M. C. Lea — Allotropic Silver. 263 



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 becween 

 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 decom- 

 position results. 



For this purpose silver chloride was precipitated with an ex- 

 cess of hydrochloric acid, after thorough washing was placed in 

 a glass tube of about a centimeter in internal diameter and one- 

 half a meter long, and was sealed up with a blast lamp. During 

 all these operations the chloride was thoroughly protected from 

 light. Five of six cubic centimeters of pure water were first 

 added to the chloride. It was intended to exclude completely or 

 almost completely the effect of pressure and to act on the chloride 

 as far as possible by heat only, and for this reason a longer tube 

 was used and one end only was immersed in the chloride of 

 calcium bath, the other end remained cold throughout the 

 operation. 



The silver chloride formed itself into a compact plug and 

 was forced by the steam which generated below it up to the 



