﻿Intelligence and Miscellaneous Articles, 551 



crease in general with the temperature, this may also be expected in 

 the case of iodide of silver; and if at a higher temperature any- 

 liberated iodine be removed and collected, the reaction for iodine may 

 possibly be obtained. Moreover at a higher temperature the far 

 darker coloration, and therefore the more powerful optical absorp- 

 tion of the silver haloid salts, renders probable, as I have already 

 remarked, a stronger chemical absorption and decomposition. 



By my previous experiments I have not been able with certainty 

 to detect iodine in the current of air which has been passed over 

 heated and insolated iodide of silver ; a decisive result for these ex- 

 periments may be expected with the use of the summer sun. 



Chloride of silver and bromide of silver may in the dark be ob- 

 tained in clear lustrous crystals from their solution in ammonia, and 

 iodide of silver from its solution in hydriodic acid. If these crystals 

 be exposed in the presence of free chlorine (or bromine or iodine) 

 in glass tubes to the action of light, no chemical change takes 

 place, though a mechanical change is observable. The crystals 

 of iodide of silver fall to powder; crystals of bromide of silver and of 

 chloride of silver become opaque and lose their lustre. The deport- 

 ment of the clear transparent layer of iodide of silver, such as is ob- 

 tained by iodizing a silver mirror, is similar. The freshly prepared 

 clear layer changes in a few minutes in sunlight to a yellowish-grey 

 rough mass, which exhibits different colours in transmitted light, 

 varying with the duration of the action : it first of all appears yel- 

 lowish brown, then dark brown and very turbid, then red and green 

 and blue, becoming considerably more transparent, and is finally of 

 a pale bluish white. The ultimate colour may, moreover, be differ- 

 ent, according to the thickness and colour of the layer and the inten- 

 sity of light. These colours arise from diffraction of light, and de- 

 pend upon the degree of pulverization of the iodide produced by the 

 action of light. If the air in the interstices of the powder be replaced 

 by another medium, the layer impregnated with lac, the colours alter 

 and mostly lose intensity. It is probable that in what are called 

 4 * coloured photographs" the colours also result from a change of 

 structure. 



Most sensitive to mechanical change is iodide of silver when it has 

 first been formed in iodine-vapour ; by lengthened keeping in the air, 

 or by immersion in dilute silver solutions or other substances which 

 absorb iodine, the sensitiveness is almost entirely removed, but is 

 restored by again fuming with iodine-vapours. The mechanical 

 change of iodide of silver is effected only by those colours of the 

 spectrum which photographically excite iodide of silver ; light which 

 has passed through a layer of iodide of silver (and has thus been de- 

 sensitized) is therefore quite inactive. 



This deportment of iodide of silver may be used for preparing 

 photographic pictures ; and by the use of a photographic negative a 

 brown picture which is positive by transmitted light is obtained. If 

 light be allowed to act further, the brown colour changes into a clear 

 blue, the shadows become bright, the positive changes into a negative. 

 By hyposulphite of soda both the coherent and the pulverulent iodide 

 of silver are obtained ; by washing with dilute solution of nitrate of 



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