on Allotropic Silver. 



339 



sufficient to alter the product formed. Thus, ferrous tartrate 

 in dilute solution acting on silver tartrate gives rise to the 

 formation of the gold-coloured substance, but when citrates 

 are substituted the blue substance is obtained. 



Production of Allotropic Silver by Inorganic Substances. — 

 For reasons which will be mentioned presently the reduction 

 of silver must take place gradually to produce the allotropic 

 form, and for a time it seemed an invariable condition that 

 an organic substance of some sort should be present. This, 

 however, proves not to be essential. In a paper presented to 

 the American Academy, and kindly read for me by Prof. 

 Remsen at the meeting in April last, I alluded briefly to 

 having found a reaction depending upon inorganic agents 

 only. It is as follows : — Sodium hypophosphite added to 

 silver nitrate does not effect reduction ; but when hypophos- 

 phorous acid is set free by the addition of phosphoric acid, a 

 red coloration appears, indicating the presence of allotropic 

 silver. The coloration is transitory, no doubt because of the 

 strong tendency of free mineral acids to convert allotropic 

 to normal silver; but red and blue stains form on the sides of 

 the vessel. 



Phosphorous acid gives similar results, though perhaps less 

 well marked. 



Action of Light on Blue Silver. — This action differs with 

 different varieties : it was more especially examined with the 

 form that is obtained from the soda-dextrine silver solution 

 already described, by pouring the solution into an equal bulk 

 of water to which sulphuric acid had been added in the pro- 

 portion of 4 cubic centiin. to each 100 cubic centim. of water. 

 This form was selected because it is easy to obtain with great 

 constancy of result, and because it is one of the forms of 

 blue silver most sensitive to light. 



Exposed to light this substance first becomes more distinctly 

 blue, losing a slight greenish shade. With continued expo- 

 sure it passes to a yellow-brown shade, and finally to a per- 

 fectly pure golden yellow of great brilliancy and lustre. This 

 last is the intermediate or crystalline form. 



The action of light on this form of silver is remarkable in 

 this respect, that its first effect is to increase the sensitiveness 

 to reagents. 



This result was so unexpected, and a priori so improbable, 

 that it was subjected to the most careful verification before 

 being accepted. The action is very easily shown by exposing 

 a film of the substance to light, covering part of the surface 

 with an opaque screen. After twenty or thirty minutes of 

 exposure to strong summer sunshine the film may be plunged 



2 A 2 



