24 



THE POPULAR SCIENCE MONTHLY. 



The chemistry of light first became, in the full sense, a branch of 

 science capable of thorough investigation when Dr. Draper devised a 

 method of measuring the force of the chemical rays, and thus brought 

 the subject within the sphere of quantitative research. He showed 

 that these rays are absorbed in the cliemical combination, and that 

 the rate of absorption corresponds to the amount of chemical change. 

 He apf)lied mixtures of chlorine and hydrogen gases for this j)urpose, 

 wliich combine under the action of the chemical radiations, the meas- 

 urable rate of combination becoming tiie index of radiant activity. 

 Professors Roscoe and Bunsen subsequently employed sensitive i^apers 

 which were blackened in certain times to certain shades, as measurers 

 of the chemical force, and these Avere used at the Kew Observatory, 



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Figs. 3 and 4. Variation of Chemical Eats at Kew. 



near London, to trace the variations of chemical activity in the solar 

 rays. For, as the chemical force is not light, neither does it follow the 

 laws of light in producing its effects. Dr. Draper had previously 

 shown that, as we go southward toward the equator, and the light 

 increases in brilliancy, there is an increasing interference with the 

 chemical rays, the yellow space of no-chemical action widening with 

 the progress southward. It is also well known that there is much 

 greater difficulty in obtaining good photographic pictures under the 

 full blaze of a tropical sun than in our own latitude. The investiga- 

 tions at Kew were accordingly directed to the variations that the 

 chemical rays undergo at different hours of the day and at different 

 seasons of the year. The graphic diagrams, Figs. 3 and 4, show the 

 results that were arrived at in 1866. The curves exhibit the rise and 

 fall of the average monthly chemical intensity with the hour of the 

 day, from 6 A. M. to 6 p. m., throughout the year. We see from these 

 curves that the raaximum of chemical action occurs at twelve o'clock, 

 and that the forenoon rise and afternoon decline are very nearly equal, 

 while the chemical intensity of July is fully seven times as great as in 

 December. 



The statements that have been made that in Mexico, where the 

 light is very intense, from twenty minutes to half an hour is required 



