36 i Mr. C. Tomlinson on the Motion of 



stances sufficiently varied to detect the regulating law, and to 

 eliminate disturbing causes. 



In this examination of Dr. Draper's results, the inquiry be- 

 came divested of some of its marvellous features, and pointed out 

 the line of inquiry to be followed, and the theory to be established. 

 Let it be granted that camphor, and other substances capable of 

 being raised in vapour at ordinary temperatures, become, on the 

 reduction of temperature, condensed on the coldest side of the 

 vessel after the manner of dew, and all these varied phenomena 

 range themselves in the most orderly manner under two well- 

 known laws, namely, radiation of heat, whereby a surface, or a 

 portion of a surface, becomes colder than the vapour in contact 

 with it, and, secondly, condensation of vapour by the contact of the 

 colder body. That is the theory which I have now to support. 



In proceeding to apply this theory to ChaptaVs experiments, 

 1 adopted certain variations in the mode of performing them ; 

 and naturally so, because, as the form of an experiment is but 

 the expression of the thought that produces it, the one will vary 

 with the other, as vessels cast in moulds of different patterns 

 will vary. Chaptal exposed his solutions to the light in glass 

 vessels, one-half of each vessel being covered with black taffeta, 

 for the purpose, as he thought, of shutting out the light, when in 

 fact he was merely preventing cooling by radiation. I obtain 

 results identical with his in transparent vessels in the full sun- 

 shine by preventing radiation and evaporation in one-half of each 

 vessel by covering it with a thin plate of glass or of mica. In 

 such cases the exposed half of the vessel had a crystalline capil- 

 lary deposit running round it, and increasing from day to day, 

 and even passing over the edge of the glass and covering the 

 outside, while on the covered portion there was no deposit what- 

 ever, or only a faint one after some days' exposure. Indeed it is 

 not necessary that the covering glass plate should be in contact 

 with the glass : if it only overshadow it, or be suspended over it, 

 the preventive action is equally produced. I have had a solution 

 of bichromate of potash in one vessel and a solution of sulphate 

 of iron in a similar vessel at a lower level. The glass, which partly 

 covered the first, projected over the second vessel without touch- 

 ing it ; and in both cases, that is, in the covered half of one, and 

 the overshadowed half of the other, there were no deposits, 

 although there were abundant ones in the uncovered sides of both 

 glasses. 



Chaptal recognized the fact that when a large number of eva- 

 porating dishes were arranged according to his method in a small 

 close room, no results were produced. He would have succeeded 

 had he contrived some method of keeping the air dry. I put a 

 solution of sulphate of copper, partly covered with a glass plate, 



