324 Mr. M. Carey Lea on 



it in the same sheet was preserved, and these two pieces, that 

 heated (after complete cooling) and that not heated, were 

 placed side by side in an oxalate developer. Comparison 

 between these developments indicated the effect of the heat. 

 The following results were obtained : — 



A piece kept for 3 minutes at 145° C. was strongly affected 

 and blackened quickly in the developer, the companion piece 

 remaining white. 



A piece kept for 15 minutes at a temperature commencing 

 at 131° C. and ending at 136° was still more thoroughly 

 affected than the foregoing, the longer exposure more than 

 making up for the lower temperature. Companion piece 

 remained white. 



A piece kept for 8 minutes at a temperature 107° to 108° 

 was distinctly but not strongly affected. Companion piece 

 as before. 



A piece kept for 17 minutes at a temperature of 100° to 

 102° was almost unaffected. A long and careful development 

 brought out a faint difference between the piece so heated 

 and its companion piece. 



It was found that to obtain accuracy in determinations 

 such as these, the paper must rest on a glass, and not a metal, 

 shelf in the desiccator, as the metal shelf is always hotter than 

 the air by which the thermometer is affected. In using a 

 metal shelf, if the paper curled by reason of the heat, the part 

 that rested on the shelf developed darker than that which was 

 simply acted on by the air. By substituting a glass shelf 

 this difference of effect disappeared. 



The result of the foregoing and other experiments was that 

 the effect of heat on AgBr commences at about 100° C, that 

 up to 108° it is still slight and acts slowly, but that at 120° 

 to 126° a strong action commences, which further increases 

 as the temperature is raised. The analogy with allotropic 

 silver is well marked. 



It may at first seem strange that a temperature of 100° C. 

 should produce a permanent change in a substance which will 

 bear a high heat without decomposition, but the explanation 

 lies in the presence of water in the former case. When silver 

 bromide is formed in paper and dried in the air, it still retains 

 moisture. Even at 100° C. this moisture is not driven off. 

 A silver haloid requires to be heated to a temperature between 

 130° and 140° for several hours before it ceases to lose 

 weight. Therefore in all the foregoing cases moisture must 

 have been present. 



It remains to be shown that by a sufficiently long exposure 

 to a moderate heat in the presence of moisture, a visible 

 decomposition results. 



