M. Carey Lea — Properties of Allotropic Silver. 239 



required years or might not have occurred at all. The 

 permanency of this substance is gi-eatly influenced by moisture, 

 so that when simply air-dried before placing in tubes it is less 

 permanent than when dried at 70° or 80° C. in a stove. Tubes 

 placed in the same box containing the blue form remained 

 unaffected by the motion, though only partly tilled and al- 

 lowed to move freely. 



When gold-colored allotropic silver is gently heated in a 

 test tube it undergoes a remarkable change in cohesion. Be- 

 fore heating, it is brittle and easily reduced to fine powder. 

 After heating it has greatly increased in toughness and cannot 

 be pulverized at all. 



Both the gold-yellow and the blue forms resemble normal 

 silver in disengaging oxygen from hydrogen peroxide. 



These two forms though differing so much in color and 

 stability and differing also in specific gravity and in their mode 

 of formation, have many properties in common, not possessed 

 by ordinary silver, and differentiating them strongly from it. 

 They show a vastly greater sensitiveness to reagents, and are 

 also sensitive to fight. The ability to form perfect metallic 

 mirrors by being simply brushed in the pasty condition over 

 glass was mentioned in a previous paper. 



Many substances which react little if at all with ordi- 

 nary silver, attack the gold-colored and the blue allotropic 

 silver with production of very beautiful colors due to the 

 formation of thin films and resulting interference of two 

 reflected rays. In my previous papers I called this the "halo- 

 gen reaction" because first obtained by the action of sub- 

 stances which easily parted with a halogen. But I have since 

 found that many other reagents will produce the same or similar 

 effects. These are 



Sulphides. Paper brushed over with either the gold, the 

 copper colored, or the bluish green substance exposed to the 

 vapor of ammonium sulphide, or immersed in a dilute solution 

 of it, assume beautiful hues, though less brilliant than those 

 obtained in some other ways. 



Potassium permanganate in dilute solution produces blue, 

 red and green colors. 



Potassium ferricyanide in moderately strong solution 

 gradually attacks allotropic silver with production of splendid 

 blue, purple and green coloration. 



Phosphorous acid produces gradually a rather dull color- 

 ation. 



The color reaction is produced finely by substances which 

 readily part with a halogen such as ferric and cupric chlorides, 

 sodium hypochlorite, hydrochloric acid to which potassium 

 bichromate has been added, and by corresponding, bromine 



