480 M. Carney Lea — Allotropic Forms of Silver. 



is a variety which is copper-colored. Insoluble in water, appears 

 to have no corresponding soluble form. 



Properties possessed by all the varieties in common and distin- 

 guishing them all from normal silver. 



All these forms have several remarkable properties in com- 

 mon. 



1. That of drying with their particles in optical contact, 

 and consequently, forming a continuous film. — If either is 

 taken in a pasty condition and is spread evenly over paper 

 with a fine brush, it takes on spontaneously in drying a luster 

 as high as that of metallic leaf. C when so treated would be 

 taken for gold leaf. But this property is much better seen by 

 brushing the pasty substance over glass. When dry, an abso- 

 lutely perfect mirror is obtained. The particles next the glass, 

 seen through the glass, are as perfectly continuous as those of 

 a mercurial amalgam, and the mirror is as good. A and B 

 form bluish-green mirrors, C, gold or copper-colored mirrors. 



2. The halogen reaction. — When any of these allotropic 

 forms of silver are brushed over paper and the resulting metal- 

 lic films are exposed to the action of any haloid in solution, 

 very beautiful colorations are obtained. The experiment suc- 

 ceeds best with substances that easily give up the halogen, such 

 as sodium hypochlorite, ferric chloride, iodine dissolved in 

 potassium iodide, etc. But indications are also obtained with 

 alkaline salts such as ammonium chloride etc, though more 

 slowly and less brilliantly. With sodium hypochlorite the 

 colors are often magnificent, intense shades with metallic reflec- 

 tions, reminding one of the colors of a peacock's tail. Blue is 

 the predominating tint. These are interference colors, caused 

 by thin films, but whether of a normal silver haloid or a hemi- 

 salt, cannot be said. When silver leaf (normal silver) is fastened 

 to paper and a comparative trial is made, the contrast is very 

 striking. — This matter will be more particularly examined in 

 the 2d part of this paper, and is mentioned here as one of the 

 reactions distinguishing allotropic from ordinary silver. 



3. The action of acids. — The stronger acids, even when 

 much diluted, instantly convert the allotropic forms of silver 

 into normal gray silver ; even acetic acid, not too much diluted, 

 does this. It is important to remark that this change takes 

 place absolutely without the separation of gas. I have more 

 than once watched the whole operation with a lens and have 

 never seen the minutest bubble escape. 



4. Physical condition. — All these allotropic forms of silver 

 are easily reduced to an impalpable powder. One is surprised 

 to see what is apparently solid burnished metal break easily to 

 pieces and by moderate trituration yield a fine powder. 



