Second Preparation. To coat the plates with gelatin, they must first of all be thoroughly rinsed with cold water, best under a tap, but without the prepared side being touched. They are then dried, and ready to be treated with gelatin. This is done in the following manner: a box having a sheet - iron bottom, and a cover of dark linen or cloth, is provided; inside, three inches above the iron bottom, is a frame, spanned with linen, exactly the size of the interior, and this is covered with filter-paper loosely laid upon it. This frame should equalize perfectly the unequal temperature of the iron below, for under the box is fitted a gas or spirit flame. Three inches under the lid are iron rods from one side of

368. Now make a second sensitive liquid, that which forms the printing film, as follows:

Gelatin,.........................

90 grammes ( 2 ozs. 7 drs.)

Water.........................

720 " (22 " 4 " )

Fish-Glue(true),.........................

30 " ( 8 " )

Water,.........................

360 " (11 ozs. 4 " )

Bichromate of Potash, pure, or of Ammonia,.............

30 " ( 8 " )

Water,.........................

360 " (11 ozs. 4 " )

The gelatin is put to swell twenty-four hours before operating, as is also the fish-glue. Then these two substances are dissolved separately in their water, over a water-bath. The gelatin will dissolve at a temperature of from 40 to 60° C. (104 to 140° F.). As to the fish-glue, it will be necessary, in order to dissolve it, to increase the heat to the boiling point, and even then a complete solution will not be attained. These two solutions are filtered in a clean recipient through a rather coarse cloth, and then the solution of bichromate is poured into the same filter.

The apparatus contrived by Mr. Brewer to filter the gelatin whilst hot is very useful, and consists of two distinct parts: 1. A glass funnel, the tube of which passes through a cork. 2. A copper receiver, forming a jacket for the funnel, and holding the water, which is heated by a gas-burner or an alcohol-lamp, as is seen in the cut (Fig. 103). In case of accidental breakage, the glass funnel is very easily replaced. - Leon Vidal.

Fig. 103.

358 Second Preparation 125

box to the other, placed horizontally,each rod being furnished with two or three holes, into which screws are fitted, upon the heads of which the glass plates rest.By turning these screws so that the heads are raised or depresssd, the plates are easily brought into a horizontal position. A thermometer suitably fitted into the side of the box indicates the temperature. Two, three, or more glass plates are laid horozontally upon the

The drawing below gives an idea of the drying - box in which are prepared the photo-tvpic plates. This is divided into two distinct parts: First the lower chamber, having a number of gas-pipes, pierced with holes, running through it at equal distances and forming a grating. Two or three of these pipes are suffi-cient to give the proper heat, even when it is necessary to raise it over 150° C. (302° F.) A stop - cock is attached to each pipe so that it can be used independently of the others. This chamber is closed by a door extending the full length to facilitate the lighting, cleaning, and repairing of the tubes. One or more openings made at the two ends of the beating chamber allow the air to enter that is necessary for the combustion of the gas. These openings are regulated according to the number of burners used, so as to obtain the desired effect without useless loss of heat. If gas is not available, a current of steam or hot - air may be used. In the latter case, for the gas tubes, a pipe is substituted leading to a stove which furnishes the heat. This pipe extends along the bottom and passes out by means of an elbow. In the last two cases it is not necessary to place openings in the box. A metallic division of strong sheet - iron is placed between the upper and the lower chambers; this is the dryer proper. This division or apartment, which is hermetically closed, is necessary to prevent the dust, which would be carried by the current of hot - air, from falling on the gelatinised plates, to the great injury of their surfaces. Towards the middle of the height of the upper box are placed two longitudinal strips, made of wood, and nailed against the two opposite sides of the box. On these strips rest transversal iron bars having three or four screws , as is seen in the cut, at suitable distances, to divide the strips into three or four equal parts. These screws serve to wedge the plates which are placed on their upper angles), and which are adjusted by means of a spirit-level. As many bars are used as are necessary to cover the entire surface of the dryer.

Now, everything being on the movable platform, commence with the first plate. It is screws, the box is closed, and the temperature raised to 30° Reaumur. In the meantime, a quarter of an ounce (7.5 grammes) of the finest French gelatin is taken; five ounces (150 grammes) of distilled water are poured upon it, and the gelatin allowed to swell for an hour. After this the gelatin is dissolved upon a water-bath, and when it has reached a high temperature (say 70° R.), fourteen grains (0.875 grammes) of bichromate of covered with the gelatinous liquid, put back in its place, and the lid instantly closed. The platform is pushed forward a length, the second lid is opened, the plate receives its preparation, put back in its place, and the second lid closed, and so on with the rest. When the dryer is regularly heated to 35° C. (95° F.), it requires at least two hours for the complete desiccation of the plates. It is well to make sure if the operation is ended by opening the lid corresponding to the last preparation. If the plate in this part of the box is dry, all the others are certainly so, and the heat should be suppressed. The plates should be allowed to cool slowly, and may be exposed as soon as cold. In the summer the operation in the dryer maybe done in the morning, and the plates exposed in the afternoon. The plates are allowed to disgorge during the same day, and on the following morning they may be printed from. The dryer should be kept in a room the glass of which is covered with yellow paper, or, what is still better, with a pellicle covered with chrysoidine confined between two sheets of glass. The interior of the dryer should be carefully cleaned, from time to time, with a damp sponge to remove the dust, and to render this operation easier it is well to line the sides of the upper chamber with zinc.