Solar Light on Combustion. 183 



before the fireplace, or of closing the shutters of the apartment, 

 may be traced to the prevalent belief, that the access of sunlight 

 to the burning materials is unfavourable to the continuance of 

 the phaenomenou of combustion. INIost physical philosophers 

 very naturally regard this opinion as a mere popular prejudice; 

 probably originating in the well-known apparent dulling or ob- 

 scuration of flames and of solid bodies in a state of ignition, 

 which takes place when they are exposed to strong light. The 

 flame of a jet of burning hydrogen is scarcely visible in the dif- 

 fused light of a clear day ; that of an ordinary alcohol lamp is 

 barely appreciable to the eye when exposed to the direct sun- 

 shine, while a portion of ignited charcoal which glows in the 

 dark appears to be extinguished when placed in the sunlight. 

 These familiar phsenomena, attributable to well-established 

 physico-physiological laws, seem to afi'ord a much more rational 

 explanation of the origin of the popular opinion, than to suppose 

 it to be based upon observations relating to the actual rapidity 

 of burning. 



About thirty-two years ago. Dr. Thomas M'Keever published 

 a series of experiments in the 'Annals of Philosophy*,^ which 

 seemed to show that there is a real foundation for the popular 

 impression, and that solar light does actually retard the process 

 of combustion. So far as I am aware, these remarkable expe- 

 riments have never been repeated. Leopold Gmelin, in his 

 'Handbook of Chemistryf/ announces Dr. M'Keever's results 

 without comment. The important bearing which they appear 

 to have on the influence of solar light on chemical processes, as 

 well as on the modern dynamical theory of the mutual convert- 

 ibility of the so-called imponderables, induced me, during the 

 months of May and June last, to undertake a series of experi- 

 ments with the view of testing the validity of Dr. M'Keever's 

 conclusions. The subjoined Table will exhibit his results in a 

 convenient form for future reference : — 



Exp. 1. — Green Wax Taper lost in 5 minutes 



In Dark, Temp. G7°, I In Sunshine, Temp. 78', I Ratio. 



9-25 grs. I 8-5 grs. | 1 : 1-088 



Exp. 2. — Taper lost, by burning 7 minutes, in 



Dark, Temp. 67°, | Sunshine, Temp. 78°, I Ratio. 



11 grs. I iOgrs. I l:I-]00 



* Annals of Philosophy, New Series, vol. x. p. 344, November 1825. 



t Leopold Gmelin's ' Handbook of Chemistry ' (Cavendish Society's 

 Translation), vol. ii. p. 3F). London, 1819. A contemporary journal, in 

 noticing these results, remarks, " It has always been considered a vulgar 

 error that the sun's light extinguishes a fire, but the following experiments 

 by Dr. M'Kcevcr put the matter beyond a doubt." (Brewster's Edinb. 

 Journ. of Science, vol. v. p. 180. 1826'.) 



