NATURAL PHILOSOPHY. 199 



eye when exposed to the direct sunshine ; while a portion of ignited char- 

 coal, which glows in the dark, appears to be extinguished when placed in 

 the sunlight. These familiar phenomena, attributable to well-established 

 physico-physiological laws, seem to afford a much more rational explana- 

 tion of the origin of the popular opinion, than to suppose it to be based 

 upon accurate observations relating to the actual rapidity of burning. 



In the year 1825, Dr. Thomas McKeever, of England, published a series 

 of experiments in the Annals of Philosoplnj, which seemed to show that 

 there is a real foundation for the popular opinion, and that solar light does 

 actually retard the process of combustion. These results were copied by 

 the contemporary scientific journals, and were considered as proving be- 

 yond a doubt, that what had previously been esteemed a " vulgar error," 

 was, in reality, an instinctive popular induction, based upon experience and 

 observation. Even Gmelin, in his Hand Book of Chemistry, announces Dr. 

 McKeever's conclusions, witlftut expressing any misgivings in relation to 

 their accuracy. 



The important bearing which these results seem to have on the influence 

 of solar light on chemical processes, as well as on certain modern theories, in- 

 duced Professor Le Conte, during the months of May and June last, to un- 

 dertake a scries of experiments with the view of testing the validity of Dr. 

 McKeever's conclusions. In order to understand what follows, it is neces- 

 sary to state that Dr. McKeever made his experiments by determining the 

 rate of burning of tapers and candles, when the combustion took place al- 

 ternately in a darkened room, and in the sunshine in the open air. In every 

 case lie found the combustion Avas more rapid in the dark than in the sun- 

 shine; the excess in the former varying from five to eleven per cent. He 

 supposes this effect to be owing to the well-known influence of the solar rays 

 on many chemical processes ; in some instances accelerating them, but in 

 others retarding them. Under this point of view, the c/temical rays may be 

 supposed to exercise a deoxidizing power, which, to some extent, interferes 

 with the rapid oxidation of the combustible matter. In confirmation of 

 this opinion, Dr. McKeever made an experiment which appears to indicate 

 that a taper burns more rapidly in the red than in the violet extremity of the 

 solar spectrum. 



In attempting a repetition of these experiments, Professor Le Contc found 

 it impossible to secure that freedom from agitation in the atmosphere which 

 the investigation demanded, so long as he pursued Dr. McKcevcr's plan of 

 exposing the burning body to the sunshine in the open air. There were, like- 

 wise, other considerations which urged him. to modify the method of inves- 

 tigation ; and among others, it occurred to him, that, as in Dr. McKeever's 

 experiments, the temperature of the air which supplied oxygen for combus- 

 tion in the sunshine, was about 12 above that in the darkened room, the 

 rarefaction produced by heat might exercise some influence in retarding the 

 rate of burning in the sunlight. He also desired to exaggerate the supposed 

 influence, by using a concentrated pencil of solar light instead of ordinary 

 sunshine. 



In conducting his experiments Prof. Le Conte endeavored to secure two 

 conditions, viz : 



