was subsequently extensively used by Bunsen and Roscoe in their 

 photo-chemical researches. In their paper, read before the Royal So- 

 ciety, in 1856, they say, " With this instrument Draper succeeded in 

 establishing experimentally some of the most important relations of 

 the chemical action of light." 



Most of the papers he had written up to 1844 were in that year 

 collected and published together, in a book bearing the title of a 

 treatise on " the Forces producing the Organization of Plants." In 

 this there are a great many experiments on capillary attraction, the 

 flow of sap, endosmosis, the influence of yellow light on plants, etc. 



His memoir " On the Production of Light by Heat," published in 

 1847, was an important contribution to spectrum analysis ; among other 

 things it gave the means for determining the solid or gaseous condition 

 of the sun, the stars, and the nebulae. In this paper he established ex- 

 perimentally that all solid substances, and probably liquids, become 

 incandescent at the same temperature ; that the thermometric point at 

 which such substances are red-hot is about 977° Fahr. ; that the 



