356 M. C Lea — Darkened Silver Chloride 



Art. XLYI. — Darkened Silver Chloride not an Oxy chloride ; 

 by M. Caeey Lea. 



About two years ago, I published a series of papers in this 

 Journal, the main object of which may be briefly stated as fol- 

 lows : — to prove that the substances which I described as 

 " photosalts " and obtained by purely chemical means were 

 identical with those produced by light, with both the visibly 

 darkened substances, and the material of the latent image. 

 Further, that all these substances consisted of a silver haloid 

 (normal chloride, bromide or iodide) combined with the corres- 

 ponding subsalt, not in equivalent proportions, but after the 

 manner of a "lake" The subsalts, being unstable substances 

 when isolated, acquiring much greater stability by the union. 



The only objections I have seen to these views were based 

 on investigations made in England by Dr. Hodgkinson : his 

 conclusions were that an oxysalt and not a subsalt was formed. 

 Although several years have elapsed since these conclusions 

 were made public, the means by which they were reached and 

 the necessary experimental proof, do not seem to have been 

 published. Mr. Meldola in his interesting "Chemistry of Pho- 

 tography," in treating of this part of the subject seems disposed 

 to accept Dr. Hodgkinson's theory and his formula Ag 4 OCl 2 

 for darkened silver chloride. Mr. Meldola adopts my views 

 that the photosalts which I described and which were obtained 

 by purely chemical means are identical with the products re- 

 sulting from the action of light on the silver haloids, but ex- 

 presses the opinion that I have not proved my theory of their 

 constitution.. 



It has always seemed to me that the whole mass of observa- 

 tion on the action of light on silver chloride tended so thor- 

 oughly to indicate the formation of subchloride, that we might 

 reasonably accept that view, at least until something in the way 

 of proof were offered for the oxychloride theory. J3ut, waiving 

 this objection, I will endeavor to show that subchloride and 

 not oxychloride is the product of the action of light on silver 

 chloride. 



The question as to the presence or absence of oxygen in 

 colored silver chloride is one that cannot be determined satis- 

 factorily by quantitative analysis. Taking for example the 

 formula just mentioned, Ag 4 OCl 2 , it would involve the presence 

 of about 3 per cent of oxygen, if the entire mass of silver chlo- 

 ride were converted into this substance. But we know that 

 even by the longest exposure, the proportion of AgCl acted 

 upon is very small. It would probably be a liberal estimate if 



