188 M. C.Lea — Gold-colored Allotropic Silver. 



unaltered or active form. The upper figure of the same plate 

 gives the effect of a very protracted exposure (as above de- 

 scribed) on pure paper (or glass) under glass. With some 

 kinds of sized paper, this effect is produced by a much shorter 

 exposure; apparently owing to the presence of traces of a 

 hyposulphite;* which appears to aid the action of light. 



These seem to be not merely new facts but to belong to a 

 new class of facts. No instance has been hitherto known of 

 an element existing in so great a variety of forms and passing 

 so readily under the influence of any form of energy from one 

 to another of them. 



It is evident that a connection must almost certainly exist 

 between these varied transformations and the changes which 

 many silver salts undergo through the action of light and other 

 forces. This connection will form the subject of the second 

 part of this paper. The present part will be concluded by a 

 somewhat fuller description of the color reaction which is espe- 

 cially characteristic of allotropic silver. 



The Color Reaction. 



When allotropic silver is immersed in a solution of a sub- 

 stance readily parting with oxygen or sulphur or with a halo- 

 gen, a film is formed which exhibits the colors of thin plates. 

 Such phenomena are familiar and are seen in the blueing or 

 yellowing of steel in tempering and the coloring of other metals 

 when covered with films of oxide or sulphide. With allo- 

 tropic silver the colors are very brilliant, probably because 

 silver is the best of all reflectors for rays having a nearly per- 

 pendicular incidence, sending back about 90 per cent of such. 

 Light gold colored silver gives the most brilliant effects. 



The substances which produce these reactions are potassium 

 ferricyanide and permanganate, ferric and mercuric chlorides, 

 alkaline hypochlorites and sulphides, mixtures of potassium 

 bichromate with hydrochloric or hydrobromic acid, solution of 

 iodine, etc. 



Potassium ferricyanide in a five or ten per cent solution is 

 the best of these reagents because its action is more distinctive. 



* The behavior of these varieties of paper led me to make inquiries of an intel- 

 ligent paper manufacturer from whom I learned that every sort of paper pulp is 

 now treated with chlorine. As any portion of the bleaching material left in 

 the paper would eventually destroy its strength, it becomes necessary to add 

 hyposulphite in excess to remove it. Accordingly every specimen of sized paper 

 that I have examined contained hyposulphite, even the purest photographic 

 papers were not free from it, though containing greatly less than most others. 

 Apparently, the only difference is that with photographic paper more care is 

 taken to avoid any considerable excess of hyposulphite. 



