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V. On the Motions of Camphor towards the Light. By John 

 William Draper, M.D., Professor of Chemistry and Physio- 

 logy in the University of New York. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



I HAVE read with interest the very ingenious communication 

 of Mr. Tomlinson, " On the Motion of Camphor towards 

 the Light," inserted in the present (November) Number of the 

 Philosophical Magazine. 



In this communication Mr. Tomlinson considers the circum- 

 stances under which camphor crystallizes on the sides of glass 

 vessels exposed to the sun, and the action of screens of tinfoil 

 and of other materials in preventing or removing those crystalli- 

 zations. 



The conclusion at which Mr. Tomlinson arrives, respecting 

 the cause of these results, is as follows :— -" I think enough has 

 been stated to prove that the motion of camphor, &c. towards 



the light is really the effect of heat The bottles exposed in 



or near a window will always have one side colder than the other, 

 and this colder surface will determine the deposit." " What- 

 ever protects the bottles from .radiation, either wholly or in part> 

 prevents the formation of deposits." This simple and satisfac- 

 tory explanation he considers to be altogether new, remarking 

 that he is "supporting a new theory against the united testi- 

 mony of many philosophers," and adding, "my presumption 

 might be pardoned if I ventured to propound an entirely new 

 theory as to the motions of camphor, &c. towards the light." 



The readers of the Philosophical Magazine are aware that 

 several years ago I published some experiments on these cam- 

 phor motions. Those experiments Mr. Tomlinson has carefully 

 examined. They constitute, in fact, the avowed basis of his 

 memoir. As to the conclusion I arrived at, Mr. Tomlinson 

 remarks, " the result of Dr. Draper's elaborate inquiry was to 

 multiply phenomena, and leave the theoiy as it was." 



If Mr. Tomlinson will turn to page 135 of the Appendix of 

 the work he has quoted, or, better still, to the Philosophical 

 Magazine for February 1840, page 84, I think he will conclude 

 that this statement is scarcely correct, and that I had done some- 

 thing more than leave the theory where it was. In a letter to 

 the Editor of the Philosophical Magazine, I enumerated briefly 

 the facts I had observed, and concluded by furnishing their ex- 

 planation in the following words : — " Now can we explain these 

 singular results on any other known principle than this — that 

 the side of the jar nearest to the sun radiates freely the heat that 

 it receives back again, while radiation is interfered with at the 



