184 Prof. J. Le Conte on the Influence of 



Exp. 3. — Mould Candle, to consume 1 inch, took in 

 Dark, Temp. 68°, I Sunshine, Temp. 80°, I Ratio. 



Se^'O^ I 59">0^ I 1:1053 



Exp. 4. — Taper, to consume 1 inch, took in 

 Dark, Temp. 67°, 1 Sunshine, Temp. 79°, I Ratio. 



4-" 30^ I S^O^ I 1:1-111. 



Exp. 5. — Taper in Sunshine lost in 10 minutes 

 In Painted Lantern, | In Uncoated Lantern, I Ratio. 



16'5 grs. I 15 grs. | 1 : 1100 



A sixth experiment, of a similar character, made in a strong 

 moonlight, indicated no such diminution in the rate of con- 

 sumption. 



The conclusion to which Dr. M'Keever came was, that solar 

 light does exercise a positive retarding influence on the process 

 of combustion. 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 chemical rays may be sup- 

 posed to exercise a deoxidizing power, which to some extent 

 interferes with the rapid oxidation of the combustible matter. 

 In confirmation of this opinion, Dr. M'Keever made an experi- 

 ment 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 Dr. M'Keever's experiments, I 

 found it impossible to secure that freedom from agitation in the 

 atmosphere, during the exposure of the burning body to the 

 influence of sunshine in the open air, which such an investigation 

 demanded. This was his method of conducting {he, first four 

 experiments given in the Table. The powerful influence exer- 

 cised by comparatively slight disturbances in the air on the 

 rapidity of combustion, I'cnders attention to this circumstance of 

 controlling importance. His method of obviating this difficulty 

 by the use of lanterns (as indicated in experiment 5) is objec- 

 tionable, from the impossibility of securing precisely identical 

 conditions in relation to the supply of air in the interior. There 

 were likewise other considerations which urged me to modify 

 his method of conducting the investigation. It occurred to me 

 that, as in his experiments the temperature of the air which sup- 

 plied oxygen for combustion in the sunshine was about 12° F. 

 above that in the darkened room, the rarefaction produced by 

 heat might exercise some influence in retarding the rate of burn- 

 ing in the sunlight. 



In conducting my experiments, I endeavoured to secure two 

 conditions, viz. — 



1. Absolute calmness in the atmosphere. 



