On the Existence of the Negative Rays of the Sun. 87 



In the above table it will be observed that the hard steel 

 bar was slightly increased in length every time that contact 

 witli the battery was broken, although a considerable dimi- 

 nution of the magnetism of the bar took place at the same 

 time. I am disposed to attribute this effect to the state of 

 tension in the hardened steel, for I find that soft iron wire 

 presents a similar anomaly when stretched tightly. 



On inspecting the tables, it will be remarked that the elon- 

 gation is, for the same intensity of magnetism, greater in pro- 

 portion to the softness of the metal. It is greatest of all in 

 the well-annealed iron bars, and least in the hardened steel. 

 This circumstance appears to me to favour the hypothesis that 

 the phsenomena are produced by the attractions taking place 

 between the magnetic particles of the bar, an hypothesis in 

 perfect accordance with the law of elongation which I have 

 pointed out. 



[To be continued.] 



XVIII. Remarks on the Existence a?id Mechanism of the Ne- 

 gative 07- Protecting Rays of the Sun. By John William 

 Draper, M.D., Professor of Chemistry in the University of 

 Nexv York*. 



IN a letter published in the Philosophical Magazine, Nov. 

 -■■ 1812, I had occasion to make some incidental remarks 

 respecting a class of rays existing in the sun-light, which have 

 the quality of exerting a negative or antagonizing action upon 

 those engaged in producing Daguerreotype results. 



In October last, MM. Foucault and Fizeau having made a 

 communication to the French Academy of Sciences to a similar 

 effect, and M. Edmond Becquerel, in criticising their results, 

 having referred to me as the original author of the fact, I may 

 on this occasion be excused for offering a few observations 

 on this, which perhaps is destined to become one of the most 

 important phaenoniena in relation to the chemical action of 

 the sun-light. 



That the opposite ends of the solar spectrum possess oppo- 

 site qualities is an idea which, far from originating with me, 

 has been floating among chemists for many years. The first 

 distinct statement with which I am acquainted in relation to 

 it occurs in a work published by Mr. B. Wilson, the second 

 edition of which dates as early as 1776. It is entitled "A 

 Series of Experiments on Phosphori." Ho shows that it is 

 the more refrangible rays which excite the phosphorescence 

 of sulpliuret of lime, but the less nfrangibtc ones extinguish it 

 Vihcn shi?iing. 



* Communicated by the Author. 



