240 Photographic Processes. 



bestowed on toning, the uniform tinting of photographs has 

 received considerable attention, as a means of improving their 

 appearance. This is brought about in various ways. If, before 

 exposure, a paper positive is placed for a short time in a solu- 

 tion of uranium, then, on being taken out of the camera, is 

 washed for a few seconds in water at a temperature of from 

 122° to 140° Fahr., and, immediately afterwards, is plunged into 

 a solution of red prussiate of potash, it will soon acquire a 

 fine red colour. Being now dipped in a solution of nitrate of 

 cobalt, and dried at the fire, it will become green ; and this 

 colour is fixed by immersing in a solution containing sulphate 

 of iron and sulphuric acid, washing with water, and drying at 

 the fire. If a solution of prussiate of potash is used instead 

 of that of uranium, and a solution of bichloride of mercury, 

 saturated in the cold, after the paper has been taken from the 

 camera, followed by a solution of oxalic acid heated to from 

 about 122' to 140° Fahr., the colour will be a beautiful blue. 



Heliochromy . Among the various processes used by Niepce 

 de Saint Victor for the reproduction of colours, the following 

 were found to be the most effective : — A plate, like that used 

 for the Daguerreotype, is immersed for ten minutes in a solu- 

 tion of chloride of copper, or of iron, saturated to a degree suited 

 to the reproduction of the mean colours of the spectrum, and 

 then gently heated with a spirit-lamp ; if light which has passed 

 through a transparent coloured picture is now thrown upon it, 

 the various tints will be produced, but will vanish immediately. 

 If, however, the bath employed consists of half proto dr sesqui- 

 chloride of iron and half sulphate of copper, the colours of 

 objects are reproduced with great vividness, with the excep- 

 tion of yellow ; and even this is obtained by using a bath of 

 hypochlorite of soda, containing some alcohol and raised to a 

 temperature between 158° and 176° Fahr., stirring the plate 

 about in the mixture, until it is nearly black, then washing 

 with water, and drying with the flame of a spirit-lamp. Before 

 exposure, and while still lukewarm, the plate is coated with a 

 varnish which consists of dextrine and chloride of lead, and 

 dried by heat. This varnish causes the colours to appear with 

 great brilliancy, and brightens the white ground, on account of 

 the chloride of silver being bleached by the chloride of lead. 

 When a bath consisting of dentochloride of iron and sulphate 

 of copper has been used, fused chloride of lead prepared 

 directly from the metal must be employed; but, when a bath 

 consisting of hypochlorite of soda, unfusrd chloride of lead, 

 that it may neutralize tin net ion of the alkaline solution, and 

 tincture of benjamin of Siam is to be added to the varnish. 

 After the picture has been obtained; the plate is to be heated, 

 gradually, to the highest point short of carbonizing the organic 



