264 M. C. Lea—Allotropic Silver. 



middle of the tube. This effect, though not intended, answered 

 very well as the chloride was kept constantly under the influ- 

 ence 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 gray, 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 aris- 

 ing from decomposition of the chloride. The water contained 

 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 gray. The action 

 of direct sunlight was quite different; fifteen minutes' ex- 

 posure changed the greenish gray 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 aiid 

 returned. Six hours of this treatment only brought the bromide 

 to the same greenish color which it would have acquired by a 

 few minutes' exposure to diffuse light. 



The conclusion to be drawn as respects both the silver 

 haloids is that they undergo actual decomposition by the action 

 of moist heat, but that this effect is much more marked in the 

 case of chloride than that of bromide.* 



Chemical action. — Dilute sulphuric acid quickly changes 

 allotropic silver to normal, and therefore if the parallelism 

 which I have indicated really exists, marks made on bromide 

 paper with dilute sulphuric acid should be capable of develop- 

 ment. 



The experiment was made by drawing characters on silver 

 bromide with a glass rod dipped into sulphuric acid diluted 

 with twice its bulk of water. After allowing the acid to 



* Light and heat act differently on silver chloride Heat can not decompose it 

 in the absence of moisture but light can. This was proved by an investigation 

 made by the writer in 1889. in which it was shown that fused silver chloride 

 poured into petroleum and exposed after cooliug to the sun's rays was instantly 

 blackened. 



