292 THE POPULAR SCIENCE MONTHLY. 



At a later period he devised the method of measuring the inten- 

 sity of the chemical action of light, afterward perfected and employed 

 by Bunsen and Roscoe in their elaborate investigations. This method 

 consists in exposing to the source of light a mixture of equal volumes 

 of chlorine and hydrogen gases. Combination takes place more or 

 less rapidly, and the intensity of the chemical action of the light is 

 measured by the diminution in volume. No other known method 

 compares with this in accuracy, and most valuable results have been 

 obtained by its use. 



In an elaborate investigation, published in 1847, Dr. Draper estab- 

 lished experimentally the following important facts : 



1. All solid substances, and probably liquids, become incandescent 

 at the same temperature. 



2. The thermometric point at which substances become red-hot is 

 about 977 Fahr. 



3. The spectrum of an incandescent solid is continuous ; it contains 

 neither bright nor dark fixed lines. 



4. From common temperatures nearly up to 977 Fahr., the rays 

 emitted by a solid are invisible. At that temperature they are red, 

 and, the heat of the incandescing body being made continuously to 

 increase, other rays are added, increasing in refrangibility as the tem- 

 perature rises. 



5. While the addition of rays so much the more refrangible as the 

 temperature is higher is taking place, there is an increase in the in- 

 tensity of those already existing. Thirteen years afterward Kirch- 

 hoff published his celebrated memoir on the relations between the 

 coefficients of emission and absorption of bodies for. light and heat, in 

 which he established mathematically the same facts, and announced 

 them as new. 



6. Dr. Draper claims, and we believe w 7 ith justice, to have been the 

 first to apply the daguerreotype process to taking portraits. 



7. Dr. Draper applied ruled glasses and specula to produce spectra 

 for the study of the chemical action of light. The employment of 

 ruled metallic specula for this purpose enabled him to avoid the 

 absorbent action of glass and other transparent media, as well as to 

 establish the points of maximum and minimum intensity with reference 

 to portions of the spectrum defined by their wave-lengths. He ob- 

 tained also the advantage of employing a normal spectrum in place of 

 one which is abnormally condensed at one end and expanded at the 

 other. 



8. We owe to him valuable and original researches on the nature 

 of the rays absorbed in the growth of plants in sunlight. These 

 researches prove that the maximum action is produced by the yel- 

 low rays, and they have been fully confirmed by more recent investi- 

 gations. 



9. We owe to him, further, an elaborate discussion of the chemical 



