(3) Blue figures on a white ground are changed into black by dipping the proof in a solution of 4 oz. common potash in 100 oz. water, when the blue colour gives place to a sort of rusty colour, produced by iron oxide. The proof is then dipped in a solution of 5 oz. tannin in 100 oz. water. The iron oxide takes up the tannin, changing to a deep black colour; this is fixed by washing in pure water.

(4) Joltrain's. Black lines on white ground. The paper is immersed in the following solution: - 25 oz. gum, 3 oz. sodium chloride, 10 oz. iron perchloride (45° B.), 5 oz. iron sulphate, 4 oz. tartaric acid, 47 oz. water. The developing bath is a solution of red or yellow prussiate of potash, neutral, alkaline, or acid. After being exposed, the positive is dipped in this bath, and the parts which did not receive the light take a dark-green colour; the other parts do not change. It is then washed with water in order to remove the excess of prussiate, and dipped in a bath containing acetic, hydrochloric, or sulphuric acid, when all the substances which could affect the whiteness of the paper are removed. The lines have now an indigo-black colour. Wash in water, and dry.

(5) Copies of drawings or designs in black and white may be produced upon paper and linen by giving the surface of the latter 2 coatings of: 217 gr. gum arabic, 70 gr. citric acid, 135 gr. iron chloride, 1/4 pint water. The prepared material is printed under the drawing, and then immersed in a bath of yellow prussiate of potash, or of silver nitrate, the picture thus developed being afterwards put in water slightly acidified with sulphuric or hydrochloric acid.

(6) Benneden states that paper, prepared as follows, costs but 1/6 as much as the ordinary silver chloride paper, is as well adapted to the multiplication of drawings, and is simpler in its manipulation. A solution of potash bichromate and albumen or gum, to which carbon, or some pigment of any desired shade, has been added, is brushed, as uniforndy as possible, upon well-sized paper by lamplight, and the paper is dried in the dark. The drawing, executed on fine transparent paper (or an engraving, or woodcut, &c), is then placed beneath a fiat glass upon the prepared paper, and exposed to the light for a length of time dependent upon the intensity of the light. The drawing is removed from the paper by lamplight, and after washing the latter with water, a negative of the drawing remains, since the portions of the coating acted on by the light become insoluble in water. From such a negative, any number of positives can be taken in the same way.

(7) Dieterich's copying-paper. The manufacture may be divided into 2 parts, viz. the production of the colour and its application to the paper. For blue paper, he uses Paris blue, as covering better than any other mineral colours. 10 lb. of this colour are coarsely powdered, and mixed with 20 lb. ordinary olive oil; 1/4 lb. glycerine is then added. This mixture is, for a week, exposed in a drying-room to a temperature of 104°-122° F. (40°-50° C.) and then ground as fine as possible in a paint-mill. The glycerine softens the hard paint, and tends to make it more easily diffusible. Melt 1/2 lb. yellow wax with 18f lb. ligroine, and add to this 7 1/2 lb. of the blue mixture, mixing slowly at a temperature of 86°-104° F. (30°-40° C). The mass is now of the consistence of honey. It is applied to the paper with a coarse brush, and afterward evenly divided and polished with a badgers' hair brush. The sheets are then dried on a table heated by steam. This is done in a few minutes, and the paper is then ready for the market. The quantities mentioned will be sufficient for about 1000 sheets of 3G in. by 20, being a day's work for 2 girls. For black paper, aniline black is used in the same proportion.

The operation must bo carried on in well-ventilated rooms protected from fire, on account of the combustibility of the material and the narcotic effects of the ligroine. The paper is used between 2 sheets of paper, the upper receiving the original the lower the copy.

(8) By means of gelatine sensitive paper any ordinary thick cardboard drawing can be copied in a few seconds, either by diffused daylight or gas- or lamplight. The copy will be an exact reproduction of the original, showing the letters or figures non-reversed. If it is desired to make a copy in the daytime, any dark closet will answer, where all white light is excluded. The tools required are an ordinary photograph printing frame and a red lantern or lamp. The sensitive gelatine paper is cut to the size required, laid with the sensitive side upward upon the face of the drawing, and pressed thereon in the usual manner, by springs at the back of the frame, which is then carried to the window and exposed with the glass side outward for 2 to 5 seconds to the light, the exposure varying according to the thickness of the drawing. If gas- or lamplight is used at night, 20 to 30 minutes' exposure is sufficient. The frame is returned to the dark closet, the exposed sheet is removed to a dark box, and other duplicates of the drawing can be made in the same way. It is thus possible to make 10 to 20 copies of one thick drawing in the same time that it usually takes to obtain one copy of a transparent tracing by the ordinary blue process.

The treatment of the exposed sheets is quite simple; all that is necessary is to provide 3 or 4 large pans or a large sink divided into partitions. The development of the exposed sheets can be carried on at night or at any convenient time, but a red light only must be used. The paper is first passed through a dish or pan of water, and then immersed in a solution, face upwards, composed of 8 parts of a saturated solution of potash oxalate to 1 of a saturated solution of iron sulphate, enough to cover the face of the paper. The latent image soon appears, and a beautiful copy of the drawing is obtained, black where the original was white, with clear white lines to represent the black lines of the drawing. With one solution, 6 to 8 copies can be developed right after the other. After development, the print is dipped in a dish of clear water for a minute, and finally immersed for 3 minutes in the fixing solution, composed of 1 part of soda hyposulphite dissolved in 6 of water. It is then removed to a last dish of water face downward, soaked for a few minutes, and hung up to dry; when dry it is ready for use.

Some very useful suggestions will be found in a little volume by Tuxford Hallatt, entitled 'Hints on Architectural Draughtsmanship.'