=Substratum.=—(1.) Water 300 parts, white of egg 300 parts, ammonia 50 parts. (2.) Water 300 parts, gelatine 10 parts, bichromate of potassium 10 parts. Filter both solutions, and mix, taking care that the temperature never exceeds 32° C.; pour upon glass plates heated to 30° C., and dry at a temperature of 22° C. After thoroughly cleaning the back of the plate, place, face downwards, upon a black cloth, and expose to light through the glass till the coating becomes brownish. To test if sufficiently exposed, dampen a portion of the film with the finger, scratch with the nail, and the scratch should stand out perfectly sharp. =The intermediate layer= is to ensure the better adhesion to the glass of the print layer proper, and to facilitate the retention of moisture while printing. It consists of gelatine 100 parts, water 1,500 to 1,800 parts, alcohol 150 parts, glycerine 5 parts, bichromate of potassium 5 parts. This is spread over the substratum, allowed to dry, and exposed to diffused light, or allowed to stand for twenty-four hours, well washed, and allowed to dry spontaneously, after which it may be kept for weeks. =The Print Layer= consists of water 1,500 parts, gelatine 100 parts, bichromate of ammonium 30 parts, complete solution being effected by means of a water bath; the drying box heated to 45° to 50° C. Place the levelling stand upon a sheet of white paper, carefully dust and accurately level the glass plate, and coat, using 30 ccm. of solution to cover 25 × 35 cm. of surface, avoiding any overflow of solution. =The Washing Water= used after printing in the copying frame should not exceed 22° C. in the summer, or fall below 10° to 11° C. in the winter. Washing should be continued until the film is free from colour, and a final rinse should be given under a rose. Allow to dry slowly. =Etching Solution.=—Water 500 parts, glycerine 500 parts, chloride of sodium 15 parts, or hyposulphite of soda 15 parts. =Rolling Up.=—If the picture take too much ink, wash off with turpentine, carefully wipe with a cloth, and continue the etching. If it take little, wash off the ink with etching fluid, and place in a hardening solution of chrome alum 1 part, water 6 parts, wash, dry, and once more etch for a quarter of an hour. =Varnish for Prints.=—Bleached shellac 100 parts, dissolved in alcohol 300 parts, adding ammonia 300 parts, further adding, with constant agitation, 600 parts of boiling water. CHAPTER XVII. PRACTICE OF PHOTO-LITHOGRAPHY. In photo-lithographic operations, quite a different set of conditions has to be considered as compared with Collotype. An explanation is, of course, not far to seek, as both the chemical and physical properties of the lithographic stone and the exposed chromated gelatine layer are so extremely different. To the better understanding of the operations, the composition of the lithographic stone and the principles underlying the process may be first considered.