328 Mr. M. Carey Lea on Allotropic Silver. 



much interest 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 im- 

 pression 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 

 energy. But allotropic silver also shows an almost exactly 

 similar capacity to respond to the influence of energy in all 

 its manfestations by undergoing changes of a like character. 



The inferences to be drawn from the foregoing seem to be 

 as follows. That silver may exist in three forms : 1st. Allo- 

 tropic silver which is protean in its nature ; may be soluble or 

 insoluble in water, may be yellow, red, blue, or green, or may 

 have almost any colour, but in all its insoluble varieties always 

 exhibits plasticity, that is, if brushed in a pasty state upon a 

 smooth surface its particles dry in optical contact and with 

 brilliant metallic lustre. It is chemically active. 2nd. The 

 intermediate form, which may be yellow or green, always 

 shows metallic lustre, but is never plastic and is almost as 



indifferent chemically as white silver. 3rd. Ordinary silver 



Further, that allotropic silver can always be converted, either 

 into the intermediate form, or directly into ordinary silver ; 

 that the intermediate form can always be converted into 

 ordinary silver, but that these processes can never be reversed, 

 so that to pass from ordinary silver to allotropic it must first 

 be rendered atomic by combination, and then be brought back 

 to the metallic form under conditions w T hich check the atoms 

 in uniting. That allotropic silver is affected by all forms of 

 energy, and that this effect is always in one direction, namely, 



