HISTORY OF PHOTOGRAPHY. 167 



If paper be washed with a mixture of the solution of aiimionio-citrnte of ii-on 

 and ferrosesquicyanate of potash, so as to contain the two salts in about equal 

 proportions, and, being then impressed with a picture, be tlu'own into water and 

 dried, a negative blue image will be produced. This pictvu-e I have found to be 

 susceptible of a very curious transformation, preceded by total obliteration. To 

 effect this it must be washed with solution of proto-nitrate of mercury, which 

 in a little time entirely discharges it. The nitrate being thoroughly washed 

 out and the picture dried, a smooth iron is to lie passed over it, somewhat hotter 

 than is used for ironing linen, but not sufficiently so to scorch or injure the 

 paper. The obliterated picture innuediately reappears, not blue, but brown. 

 If kept for some weeks in this state between the leaves of a portfolio, in com- 

 plete darkness, it fades and at length almost entirely disappears. But, what is 

 very singular, a fresh application of the heat revives and restores it to its full 

 intensity. 



This curious transformation is instructive in another way. It is not op- 

 erated by light; at least not by light alone. ^1 certain temperature must be 

 attained, and that temperature suffices in total darkness. Nevertheless, I And 

 that on exi)Osing to a very concentrated spectrum (collected by a lens of shoi't 

 focus) a slip of paper duly prepared as above — that is to say, by washing with 

 the mixed solutions, exposure to sunshine, washing, and discharging the imi- 

 form blue color so induced, as in the last article — its whiteness is changed to 

 brown over the whole region of the red and orange rays, but not beyond the 

 luminous spectrum. Three conclusions seem unavoidable : First, that it is the 

 heat of these rays, not their light, which operates the change ; second, that 

 this heat possesses a peculiar chemical quality which is not possessed by the 

 purely calorific rays outside of the visible spectrum, though far more intense ; 

 and, third, that the heat radiated from obscurely hot iron abounds especially 

 in rays analogous to those of the region of the spectrum above indicated. 



Sir John Herschel then proceeds to show that, whatever be the state 

 of the iron in the double salts in question, its reduction by blue light 

 to the state of protoxide is indicated by many other reagents. Thus, 

 for example, if a slip of paper prepared with the ammonio-citrate of 

 iron be exposed partially to sunshine and then Avashed with the 

 bichromate of potash, the bichromate is deoxidized and precipitated 

 upon the sunned portion, just as it would be if directly exposed to the 

 sun's rays. 



I have proved this fact with a great number of preparations of 

 cobalt, nickel, bismuth, platinum, and other salts which have been 

 thought hitherto to be insensible to solar agency ; but if they are par- 

 tially sunned, and then washed with nitrate of silver and put aside 

 in the dark the metallic silver is slowly reduced upon the sunned 

 portion. In many instances daj's w^ere required to produce the visible 

 picture; and in one case paper, being washed with neutral chloride 

 of platinum, were ^sunned, and then washed in the dark with nitrate 

 of silver; it was some weeks before the image made its appearance, 

 but it was eventually perfectly developed. This sj)ecimen has been 

 kept for several years, and continues constantly to improve in clearness 

 and definition. 



