towards the Light. 115 



which is nearest to him may actually be the coldest ; such an 

 opinion, it is evident, rests on no sufficient grounds." He then 

 describes the experiment referred to by me in my paper, in which 

 a differential thermometer, enclosed in a jar and exposed to the 

 sun, proved that the side of the jar nearest to the sun was the 

 warmer, and adds, " Hence we know that in all cases where 

 crystals of camphor, dew of water, &c. are deposited on the side 

 next the sun, they are so deposited in opposition to an energetic 

 force which tends to remove them." 



This is pretty strong evidence that Dr. Draper's theory is very 

 different from mine. 



Again, he says, p. 120, "The sun's rays have the power of 

 causing vapours to pass to the perihelion side of vessels in which 

 they are confined, but, as it would appear, not at all seasons of 

 the year." He then describes a case in which, during December, 

 January, and part of February, " a deposit was uniformly made 

 towards the sun ; during the months of March, April, and part 

 of May next following, although every part of the arrangement 

 remained to all appearance the same, yet the camphor was de- 

 posited on the side furthest from the sun. It does not appear 

 that any immediate cause can be assigned for this waywardness." 



Could Dr. Draper have put forward such a statement as this 

 in 1844, if he had had the slightest idea of the true theory of 

 the motion of camphor towards the light ? 



Again, p. 122, when a circular plate of glass was put into a 

 glass receiver above the camphor, no deposit was made on the 

 plate. Dr. Draper says, "It was not without surprise I observed 

 that, however long the plate was continued in the beams of light, 

 no crystallization would ensue." Cases of this kind are perfectly 

 explicable on my theory, and, indeed, I give several such. 



Commenting on such a case, Dr. Draper says that " to reduc- 

 tion of temperature we cannot look for an explanation." I 

 prove that reduction of temperature is the only means of ex- 

 planation. 



Again, at p. 122, Dr. Draper says, " Why does this con- 

 densation take place on the hottest surface, the side nearest 

 to the plate ? We cannot admit that the rays of heat have any 

 active part in bringing about the phenomenon. On the other hand, 

 they ought rather to exert a contrary effect, antagonizing the 

 powers that solicit the camphor crystals to form, and driving 

 them to the coldest surface. We are therefore reduced to the 

 supposition that, when the light of the sun impinges on a sur- 

 face of glass, it places that surface in such a condition that it 

 exerts a pressure on the adjacent medium, immediately followed 

 by a condensation of that medium." 



I have no idea what this means ; but I would ask how it is 



12 



