Development. 293 



totally free from, the imperfections of the ordinary kind ; it has 

 no inequality of pores ; it is quite homogeneous, being, what 

 no other paper is, pure cellulose ; and it greatly exceeds albumen 

 in sensitiveness. It loses this sensitiveness, however, with 

 great rapidity by becoming dry ; but this imperfection is not 

 quite irremediable, since, if a certain quantity of albumen is 

 mixed with it, rapidity is in a great degree preserved, and the 

 plates may be kept for some days without being used. Collo- 

 dion is the more effective, since the silver salt is diffused through 

 its whole substance. The reddish tint of old collodion is due 

 to iodine which is set at liberty; this causes it to have less ra- 

 pidity, and a greater tendency to solarization than new, but it 

 gives a more intense image. The addition of carbonate of soda 

 in solution not only restores the collodion thus reddened, but 

 augments sensibility, and increases the density of the image. 



The time required for exposure with the so-called dry collo- 

 dion is diminished by washing the plate after it is taken from 

 the sensitizing bath, then dipping it in tannin, and again wash- 

 ing it. The tannin keeps the pores of the sensitive coating 

 open, and also exerts a stimulating action on the iodide of sil- 

 ver, so as to render the effect of the light more intense and 

 more complete ; but its chief advantage is during the develop- 

 ment. A merely bromurated collodion, with tannin, is quicker 

 than one with iodine and bromine without it. The contrary is 

 the case with moist collodion. The latter owes its sensibilty 

 to the nitrate of silver, which, in drying, not only crystallizes, but 

 forms a double salt with the iodide. Hence, to obtain a quick 

 dry collodion, we must employ a bromo -iodide, and remove the 

 nitrate by washing. If glycerine is mixed with the nitrate 

 used in sensitizing collodion, as it does not dry, it will keep the 

 surface moist, so that the plate may be left for at least a day 

 without being used. If the glycerine is applied after exposure, 

 the developing and finishing may be postponed. 



Development. In developing negatives, the bromide and 

 iodide are reduced in those places where the light has acted, by 

 protoxide of iron and gallic acid ; the latter seizes the oxygen 

 of the oxide, and its affinity for this element is so strong that 

 it will precipitate metals, in a pure state, from even powerful 

 acids. The substitution of formic for acetic acid, in a developing 

 fluid, seems to give greater intensity to the picture, -and, by 

 consequence, shortens the time required for exposure. It has 

 been found that when formic acid is used, a picture may, in fa- 

 vourable circumstances, be taken in less than a second. A nega- 

 tive is changed into a positive picture, by rendering the reduced 

 silver white and brilliant, instead of allowing it to retain the 

 appearance it presents in a negative. For the purpose of judg- 

 ing whether or not a negative will give a good positive,, it may 



