-60 SCIENTIFIC MISCELLANY. 



ink ; that is to siiy, where the light has acted, just as where greasy ink has 

 ibeen used, water is repelled ; where light has not acted, water is absorbed 

 and grease repelled ; and where light has partly acted, as in the half-tones 

 •of a negative, water is pai'tly absorbed, and grease partly repelled. This 

 important fact must also be noted, viz : that the gelatine sheet thus treated 

 has capabilities far beyond those of the lithographic stone, the latter being 

 .able to deal only with the contrasts of black and white (or lines), while the 

 gelatine is able to render all the infinite gradations of shade (or half-tints) 

 which appears in any photographic negatives taken from nature or life. 

 Ordinary gelatine is dissolved in warm water, and a sufficient quantity of 

 bi-chromate of potash is added to render it sensitive to light, and of alum to 

 make it very hard and durable. This solution is poured on a level plate, 

 previously rubbed over with wax, and is dried by means of heat. As soon 

 as dry, or when required for use, the sheet of gelatine is stripped from the 

 jilate, and printed under a photographic negative. When the picture ap- 

 pears sufficiently plainly, the sheet of gelatine is taken from under the neg- 

 ative, and made to adhere to a metal plate. The method of adhesion used 

 is that of atmospheric pressure. The sheet of gelatine and the metal plate 

 are put together under water ; as much of the water as possible is got rid 

 off from between the two surfaces ; the gelatine absorbs the remainder, so 

 that a vacuum is created, and the picture is thus attached to the plate b}' 

 the weight of the atmosphere. The superfluous chemicals are soaked out 

 with water, and the plate, Avith the printing surface of gelatine attached, is 

 placed on an ordinary platen printing-press, and inked up with ordinary 

 ink. In printing, it is necessary occasionally to dampen the plate with wa- 

 ter. A mask of paper is used to secure white margins for the prints ; and 

 the impression is then pulled, and is readj^ for issue. Two or more inks are 

 sometimes used in the j)roduction of on(5 picture, as it is found that, where 

 the light has acted deeply, a stiff ink is required ; but, where it has acted 

 not so deeply, — that is, in the half-tones, — a thinner ink may be used. So 

 that a stiff ink is first used for the shadows, and a thinner ink afterward for 

 the half-tones. In this manner three or four inks may be used in printing 

 one impression. The effect of India or other colored tint is obttaued by 

 using, instead of ordinary water for dampening the plate, water with some 

 color in it. The paper absorbs a certain amount of water out of the plate, 

 and with it a certain amount of color. The ordinary rollers are not found 

 to be satisfactory ; and a mixture of gelatine, glycerine, and castor oil is 

 used. 



By a modification of the process and from subjects in line, electrotypes are 

 produced capable of being worked on any printing -press with or without 

 type. 



T. A. Edison, in an article in the American Chemist, has the following 

 Laboratory Notes, which are of much interest, and may be found valuable: 



"Hard rubber or vulcanite, placed for several weeks in nitrobenzol, be- 

 comes soft and pliable like leather, and easily broken. 



"The vapor of chloral hydrate is a solvent of cellulose. I have found 

 the corks of bottles containing the crystals eaten away to the depth of a 

 quarter of an inch, the cork being resolved into a black semi-liquid. Cer- 

 tain kinds of tissue paper are jDartially dissolved in time, if thrown in a 

 Taottle containing the crystals. 



"A very difficult substance to dissolve is gum copal. I have found that 

 uniline oil dissolves it with great facility. 



"Hyposulphite of soda is apparentl}^ soluble to a considerable extent in 



