M. Carey Lea — Allotrojnc forms of Silver. 49 



solutions, under the influence of which the substance itself 

 slowly but continually changes. Next, the saline solution used 

 for washing must be removed by alcohol. During this treat- 

 ment, the substance, at first very soluble, gradually loses its 

 solubility and when ready for analysis, has become wholly 

 insoluble. It is impossible at present to say whether it may 

 not have undergone other change : this is a matter as to which 

 I hope to speak more positively later. It is to be remarked, 

 however, that these allotropic forms of silver acquire and lose 

 solubility from very slight causes, as an instance of which may 

 be mentioned, the ease with which the insoluble form B re- 

 covers its solubility under the influence of sodium sulphate and 

 borate and other salts as described in the previous part of this 

 paper. 



The two insoluble forms of allotropic silver which I have 

 described as B and C ; B, bluish green, C rich golden color, 

 show the following curious reaction. A film of B, spread on 

 glass and heated in a water stove to 100° C. for a few minutes 

 becomes superficially bright yellow. A similar film of the 

 gold-colored substance C treated in the same way, acquires a 

 blue bloom. In both cases it is the surface only that changes. 



Sensitiveness to Light. — All these forms of silver are acted 

 upon by light. A and B acquire a brownish tinge by some hours' 

 exposure to sunlight. With C the case is quite different, the 

 color changes from that of red gold to that of pure yellow gold. 

 The experiment is an interesting one, the exposed portion 

 retains its full metallic brilliancy, giving an additional proof 

 that the color depends upon molecular arrangement, and this 

 with the allotropic forms of silver is subject to change from 

 almost any influence. 



Stability. — These substances vary greatly in stability under 

 influences difficult to appreciate. I have two specimens of the 

 gold yellow substance C, both made in Dec. 1886, with the 

 same proportions, under the same conditions. One has passed 

 to dazzling white, normal silver, without falling to powder, or 

 undergoing disaggregation of any sort ; the fragments have 

 retained their shape, simply changing to a pure frosted white, 

 remaining apparently as solid as before, the other is unchanged 

 and still shows its deep yellow color, and golden luster. An- 

 other specimen made within a few months and supposed to be 

 permanent has changed to brown. Complete exclusion of air 

 and light is certainly favorable to permanence. 



Physical condition. — The brittleness of the substances B 



and C, the facility with which they can be reduced to the finest 



powder, makes a striking point of difference between allotropic 



Am. Jour. Scr.— Third Series, Vol. XXXVIII, No. 223.— July, 1889. 

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