172 Miscelianies. 
ened parts will be acted upon in a different manner from those which re- 
tain their original color, and after such action they will probably be no 
longer affected by the rays of the sun, or, at all events, will have no ten- 
dency to assimilate by such exposure; and if they remain dissimilar, the 
picture will continue distinct, and the great difficulty be overcome. 
The first trials of the inventor to destroy the susceptibility of the metal 
- lic oxide were entirely abortive; but he has at length succeeded to an 
extent equal to his most sanguine expectations. ‘The paper employed by 
Mr. Talbot is superfine writing paper; this is dipped into a weak solution 
of common salt, and dried with a towel till the salt is evenly distributed 
over the surface: a solution of nitrate of silver, is then laid over one side 
of the paper, and the whole is dried by the heat of the fire. It is how- 
ever, necessary to ascertain by experiment the exact degree of strength 
requisite in both the ingredients, for if the salt predominates, the sensi- 
bility of the paper gradually diminishes, in proportion to this excess, till 
the effect almost entirely disappears. 
In endeavoring to remedy this evil, Mr. Talbot discovered that a Te 
newed application of the nitrate not only obviated the difficulty, but ren- 
dered the preparation more sensitive than ever: and by a repetition of the 
same process the mutability of the paper will increase to such a degree, 
as to darken of itself without exposure to the light. This shows that 
the attempt has been carried too far, and the object of the experimenta- 
list must be to approach, without attaining this condition. Having pre- 
pared the paper and taken the sketch, the next object is to render it per- 
manent, by destroying the susceptibility of the ingredients for this purpos® 
Mr. Talbot tried ammonia and several other re-agents with little success, 
till the iodide of potassium, greatly diluted, gave the desired result; this 
liquid, when applied to the drawing, produced an iodide of silver, @ sub- 
stance insensible to the action of light. This is the only method of pre 
serving the picture in its original tints, but it requires considerable nicetY, 
and an easier mode is sufficient for ordinary purposes. It consists 12 
immersing the picture in a strong solution of salt, wiping off the super 
fluous moisture, and drying it by the heat of the fire; on exposure to the 
sun, the white parts become of a pale lilac, which is permanent and 
immoveable. Numerous experiments have shown the inventor that the 
depth of these tints depends on the strength of the solution of salt; he 
also mentions that those prepared by iodide become a bright yellow under 
the influence of heat, and regain their original color on cooling. Without 
the application of one of these preservatives the image will disappe by 
the action of the sun ; but if inclosed in a portfolio, will be in no danget 
of alteration: this, Mr. Talbot remarks, will render it extremely cnr 
nient to the traveller, who may take a copy of any object he desires, 
apply the preservative at his leisure. In this respect Mr. Talbot's syst™ 
is greatly superior to that of M. Daguerre, since it would be scarcely PO 
