﻿550 Intelligence and Miscellaneous Articles, 



violet of the solar spectrum ; they are variously so for the visible 

 part of the spectrum. 



Chloride -of -silver collodion is only sensitive for the extreme violet, 

 as far as about halfway between Fraunhofer's lines H and G. 



Iodide- of -silver collodion is sensitive to beyond the line G. 



Br omide-of- silver collodion is sensitive nearly to the line F. 

 Mixed iodide and bromide-of-silver collodion is sensitive to the line E, 

 as well as mixed iodide and chloride of silver. The very remarkably 

 greater delicacy of mixtures of bromide and iodide of silver has long 

 been used in practical photography. 



The optical absorption of transparent plates of these substances is 

 seen, on being examined spectrally, to be accurately restricted to 

 the limits given of chemical action. This is specially the case with 

 mixtures of iodide and bromide of silver. Chloride of silver is colour- 

 less, iodide of silver bright yellow and transparent, bromide of silver 

 of a somewhat deeper yellow ; but the mixture of the two obtained 

 by melting is orange-yellow. This difference in colour, which indi- 

 cates the different delicacy of light, can be recognized by the preci- 

 pitates : pure iodide of silver and pure bromide of silver are pale 

 yellow ; the precipitate with the mixed iodides and bromides, and 

 with the iodides and chlorides is intense yellow. 



I have formerly shown that the ultra-red heat-rays, which do not 

 act photographically, are not at all absorbed by the haloid silver- 

 salts; so that, including the heat-rays, the principle holds: — The ha- 

 loid compounds of silver are chemically changed by all rays which they 

 absorb in appreciable strength. 



This connexion of absorption of light and chemical action pro- 

 bably holds for all substances sensitive to light. Moreover Herschel 

 (from experiments on bleaching vegetable colours) and Draper (from 

 experiments on the decomposition of ferric citrate) have called atten- 

 tion to the reciprocity of chemically active and transmitted light. — 

 Berichte der Berliner Chemischen Gesellschaft, No. 4, 1871. 



CHEMICAL AND MECHANICAL CHANGES IN HALOID SILVER- 

 SALTS BY LIGHT. BY C. SCHULTZ-SELLACK. 



Chloride and bromide of silver are decomposed in light, chlorine and 

 bromine being liberated in such quantities that they are recognizable 

 by the odour and by chemical reagents ; the dark-coloured product 

 of decomposition is therefore probably a subchloride and subbromide. 

 If a greater excess of chlorine- or bromine-vapour be admitted, the 

 colour disappears on continuing the illumination. In the action of 

 light upon iodide of silver a dark coloration also sets in, although no 

 free iodine could be shown ; by a slight addition of free iodine the 

 coloration is prevented. 



As these decompositions depend upon the tension of the vapour of 

 free chlorine, bromine, and iodine, they can be regarded as phenomena 

 of dissociation effected by light. The tension of dissociation of iodine 

 in the case of iodide of silver is then so small that iodine cannot be 

 directly detected in the vicinity. As the tensions of dissociation in- 



