PHOTOGRAPHY. 29 



was the most easily operated upon. If we take a piece of 

 paper and brush over it a mixed solution of these two salts, 

 and when dry expose it to light beneath a negative for a short 

 time (about two minutes), we shall obtain a blue image after 

 treating it with a solution of potassium ferri-cyanide>. This 

 blue colour can only result from the contact of the potassium 

 ferri-cyanide with some ferrous compound. Here then we 

 have a demonstration of the change effected by light ; the 

 ferric compound is reduced to a ferrous state. 



I will endeavour to produce such a print before you, but I 

 must tell you that these iron salts are most objectionable for 

 lecture experiments. If you expose a piece of paper which 

 has been coated with this ferric compound, causing an image 

 to be formed of a ferrous salt, and put it away in the dark, 

 it rapidly loses the impression altogether. The ferrous 

 becomes reconverted into the feme salt. This is exceed- 

 ingly tantalizing. I had some sheets of prepared paper 

 exposed only this morning, and on developing them by the 

 method already indicated, you see the image is very weak, 

 due to this reactionary cause ; had it remained undeveloped 

 a few hours longer we should have had no image at all. 



An iron print can also be developed by means of silver 

 nitrate. A ferrous salt of iron will reduce silver nitrate 

 to its metallic state, as already shown in yesterday's lecture ; 

 and if I bring a solution of the latter on to the exposed 

 print, you will see that the silver deposits on those parts 

 affected by light. 



The next salts to which I must call your attention are the 

 uranium salts, of which there are specimens on the card 

 which you see before you. Uranium nitrate is sensitive to 

 light in he presence of organic matter, being thus reduced 

 to the state of an oxide. This oxide precipitates silver and 

 other metals from their solutions, and with potassium ferri- 

 cyanide forms a nearly insoluble brown compound. 



We will now develop a picture with the ferri-cyanide, and 

 you will note its appearance. I have also another photo- 

 graph printed with uranium, which is now being placed in 

 a solution of, silver nitrate, to which a little gallic acid 

 has been added. The silver is gradually being reduced by 

 uranium oxide, and a metallic image is being built up. After 

 passing the paper through sodium hyposulphite and wasliing, 

 the picture is permanent. 



