HISTORY OF PHOTOGRAPHY. 187 



and when dry washed with a solution of the ferrocj^anide of potas- 

 sium, became extremely sensitive to light, changing- from a light 

 brown to a full black by a moment's exposure to sunshine. Follow- 

 ing out this result, it was discovered that perfectly pure iodide of 

 silver was acted on with even greater rapidity, and thus became easy 

 to form an exquisitely sensitive photographic paper. 



The method recommended is the following : 



Highly glazed letter paper is washed over with a solution of 1 

 dram of nitrate of silver to an ounce of d-istilled water; it is 

 ([uickly dried and a second time washed with the same solution. It 

 is then, when dry, placed for a minute in a solution of 2 drams 

 of the iodide of potassium in (> ounces of water, placed on a smooth 

 board, gently washed by allowing some water to flow over it, and 

 dried in the dark at common temperatures. Papers thus prepared 

 may be kept for any length of time, and are at any time rendered 

 sensitive by simpl}^ washing them over with a solution formed of 

 1 dram of the ferrocyanide of potassium to an ounce of water. 



These papers, washed with the ferrocyanide and dried in the dark, 

 are, in this dry state, absolutely insensible, but they may at any 

 moment be rendered sensitive by merely Avashing them with a little 

 cold clean water. 



Papers thus prepared are rendered quite insensible by being washed 

 over with the above hydriotic solution. They are, however, best 

 secured against the action of time by a solution of ammonia. 



THE rLUOROTYPE. 



The fluorotype, so called from the introduction of the salts of 

 fluoric acid, consists of the following process of manipulation : 



(Bromide of pota.ssinin 20 grains. 



[Distilled water 1 fluid ounce. 



IFluoride of sodium 5 grains. 



(Distilled water 1 fluid ounee. 



Mix a small quantity of these solutions together when the papers 

 are to be prepared, and wash them once over with the mixture, and, 

 when dry, apply a solution of nitrate of silver, 60 grains to the ounce 

 of water. These papers keep for some weeks without injui-y and 

 become impressed with good images in half a minute in the camera. 

 The impression is not sufficiently strong when removed from the 

 camera for producing positive pictures, but may be rendered so by 

 a secondary process. 



The photograph should first be soaked in water for a few minutes 

 and then placed upon a slab of porcelain, and a weak solution of the 

 protosulphate of iron brushed over it; the picture almost innne- 



