M. C. Lea — Allotropic Silver. 261 



Bearing this capital fact in mind and considering the respec- 

 tive properties exhibited by the three forms of silver, it may 

 be allowable to adopt as a working hypothesis the view that 

 they may represent the three possible molecular forms of silver, 

 viz: atomic, molecular and polymerized. 



As silver in its compounds and in its saline solutions exists 

 in the atomic form, it is easily conceivable that when it sepa- 

 rates from such solutions by reduction, the atoms may or may 

 not unite to molecules. Usually elementary atoms do so unite, 

 but the phenomena of nascent action indicate that this union 

 does not take place at the instant of separation, and it is at 

 least conceivable that under particular circumstances this union 

 may be prevented. In some cases no such union takes place. 

 At least four metals exist in the form of vapor in the atomic 

 state. Whether this state continues after condensation we do 

 not know, but there is no impossibility but what such may be 

 the case. Similarly allotropic silver may represent an atomic 

 form : if this were so it should exhibit more active affinities 

 for oxygen and the halogens than the ordinary form ; also 

 it should readily pass into the ordinary form. And these 

 properties are undoubtedly exhibited by allotropic silver. 



There is no branch of chemical statics in which our knowl- 

 edge is so defective as it is in relation to the molecular con- 

 stitution of solids and more especially of the metals. All that 

 can be said is that in metals, as we ordinarily know them, this 

 constitution is probably very complex, the molecules containing 

 many atoms. When substances assume a variety of forms dif- 

 fering from each so much as do the forms of silver, we must 

 either adopt a theory of the character now suggested or else we 

 must suppose that the different forms are differently poly- 

 merized. To decide which is the most probable of these two 

 views it is best to examine as to whether an analogy can be traced 

 between these allotropic forms of silver and silver where it is 

 known to exist in an atomic form, namely, in its compounds. 

 For this comparison the silver haloids (and chiefly silver 

 chloride) will be taken. 



Action of Forms of Energy on Silver Haloids. Parallelism icith 

 Allotropic Silver. 



It is a familiar fact that certain forms of energy, light 

 especially, affect the silver haloids. In view of what has been 

 already said as to the action of all forms of energy on allotropic 

 silver, it seemed desirable to make a general examination as to 

 their action on the silver haloids and thus to determine how 

 far a parallelism could be traced. 



