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



Third Order. 



Reddish purple. 

 Bluish purple. 

 Rich green. 



The fourth order is not reached, for after this the colors 

 become much mixed, probably the action is no longer suffi- 

 ciently uniform. The other differences, beside the absence of 

 the fourth order, as will have been observed, are that in place 

 of the yellow of the second order corresponding to a thickness 

 of air of 0*000432 mm , there is a green though of a more russet 

 shade than that of the third order. 



In the third order there is at no time a pure blue corres- 

 ponding to 0"000602, but only a succession of beautiful red and 

 blue purples, gradually passing into green. 



There are few more beautiful experiments than to watch 

 these changes. Purity of color, however, depends much on 

 the purity of the paper employed. Want of this purity will 

 often cut short the changes at the pale blue of the second 

 order. 



I have endeavored to give some idea of these colors in the 

 Plates which accompany this paper, but it has proved to be a 

 most difficult task. The colors represented are, 



Plate I, purple and blue of the 2d order. 



Plate II, purple of the 2d order and green of the 3d order. 



Plate III, brown red of the 1st order. 



It has not, however, been found possible to correctly repro- 

 duce the brilliancy and depth of color of the originals. 



Philadelphia, Jan. 24, 1891. 



(To be continued.) 



Explanation of the Plates. 



In each pair of figures the upper one represents the effect of exposing allo- 

 tropic silver to some form of energy. The changes are in all cases similar in 

 character. 



In the lower of each pair of figures the effect is represented which would be 

 produced by immersing the upper one in a solution of potassium ferricyanide. 

 This affords proof of the completeness of the change by showing that gold-colored 

 silver in passing into the ordinary form has lost its power of reacting with a 

 ferricyanide. In these lower figures an attempt has been made to show some of 

 the colors produced in this way. But they fall far short of the originals in bril- 

 liancy and intensity. These last are so remarkable that the lithographer who 

 executed the work remarked that even an artist with a brush and palette of 

 colors could not imitate them, and that therefore it was hopeless to expect to 

 reproduce them by lithography — in other respects the Plates represent fairly well 

 the changes that take place. 



