386 31. C. Lea — Combinations of Silver Chloride, etc. 



being compounds of two chlorides of silver not combined in 

 definite proportions, by establishing the existence of other 

 analogous compound chlorides. 



Again, the sensitiveness to light of AgCl is so strongly 

 modified by mere traces of ferric chloride, that evidently a 

 quantity of the latter substance, quite too small to visibly affect 

 the color of the AgCl, may materially change its sensitiveness^ 

 thus affording an illustration of what takes place in the 

 latent image, where the presence of a quantity of Ag 2 Cl, too 

 minute to be visible, is sufficient to powerfully influence the 

 substance combined with it. 



It also furnishes an explanation for a well-known fact that 

 has hitherto seemed anomalous. It has long been known that 

 a trace of mercuric chloride suffices to greatly diminish the sen- 

 sitiveness of silver chloride to light. This isolated fact now be- 

 comes simply one of a series; AgCl combines with a small pro- 

 portion of mercuric chloride just as it does with other metallic 

 chlorides, those of the iron triad for example, and does not 

 give it up when washed. 



Debray (quoted in Gmelin-Kraut, article Silver), has indeed 

 expressed the opinion that the mercuric chloride can be entirely 

 removed by washing with water. To fix this point I have 

 sought for a convenient means of detecting small quantities of 

 mercuric chloride in presence of AgCl, and have found it in a 

 solution of stannous chloride made strongly acid with HC1, 

 Pure AgCl is not darkened by this reagent, provided that light 

 is carefully excluded, but if mercuric salt is present, a brown or 

 brownish black coloration results. Long washing has with me 

 wholly failed to remove the mercuric salt. I therefore look 

 upon the combination as one of considerable stability. 



All these combinations diminish the sensitiveness of AgCl,. 

 but this effect is greatly stronger with those chlorides which easily 

 part with one equivalent of chlorine, as we saw in the case of 

 ferric chloride. Mercuric chloride acts in the same energetic 

 way. 



It seems indeed that the reduction of sensitiveness in these 

 cases is somewhat out of proportion to the amount of chlorine 

 that could be yielded up by the trace of the foreign chloride 

 which is combined with the AgCl. But this is, perhaps, to be 

 explained by this trace of chlorine holding in check the initial 

 movement toward reduction. 



It is worth observing that experiment and observation are 

 constantly tending to enlarge the number of substances with 

 traces of which the silver haloids show themselves capable of 

 uniting, with great modification of their properties as a conse- 

 quence. 



The reduction of sensitiveness that results from the presence of 



