HISTORY OF PHOTOGRAPHY. 181 



is done by washing in jnire Avater. If the water contains any mnri- 

 ates the picture sutlers, and long soaking in such Avater obliterates it, 

 or if a few grains of common salt are added to the water the appar- 

 ent destruction is very rapid. The picture is, however, capable of 

 restoration, all that is necessary being to expose it to sunshine for a 

 quarter of an hour, when it revives, but instead of being of a red 

 color it becomes lilac, the shades of color depending upon the quan- 

 tity of salt used to decompose the chromate of silver which forms the 

 shadow parts of the picture. 



Mr. Bingham remarks on this process, that if Ave substitute sul- 

 phate of nickel for the sulphate of copper, the paper is more sensitiA^e 

 and the picture is more clearly de\"eloped by nitrate of silver. 



The following modification of this process possesses some advan- 

 tages. If to a solution of the sulphate of copper we add a solution 

 of the neutral chromate of potash, a very copious brown precipitate 

 falls, Avhicli is a true chromate of copper. If this precipitate, after 

 being Av^ell Avashed, is added to Avater acidulated Avith sulphuric acid, 

 it is ciissoh^ed and a dichromatic solution is formed, Avhich Avhen 

 spread upon paper is of a pure yelloAv. A A^ery short exposure of 

 the paper AAashed Avith this solution is quite sufficient to discharge 

 all the yelloAv from the paper and giA^e it perfect Avhiteness. If an 

 engraA'ing is to be copied Ave proceed in the usual manner; and Ave 

 may either bring out the picture by placing the ])aper in a solution of 

 carbonate of soda or potash, by Avhich all the shadows are represented 

 by the chromate of copper, or by washing the paper Avith nitrate of 

 silver. It may sometimes happen that, oAving to deficient light, the 

 photograph is darkened all oA^er Avhen the sih^er is applied; this 

 color, by keeping, is gradually remoATul and the picture comes out 

 clear and sharp. 



If the chromate of copper is dissoh^ed in ammonia, a beautiful 

 green solution results, and if applied to paper acts similarly to those 

 just described. 



The chromatype pictures, under certain conditions aflE'ord a beauti- 

 ful example of the changes Avhich take place sloAvly in the dark 

 from the combined operations of the materials employed. 



If Ave take a chromatype picture after it has been deA^eloped by 

 the agency of either nitrate of silver or of mercury and i^lace it 

 aside in the dark, it Avill be found after a few Aveeks to have darkened 

 considerably both in the lights and shadoAvs. This darkening sloAvly 

 increases, until cA^entually the picture is obliterated beneath a film 

 of metallic silver or mercury ; but Avhile the picture has been fading 

 out on one side it has been dcA'eloping itself on the other, and a A'ery 

 pleasing image is seen on the back. After some considerable time 

 the metal on the front gives Avay again, the paper slowly Avhitens, 



