282 J. C. Blcike — Colors of Allotropic Silver. 



Art. XXYII. — The Colors of Allotrojnc Silver ; bj J. C 



Blake. 



[Contributions from the Kent Chemical Laboratory of Yale University — CXIX.] 



A GREAT deal has been written concerning the allotropv of 

 silver, perhaps the most remarkable phenomena described being 

 the beantif al color effects obtained by Care}^ Lea."^ No satis- 

 factory explanation of such effects has ever been published, 

 however. I have repeated most of the work described in the 

 literature relating to " allotropic " silver, " colloidal " silver, 

 and, to some extent, the so-called " sub-salts " of silver, and 

 have arrived at the conclusion that all of the color effects 

 observed may be explained by assuming the existence of three 

 or possibly four allotropic forms of silver. Before describing 

 these forms it will be necessary to point out specifically that 

 each of them has a characteristic color in reflected light, and 

 another, very nearly complementary color in transmitted light. 

 In the case of mirrors transparent to certain wave-lengths of 

 light, the colors of the reflected and transmitted light are com- 

 mingled when such mirrors are viewed in ordinary light, to 

 mutual obliteration. The confusion of colors resulting from 

 the commingling of the various allotropic forms of silver among 

 themselves, as well as the blending of colors due to the inter- 

 mixture of allotropic silver and foreign colored substances, 

 es])ecially when such intermixtures are present in solution 

 together, must be constantly borne in mind. 



These four assumed allotropic forms of silver, together with 

 their most pronounced characteristics and convenient modes of 

 preparation, are as follows : 



Form of silver, named 

 after the color most Color in transmitted 



readily observed. Color in reflected light. light. 



« WhitP sil vpr " T^Parl V whitP Nearly opaque, even 



W nite siU ei JN eai ly tv mte • ^^ ^^^ thinnest films 



" Blue silver " Golden yellow Blue 



"Red silver" Indigo-blue Red 



"Yellow silver" Indigo-blue Yellow 



The transmitted colors are easily observable. They are the 

 colors seen when the substances are in pseudo-solution — per- 

 manent suspension or colloidal solution, — and in mass, provided 

 no mirror surface has been- formed. The colors in reflected 

 light, on the other hand, are observable only when a mirror 

 surface has been formed, either by deposition on glass, by 



*This Journal, xxxvii, 476, and many articles closely foUovsring. Phil. 

 Mag., xxxi, 238, 320; xxxii, 337. 



