266 M. V. Lea— Allotropic Silver. 



while the unstable compound resists many days' exposure to 

 the strongest sunlight. 



In examining the action of light upon allotropic silver (see 

 Part I) an equally remarkable effect was described. Although 

 all the other forms of energy applied readily and quickly con- 

 vert allotropic to ordinary silver, light (at ordinary tempera- 

 tures), fails to effect this change even by exposures lasting for 

 several months. If we conceive that the atomic form of silver 

 which exists in AgCl corresponds to the allotropic form, and 

 that the more condensed form of subchloride corresponds to 

 the " intermediate form," we shall obtain a reasonable explana- 

 tion of the action of light. 



The inability of light to carry the change which it produces 

 in allotropic silver beyond the u intermediate form" exactly 

 corresponds to its inability to carry the decomposition of silver 

 chloride further than to subchloride or rather to photochloride. 

 (It is understood that the silver chloride here spoken of is that 

 which is formed by precipitation with excess of hydrochloric 

 acid). This explanation appears to remove a real difficulty,, 

 and at the same time establishes a perfect parallelism between 

 that action and the action of light on allotropic silver. 



Although the foregoing study of the silver haloids was made 

 for the purpose of fixing the relations which exist between them 

 and allotropic silver, the results nevertheless have much inter- 

 est in relation to the haloids themselves and place their nature 

 in a somewhat new light. For it is shown that these haloids, 

 though substances in some respects of very great stability have 

 their equilibrium so balanced as to respond to the slightest 

 influence, not merely of light, but of any form of energy, not 

 receiving a momentary but a permanent impression which, 

 though so slight as to be invisible, still greatly increases the 

 tendency of the molecule to fall to pieces under the action of 

 a reducing agent. Further, four of these forms of energy* 

 light, heat, electricity and chemical action, when more strongly 

 applied totally disrupt the molecule. One form of energy, 

 mechanical force, though capable of producing the invisible 

 effect makes an apparent exception in respect of this ability to 

 disrupt. This matter is now under examination and it will 

 probably be shown hereafter that the analogies are complete 

 and without exception. 



The same completeness holds with regard to the analogies 

 which form a principal subject of this paper, namely, those 

 existing between allotropic silver and the metal as it exists in 

 the salts of silver. No other salts but those of silver show 

 this wonderfully balanced equilibrium, sensitive to all forms of 



