362 Mr. C. Tomliuson on the Motion of 



of each other, gave contradictory evidence. They were both 

 placed under the same circumstances as to light, but with this 

 important difference as to weather — that one was protected from 

 it, and the other exposed to it. In fact there was no escaping 

 from the conclusion, notwithstanding Dr. Draper's protest, that 

 heat is largely concerned in the production of the phenomena. 

 It appeared in these early trials to be clearly made out that the 

 vapour of camphor which filled the bottle was disposed of on the 

 coldest surface of the glass, which might or might not be fur- 

 thest from the source which supplied both the light and the 

 heat, where it condensed after the manner of dew. 



To test this idea, all that seemed necessary was to expose the 

 vessel containing the camphor to the heating action of the sun 

 apart from its light. Accordiugly two cylindrical glass jars, A 

 and B, 7 inches high and 2-J inches in diameter, were charged 

 with about 100 grains of crude camphor: their mouths were 

 closed with bungs covered with tinfoil, and so enclosed in well- 

 fitting canisters of tinned iron. A was placed in the west, and 

 B in the east window, on the morning of the 8th of May. They 

 were examined at 3 p.m., the temperature in the west window 

 being 70°. A exhibited a faint deposit of crystals on the side of 

 the glass furthest from the light, that is, on the coldest part of 

 the jar. B gave no result, the sun having left the window before 

 this canister was placed. The next morning B was again ex- 

 amined, and a faint deposit was found furthest from the light. 



A result being thus obtained by heat alone, the experiment 

 was varied in the following manner. Four 8- ounce phials, 

 A, B, C, D, were charged, A with refined, and the others with 

 crude camphor. A was covered with tinfoil, B was enclosed in 

 a tin canister, C was covered with brown paper, and D was left 

 naked. They were all arranged around a heated cannon-ball in 

 a darkened room, at such distances as to be exposed to an initial 

 temperature of nearly 90°. In less than half an hour C and D 

 exhibited copious deposits of small crystals furthest from the 

 source of heat. In an hour and a half A had a very faint 

 furthest deposit, B no deposit. 



1 could now understand why the glasses in the tin canisters 

 exposed to the sun exhibited only faint deposits. The canisters 

 being good conductors of heat, made the glass of nearly the same 

 temperature all round, and my theory required that one part 

 should be colder than the other, and hence the faint deposit on 

 the furthest side where it was only slightly colder; whereas brown 

 paper being a bad conductor, would keep the glass warm on the 

 side nearest the source of heat, and much colder on the furthest 

 or opposite side, as was proved by the copiousness of the deposit. 

 But how did these results agree with Dr. Draper's assertion, 



