360 M. C. Lea — Darkened /Silver Chloride, etc. 



2. When pure moist silver chloride is exposed to light, the 

 darkening goes on steadily to a certain point and then virtually 

 ceases. Although the mass may be constantly agitated so as to 

 expose fresh portions whilst it is kept moist, or better, kept 

 under water, the action of light after a few days exposure, 

 ends almost wholly, and though the chloride now seems per- 

 fectly black, analysis shows that the amount of chloride altered 

 has been very small, perhaps a twentieth. For this well 

 known and most remarkable fact I believe no explanation has 

 ever been offered. 



It is to be explained I think as follows : When light acts on 

 moist silver chloride, acid products are formed, the nature of 

 which has not been fully made out, but whose presence is cer- 

 tain. (If the exposure is made under water, the water red- 

 dens litmus, it does not bleach it.) These acid products would 

 instantly destroy silver subchloride isolated. As long as the 

 quantity of subchloride present and combined with normal 

 chloride is small, the protecting power of the normal chloride 

 saves the subchloride from decomposition. The ability of 

 normal chloride to protect the subsalt rapidly diminishes as 

 already mentioned as the proportion of the latter increases; 

 these forces in time find an equilibrium (as in the case of many 

 other chemical reactions), in which the quantity of subchloride 

 becomes constant, and any excess that is formed by light 

 beyond the quantity which the chloride can protect is instantly 

 destroyed Consequently even the most protracted exposure 

 fails to increase the proportion of subchloride beyond its limit. 



The complete cessation of the reducing action of light after 

 a certain very small fraction of the chloride present has been 

 attacked cannot be explained by the oxysalt theory. For, if a 

 small portion of the chloride has been converted into Ag 4 OCl 2 

 there is no conceivable reason why a continuance of the same 

 agencies at work should not affect the residue and so go on 

 until the whole of the silver chloride passes into the new con- 

 dition. — JSTo theory deserves serious consideration that does not 

 offer a satisfactory explanation of these two conspicuous facts : 

 the pause at the outset, and the cessation of action as soon as a 

 certain small proportion of chloride is reduced. 



A confirmation of the explanation here offered is to be found 

 in the fact that black forms of photochloride chemically 

 formed are almost wholly unacted upon by light. By treating 

 metallic silver in a state of fine division* with sodium hypo- 

 chlorite, black photochloride is easily formed which, after two 



* For such purposes, metallic silver is best obtained by precipitating the 

 nitrate with sodium hydroxide in excess, and without washing, adding a solution 

 of milk sugar. In a few hours the reaction is complete (Levol's method.) If the 

 presence of any undecomposed oxide is feared the product may be washed with 

 ammonia. 



