Action erf' the Atmosplicrc vjion ncichj-dcepcited Soil. 423 



afFect only the finest surface of bodies, and by no means to pene- 

 trate the usually so extremely thin layer of iodide of silver." 



He shows that there is a condition of iodide of silver in which 

 all the colours exert an influence upon it. The blackened state, 

 which results from the action of lig-lit upon iodide of silver, is a 

 composition not understood, but he considers it hifj^hly probable, 

 if not fully proved, from a number of facts, that liji^ht produces 

 no decomposition of iodide of silver. (^Scientijic Memoirs, iii. 

 423.) Some of Moser's conclusions have however been disputed. 



That light, however, does exert a decided influence, which may 

 be termed chemical, on many bodies, appears from a number of 

 well-ascertained facts. Mr. Hunt, in the Report of the British 

 Association for 1^50, has collected a long list of those bodies 

 which have been observed to be more or less susceptible of 

 chemical chan2;e under the influence of the solar radiations, 

 embracing various salts and compounds of silver, gold, platinum, 

 mercury, iron, copper, manganese, lead, tin, and other less fre- 

 quent metals ; also combinations of phosphorus and ammonia, 

 chlorine and hydrogen, nitric acid, and many resinous bodies, 

 upon which he remarks as follows ; — " From the extensive list 

 wliich lias been given it will be seen that the action of the solar 

 radiations — so far from being confined, as it was formerly thought 

 to be, to a few peculiar chemical compounds, which, existing in 

 a state of exceedingly nice equililjrium, were liable to have 

 their affinity disturbed by the operation of any external force — is 

 so extensive that scarcely any body in nature, organic or inorganic, 

 is independent of the solar influences, altliough their scales of 

 sensibility to them are widely diflerent." A somewhat contrary 

 verdict tliis to Moser's. 



Gay Lussac and Tlienard observed that hvdrogen and chlorine 

 combine so rapidlv in sunshine as sometimes to explode, altliough 

 they do not unite at all in the dark, and only slowly in diffused 

 light; and Dr. Draper, of New York, has stated tlie remarkable 

 fact tliat chlorine whi<!h has been exposed to davlight or sun- 

 shine possesses qualities which are not found in clilorinc made 

 and kept in tlie (hirk. 



The investigalioiis of Dr. Daiibonv and others have shown 

 that the decomposition of carlionic acid by plants goes on witli 

 greatest rapidity under the influence of the most luminous rays, 

 being most remarkable in the yellow ; and Lindley states that the 

 action of llglit ujjon < uttings of phints is little inferior to that of 

 heat, while the imjiortant influence of the same agencv on vital 

 power is acknowlcd^a-d on all hands. 



With regard to the phciioiucna of ■phosphorescence induced by 

 sunlight, Becquerel is of opinion that everytliing seems to prove 



