296 Deterioration of Photographs. 



perclia bath, continues to decompose the nitrate, until it is 

 saturated with the salt ; and some kinds of gutta-percha never 

 become fit for use. Gutta-percha may have an injurious effect, 

 without either causing a deposit, or producing turbidity. 



If collodion contains too little water, not only is its 

 sensitiveness diminished, but several imperfections are pro- 

 duced in the picture- As to perfectly anhydrous collodion, 

 it ,is difficult to obtain, and still more difficult to preserve un- 

 changed; since, while being used, it attracts moisture from 

 the atmosphere. If the collodion cracks and separates from 

 the plate, the evil is prevented by the addition of honey ; which 

 also renders collodion that has lost its sensibility by age fit 

 for use. Great solidity is imparted to collodion by adding 

 ether, by itself, or along with a small quantity of alcoholic 

 solution of iron, and by albumen mixed with water charged 

 with dextrine. 



After a negative has been exposed for, apparently, a 

 sufficient length of time, it may be found that the sensitive 

 coating has not been acted upon by the light to such a depth 

 as that the reduced silver will be large enough in quantity to 

 intercept the light so as to afford a good positive ; and if the 

 ordinary means of intensifying are employed, the collodion 

 may be softened, or the details may be obliterated. These 

 inconveniences can all be prevented, by causing light to 

 continue the work which was begun in the camera. For this 

 purpose, when the picture has been brought out, whether by 

 gallic acid or by protosulphite of iron, it is to be washed, and 

 then subjected to the action of light, which causes the picture 

 to go on acquiring vigour in a surprising manner. The light 

 affects only those parts on which it acted when the plate was 

 in the camera ; since the reducing agent has taken away all the 

 silver, except that forming the picture. Such an effect cannot 

 be produced after fixing, because the hyposulphite removes all 

 the iodide of silver which has not been sufficiently acted upon. 



Gradual deterioration of Photographs. As yet, we cannot be 

 certain, whatever care we take, that a photograph will not 

 deteriorate by time ; for it has been ascertained that the very 

 substances which are used to prevent light from further action, 

 may sometimes, themselves, attack the shadows and half- 

 shadows. Moisture is one of the chief causes of the deterio- 

 ration of paper photographs. It is avoided by using paper 

 which has been dipped in a liquid that consists of alcohol, ben- 

 jamin, and chloride of cadmium, and drying; then sensitizing^ 

 in the way usually adopted with albumenized paper. Prepared 

 in this manner, paper has its sensibility increased. Benjamin, 

 either by itself, or mixed with another resin, may be sub- 

 stituted for albumen, etc., in the preparation of paper for 



