Allotropic Silver. 325 



For this purpose silver chloride was precipitated with an 

 excess of hydrochloric acid, after thorough washing was 

 placed in a glass tube of about a centimetre in internal 

 diameter and half a metre long, and was sealed up with a 

 blast lamp. During all these operations the chloride was 

 thoroughly protected from light. Five of six centimetres 

 cube of pure water was 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 middle of the tube. This effect, though not intended, 

 answered very well, as the chloride was kept constantly under 

 the influence of steam at about 100°. It soon began to 

 darken and at the end of three or four hours all the lower 

 part was violet-brown, the upper part grey, the change taking 

 place entirely through the mass. Some thin smears of silver 

 chloride on the lower inside part of the tube were completely 

 blackened. 



On opening the tube next day there was no escape of gas. 

 The water sealed up with the silver chloride had acquired a 

 faint but distinct alkaline reaction, showing that enough alkali 

 had been dissolved from the glass to overcome any acidity 

 arising from decomposition of the chloride. The water con- 

 tained traces of alkaline chloride. 



A similar examination was made with silver bromide pre- 

 cipitated with excess of hydrobromic acid and thoroughly 

 washed with distilled water. The action of diffuse light on 

 silver bromide is very different from that on silver chloride. 

 A portion of that prepared as above mentioned changed 

 in diffuse light very quickly from yellow to greenish yellow, 

 but after that first change the alteration was extremely slow 

 and in an hour had only reached to a dirty greenish grey. 

 The action of direct sunlight was quite different : fifteen 

 minutes' exposure changed the greenish grey to dark chocolate- 

 brown. 



In the tube the silver bromide did not form a plug like the 

 chloride, but separated into balls which remained in the bottom 

 of the tube. By keeping the chloride-of-calcium bath con- 

 siderably above 100° C, the water in the tube was kept 

 actively boiling : it condensed in the upper part of the tube 

 and returned. Six hours of this treatment only brought the 

 bromide to the same greenish colour which it would have 

 acquired by a few minutes' exposure to diffuse light. 



Phil. Mag. S. 5. Vol. 31. No. 191. April 1891. 2 A 



