34.2 Dr. Draper on the Motions of Camphor towards the Light, 



bulb thermometer was then inserted, and the depressions were 

 found as follows : — at the top 2°"4, at the middle 2°*4, and 

 at the bottom 2°*2. Hence by Glaisher's factors we find 

 the differences between the temperature of the air and the 

 temperatures at the dew-points to be 4°'6, 4°'6, and 4 0, 2 

 respectively. 



A cover being placed upon the jar, it was allowed to stand 

 for four hours, and then it was found that the whole of the air 

 in the jar had become saturated with vapour; for then the wet- 

 bulb thermometer indicated the same temperature as the naked 

 bulb. The cover was then taken off, and after the lapse of 

 15 minutes it was found that nearly the whole of the vapour 

 in the jar had become diffused in the atmosphere in that time ; 

 showing that vapour already formed diffuses itself in the atmo- 

 sphere much more rapidly than it is formed from the surface of 

 the water. In the former case no change of specific heat takes 

 place, whereas in the latter case there is a great change of 

 specific heat in the passage of the liquid to the state of vapour,, 

 which tends to retard the process of vaporization. These results 

 explain the following phenomena : if the covered jar be left 

 over night, a copious deposit of dew will be found on the sides 

 of the jar; but, on the contrary, if the cover be off, no dew will 

 be formed. 

 Hastings, March 2, 1863. 



XLVI. On the Motions of Camphor towards the Light, and on 

 Variations in the Fixed Lines of the Solar Spectrum. By John 

 William Draper, Professor of Chemistry and Physiology in the 

 University of New York. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IT has so happened that I have not seen the February Number 

 of the Philosophical Magazine until today. 



I regret to find that Mr. Tomlinson has not frankly done 

 me the justice which was due, as respects the explanation of 

 the motions of camphor to the light, but has endeavoured to 

 strengthen his case by, as I trust, an unconscious misstate- 

 ment of dates. 



Mr. Tomlinson admits that I published in the Philosophical 

 Magazine for February 1840, the true explanation of these mo- 

 tions — that camphor crystallizes on the side of a glass vessel 

 nearest the sun because that side is the coldest. He endeavours, 

 however, to show that I had so little faith in this explanation, as 

 to publish in 1 844 other experiments to prove that it is incorrect. 



