Camphor towards the Light. 363 



that reflected light produces furthest deposits ? Does it ? To 

 test this, a camphor-bottle was furnished with a brown paper 

 hood so as completely to cover it. A slit was then cut out 3^ 

 inches in length and J inch in width, extending from near the 

 top to the bottom of the bottle. A looking-glass was placed 

 near to and facing an east window, and, a few inches off, the 

 covered bottle with the slit opposite the looking-glass. The 

 morning was wet and cloudy with occasional bursts of sunshine ; 

 but in the course of two hours a deposit was formed nearest the 

 slit, and consequently nearest the reflected light. The experi- 

 ment was repeated over and over again with the same result — 

 modified in this way, that, if the morning sun were unusually hot, 

 the deposit was scattered over a considerable portion of the inte- 

 rior of the bottle, but in such case the largest crystals were 

 opposite the slit, where in fact the bottle was coldest. A bottle 

 was wrapped up in tinfoil, and a slit only y^th of an inch wide 

 cut out. The deposit was in this case confined to the slit and 

 its vicinity. The results of these and many experiments satisfied 

 me that the te remarkable modification " which Dr. Draper sup- 

 poses reflected light to have undergone, whereby it produces 

 only aphelion deposits, is really an effect of heat, capable of easy 

 explanation. 



The experiments on the protecting action of tinfoil rings, 

 though not very original in their conception, produce admirable 

 results. Had Dr. Draper been acquainted with Prevost's expe- 

 riments on dew*, and those of Carena on hoar frost f, he would 

 have seen how tinfoil favours or prevents the deposition of 

 moisture on glass according as its position favours or obstructs 

 the radiation of heat. He would, moreover, have seen that his 

 electrical theory is of no value in explaining phenomena which 

 fall under the operation of those great laws which regulate the 

 formation of dew. 



It is true also that vapour of water and of iodine move towards 

 the light, but only under certain circumstances. Dr. Draper's 

 method of performing the iodine experiment is, I think, charac- 

 teristic of his general mode of inquiry. He raises the iodine in 

 vapour, and then places the vessel containing it in the sunshine ; 

 that is, he hastens the result, and is satisfied when it is obtained. 

 Had the iodine been exposed to the varying influences of heat 

 and cold, sunshine and shade, it would have been found that the 

 deposit, like that of camphor, and also of water, &c, is sometimes 

 made towards the light, and at other times away from it. An 

 experiment should speak with many tongues ; and in order to 

 enable it to do so, it must be repeated many times, under circum- 



* Annates de Chirnie, an xi. 1802-3. 



t Memoires de I'Academie Royale des Sciences de Turin, 1813-14, 



2B2 



