178 HISTORY OF PHOTOGRAPHY. 



autumn, and the juices of those flowering phmts which have been 

 cnhivated under the artificial circumstances of a storehouse or con- 

 servatory are more readily affected than such as are grown in the 

 open air. Many of the experiments just described furnish very in- 

 structive examples of the operations of the solar rays upon organic 

 bodies, from which we may deduce important truths connected with 

 natural phenomena. 



II. Miscellaneous Processes. 



MR. ponton's process (BICHROMATE OF POTASh). 



Under the general term of the chromatype I would propose to in- 

 clude all those processes which involve the use of any of the salts of 

 chromium. It was originally introduced to distinguish a particular 

 process which I discovered, and published at the meeting of the 

 British Association at Cork, in August, 1843; but it appears very 

 convenient to adopt the principle introduced by Sir John Herschel, 

 of grouping the phenomena of photography under special terms 

 derived from the most prominent chemical preparation employed. 



There are many preparations which are affected by light in a sim- 

 ilar manner to the salts of silver. Several have been tried as photo- 

 graphic materials, but as yet wdthout much success, with the excep- 

 tion of the bichromate of potash, which was first announced as a use- 

 ful photographic agent by Mr. IVlungo Ponton in the Edinburgh New 

 Philosophical Journal, from which I quote Mr. Ponton's own account : 



When paper is Immersed in the bichromate of potash it is i>owerfiiIly and 

 rapidly acted on by the sun's I'ays. When an ol)ject is hiid in the usual way on 

 this paper the portion exposed to the light speedily becomes tawny, passing 

 more or less into a deep orange, according to the strength of the light. The 

 portion covered by the objects retains the original bright yellow tint which it 

 had before exposure, and the object is thus represented yellow upon an orange 

 ground, there being several gradations of shade or tint, according to the 

 greater or less degree of transparency in the different parts of the object. 



In this state, of course, the drawing, though very beautiful, is evanescent. 

 To fix it, all that is required is careful immersion in water, when it will be 

 found that those portions of the salt which have not been acted on by the light 

 are readily dissolved out, while those which have been exposed to the light are 

 completely fixed on the paper. By the second pi'ocess the object is obtained 

 white upon an orange ground and quite permanent. If exposed for many hours 

 together to strong sunshine, the color of the ground is apt to lose in depth, but 

 not more so than most other coloring matters. This action of light on the bi- 

 chromate of potash differs from that upon the salts of silver. Those of the 

 latter which are blackened by light are of themselves insoluble in water, and 

 it is difficult to impregnate paper with them in a luiiform manner. The black- 

 ening seems to be caused by the formation of oxide of silver. 



In the case of the bichromate of potash, again, that salt is exceedingly solu- 

 ble, and paper can be easily saturated with it. The agency of light not only 

 changes its color, but deprives it of solubility, thus rendering it fixed in the 



