The albumen is the white of eggs, and if the operator is not accustomed to separating the whites from the yolks, it is as well to break each egg separately into a cup and then strain off the white into the bulk. The whipped and filtered albumen will keep for two or three months in a well-corked bottle.

The glass should be flowed over with this solution, drained for about thirty seconds and then placed on a leveled slab to dry in a place free from dust. The plates should then be heated for two minutes to 60° C. (1400 F.) and allowed to cool. This heating may be effected by placing the plates on two or three thicknesses of blotting paper on top of an iron plate, or what is more convenient, an empty cracker tin of fair size. The plates maybe placed in a rack and with an ordinary Bunsen burner the temperature will soon be raised inside the tin, if the lid be put on. A thermometer can be inserted through a hole close to the plates. About four minutes of this treatment will suffice. This is less likely to crack the glass. A stock of plates can be prepared and packed face to face, wrapped in pairs in tissue paper, and will keep indefinitely in a dry place. To sensitize them, the plates should be immersed in:

Silver nitrate 100 g.

Glacial acetic acid 100 ccm.

Distilled water to 1000 ccm.

This may be used in a dish, preferably of glass. If porcelain be used, a new tray should be taken into service to avoid any possible contamination from previously absorbed solutions. About three minutes immersion is sufficient. They should then be drained for about one minute and immersed in distilled water for five minutes, the water poured off and fresh applied, and this repeated six times. They may then be color-sensitized, and although their sensitiveness is very low only a green light should be used. The sensitizer may be:

Glycin red, 1: 500 alcoholic solution 8 ccm.

Cyanin, 1: 500 alcoholic solution 2 ccm.

Ammonia, 2 % solution 15 ccm.

Distilled water to 1000 ccm.

Bathe for two minutes, rinse in distilled water once, and dry at 60 ° C. (140 ° F.).

Valenta in 1892 was the first to publish a gelatine emulsion formula, and various modifications have been given; the following, one of the simplest and best, was later also given by Valenta:

A. Gelatine 10 g.

Silver nitrate 6 g.

Distilled water 300 ccm.

B. Gelatine 20 g.

Potassium bromide 2.4 g.

Sodium chloride, pure 1.5 g.

Distilled water 300 ccm.

Soak the gelatine in water for half an hour, then melt by the aid of a water bath heated to 40 ° C. (104 ° F.). When complete solution is obtained, add the salts and cool the solutions to 35 ° C. (95 ° F.). Then pour A into B rather slowly, stirring all the time, and immediately pour into 1000 ccm of alcohol with constant stirring. The emulsion separates out into flocks; these should be collected on a piece of clean linen (an old handkerchief is excellent), the ends brought together so as to form a bag, tied, and hung over a stick across a beaker filled with water. This should be changed every five minutes for half an hour. The author has found a slight modification of this more convenient: a good sized glass funnel to hold about 2 liters is obtained and a short piece of rubber tubing furnished with a pinchcock attached to its stem. A piece of cloth large enough to completely cover the top of the funnel and lap well over the sides is used. The emulsion is collected in this from the alcohol bath and it is then gathered into a bag and swung round at arm's length for a minute or two, which gets rid of most of the alcohol. The cloth is then stretched over the funnel and the center pushed down, the funnel filled with water and the emulsion stirred up with a glass rod for two minutes and then allowed to soak for five. Then the pinch cock is opened, the water run out, and the funnel filled up again so as to completely cover the shreds. About six changes of water will thus wash it perfectly. Finally the ends of the cloth are again gathered together and the bag swung round three or four times. The emulsion should now be put into a marked beaker, and melted in a water bath, the temperature of the emulsion not being allowed to rise over 35 ° C. (95 ° F.), and the volume made up to 600 ccm.

An alternative method is to make the emulsion as described above, coat the plates immediately after filtering through the wool, place the plates on a level slab to set and then wash in running water for fifteen minutes, and dry.

These emulsions are, like all others, wanting in color sensitiveness. The original plan of sensitizing them was to bathe the coated plates in dye solutions and it is still the best plan, but it may be more convenient, particularly if the emulsion is not washed until after coating, to add the dye to the emulsion. Both methods will be described, but we must deal with the coating first. It was originally supposed that a very thin film of emulsion was necessary, and the plates were coated and then whirled to sling off all but a very thin film. It has now been discovered that such thin coats, while they are rather faster, are not essential. The easiest plan in coating is to place the plates in a stack against the wall or other convenient place, pick up the first with a pneumatic bulb, pour over the emulsion to freely cover the surface, drain for fifteen seconds, and then place on a level slab to set. Practically, the thinner the coat the faster the plate, and if the above method gives too slow a plate, then the glass may be warmed, which will cause more emulsion to drain off.