Take one ounce of the best transparent gelatine, and throw it into a pipkin in which has been previously placed one pint of distilled water; set this on a slow fire, or over a lamp, until the gelatine is completely melted; then weigh out half ounce of pure citric acid and dissolve it in two ounces of distilled water; add this to the solution of gelatine, stirring it during the addition with a glass rod. The solution in the pipkin is now to be gently boiled for about half an hour: remove it from the fire, and add sufficient distilled water to make up the bulk of liquid to one pint. When quite cold, the liquid in the pipkin is to be filtered through two thicknesses of pure white blotting paper into a bottle perfectly dry and clean. We now add to every pint of filtered preservative solution, two ounces of alcohol of the specific gravity of ·840.
The solution thus prepared is ready for use, and should be of a pale amber colour, without any signs of insoluble particles floating in it; should any appear after it has been prepared for some days, a second filtration will remove them, and render the liquid again bright and clear.
It will be found better to prepare this solution only in the quantity indicated above, unless the consumption be large, for, although it will keep good for a month or more, my experience points to the fact, that the most successful results follow the use of Preservative Solution freshly prepared.
The bath for rendering the plates sensitive does not differ from that recommended for taking negatives with wet Collodion. The formula for its preparation may not be out of place, however, and may assist those whose knowledge of the matter is not perfect.
Nitrate of Silver (fused) 1¼ ounces. Distilled Water 1 pint. Iodide of Cadmium 3 grains.
Dissolve the nitrate of silver in half the water and then add the iodide of cadmium; thoroughly agitate the mixture for five or ten minutes, then add ½ ounce of alcohol, sp. gr. ·840, and fill up with the remaining half of the distilled water; further agitation, and subsequent filtration through two thicknesses of white bibulous paper, will put us in possession of a negative bath. The nitrate of silver being fused, and consequently, pure and neutral, and as it is essential to obtain clean pictures that the bath should be slightly acid in its reaction, we find it necessary to add 5 or 6 minims or drops of pure glacial acetic acid to a bath of 20 ounces, in order that the above condition may obtain.