Photographic Processes. 235 



has caused many attempts to substitute other substances for it : 

 of those which have been tried, chloride of platina with acetate 

 of soda gives the best results — it is nearly as effective as 

 chloride of gold. 



Albumenized Glass. A combination of several processes has 

 been found most successful with albumen on glass. The albu- 

 menized plate is washed with a weak solution of nitrate of sil- 

 ver to which alcohol has been added, then with a mixture of 

 protoiodide of iron, and afterwards with a strong solution con- 

 taining nitrate of silver and acetic acid. After exposure, it is 

 developed with protosulphate of iron, and fixed in the ordinary 

 way. Alhmmenized glass, usually very slow, is by this method 

 rendered extremely sensitive. 



Waxed Paper. Suitable paper is carefully saturated with 

 liquid wax, the excess of which is removed by blotting paper, 

 and a moderately hot smoothing-iron. It is then immersed, 

 for a considerable time, in a solution containing iodide and 

 bromide of potassium, after which it is dried. When required 

 to be used, it is sensitized with nitrate of silver and acetic acid : 

 it is developed with a mixture containing gallic acid and nitrate 

 of silver, and fixed with hyposulphite. 



Moist Collodion Process. Common gun cotton is almost en- 

 tirely dissolved by a mixture consisting of about nine parts 

 ether and one alcohol : the solution is Collodion. Alcolene is 

 a collodion containing no ether. It is obtained by dissolving a 

 gun cotton which has been prepared with 100 parts by weight of 

 concentrated sulphuric acid and 90 parts nitric acid, density 

 1*4 in alcohol, spec. grav. 0*803, and diluting the result, a 

 thick gummy mass, with absolute alcohol, having an iodide in 

 solution. If weaker acids are employed ether will be required 

 for solution of the gun cotton. The best iodide for preparing 

 a quick and stable collodion is that of cadmium; almost all others 

 colour the collodion, and therefore diminish its sensibility. 



A perfectly clean glass plate having been coated with the 

 iodized collodion, which is allowed to solidify, but not to be- 

 come dry, it is immersed in a solution of nitrate of silver, and 

 then placed in the camera. The picture is developed with a 

 mixture of pyrogallic and acetic acids, diluted with water, and 

 is fixed in the usual way. 



Protosulphate of iron may be used in the developing mix- 

 ture, or — which is very much better — double sulphate of iron 

 and ammonia, instead of pyrogallic acid ; if formic acid is sub- 

 stituted for the acetic, the rapidity is augmented, and the pic- 

 ture is rendered more intense. Very good results are obtained 

 if the picture is developed with protosulphate of iron, and in- 

 tensified with pyrogallic acid and nitrate of silver, before the 

 fixing or, if there is a tendency to fogging, after it. 



