as Photographic Agents, 207 



Colour of Picture. 

 n. Hydrochloric acid .... Red which blackens. 

 o. Hydrochloric aether ... Black. 



p. Aqueous chlorine Red, deepens a little. 



q. Phosphoric acid Very variable. 



14. When papers prepared with any of the above, except 

 I and q, are soaked for a little time in water, and dried in 

 the sunshine, the picture produced — it matters not what hy- 

 driodate is used — is rendered peculiarly red, and does not 

 change by re-exposure: washing either of the papers b, c, or 

 d with weak solution of ammonia, occasions this peculiarity 

 in a striking manner. 



15. The Solution of Silver. — Take of crystallized nitrate of 

 silver 120 grains, distilled water 12 fluid drachms; when the 

 salt is dissolved, add of alcohol 4 fluid drachms, which ren- 

 ders the solution opake. After a few hours a minute quan- 

 tity of a dark powder — oxide of silver? — is deposited, and 

 must be separated by the filter. 



16. The addition of the alcohol to the solution was adopted 

 from an observation I made of its influence in retarding the 

 chemical action of the hydriodates on the salt of silver, which 

 goes on in the shade. Its use is therefore to make the action 

 depend more on luminous influence than would be the case 

 without it. 



17. Nitric aether and acetic aether not only check the 

 bleaching process in the shade, but actually act with the hy- 

 driodic salts in exalting the oxidation of the silver. In copying 

 lace or feathers, they are very valuable agents, but for any 

 other purposes they are useless, as all the faintly lighted parts 

 are of the same tint. 



18. The hydrochloric aether, which I use as the solvent of 

 the silver, and apply without any saline wash, has a similar 

 property to the nitric ; but as it is readily affected by faint 

 light, it is of greater value. However, papers prepared with it 

 must be used within twenty-four hours, as after that they 

 quickly lose their sensitiveness, and soon become nearly use- 

 less. 



19. The Hydriodic Solutions. — To fix with any degree of cer- 

 tainty the strength of the solution of the hydriodic salts, which 

 will in all cases produce the best effect, appears to me im- 

 possible ; every variety of paper, either as regards its com- 

 position, or the intensity of light to which it has been ex- 

 posed to darken, requiring a solution of different specific 

 gravity. 



20. Hydriodates of Poiassa and Soda, — The former of 

 these salts being more easily procured than any other of the 



