288 J. C. Blake — Colors of Allotropic Silver. 



reagents are added is usually immaterial, although if a non- 

 electrolyte which alone does not cause reduction of the silver 

 compound is to be added, it is best to mix it with the silver 

 solution before adding the final reducer in order to take advan- 

 tage of the power which non-electroljtes possess of inhibita- 

 ting the action of electrolytes toward colloidal solutions, as 

 already pointed out. The resulting colloidal solutions were 

 generally clear in ordinary light, but all gave the Tyndall 

 effect when the light was concentrated with a lens. Except 

 when otherwise stated the colors refer to transmitted light. 

 A -f sign indicates that the phenomenon takes place without 

 special characteristics or conditions, 



In the light of the foregoing discussion most of these results 

 will be readily understood. It will be noticed that the gray 

 effects (" white silver ") were obtained only in acid solutions. 

 In solutions sufficiently acid such effects are always obtained. 

 The tinge of yellow noted in experiments (40) and (41) was 

 doubtless due to the golden reflection of intermixed ''blue sil- 

 ver," formed while the solution was still alkaline, as in experi- 

 ment (2). The blue solutions obtained in experiments (3) and 

 (10) were especially stable. In most cases the blue color was 

 preceded by a red, brown, green, or purple color. It was 

 found impossible to prepare red silver solutions which were 

 stable except in the presence of organic matter or typical inor- 

 ganic colloids, as stannic acid. " Yellow silver " was never 

 obtained in stable solution. Except for the fact that glass is 

 colored yellow by silver and not red, this ''yellow Silver" 

 might be regarded as a variety of "red silver." The other 

 forms of silver — "white," "blue," "red" — seem sufficiently 

 distinct to be regarded as allotropic forms until their exact pro- 

 perties can be investigated. So far as that work has progressed 

 it fully bears out the idea that silver does exist in allotropic 

 forms.* Grimm worked with silver mirrors which trans- 

 mitted blue-gray light, and found that the increase in electrical 

 conductivity induced in them by lapse of time, by heat, light, 

 burnishing, and other agencies, all indicated a gradual change 

 from " molecular " to "normal" silver, corresponding, in the 

 terminology here used, to the change from " blue silver " to 

 " white silver." 



The kind advice and helpful assistance of Prof. F. A. Gooch, 

 who suggested this subject for investigation, is gratefully 

 acknowledged. 



* Grimm, Drude's Ann. [4] v, 448. 



