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224 THE ADVANCE OE PHOTOGRAPHY
steam is used to convey the sand, at a pressure of 60 to 120 lbs. to the square inch, the effect is wonderful. Sand blown with such power through a narrow pipe bores deep holes into the hardest stones, and even into glass. The process has been used to bore stone and metal plates. Figures cut in a cast-iron screen placed over a stone surface can be deeply engraved in the stone in a short time. The iron plate is also affected, but much more slowly than the stone slab. A cast-iron plate f% of an inch thick is only reduced -^ of an inch in the same time that a cut 300 times deeper is made in marble. India-rubber endures the sand stream almost as well as iron. Marble, protected by a perforated screen of india-rubber, may be cut 200 times as deep as the screen is thick without perceptibly affecting the india-rubber.
With the pressure of 100 lbs. such a sand stream can penetrate in one minute 1J inches deep into granite, 4 inches into marble, and 10 inches into soft sandstone.
The circumstance that soft bodies can be used as shields has led to elegant applications of this method in the industrial arts. For example, if glass be covered with lace pattern and a sand-blast be directed on it, the glass becomes ground in the meshes, and a copy of the lace is obtained on glass. In the same manner, painting with a gum colour upon glass can be produced clear on an unpolished ground by the sand-blast. This circumstance led immediately to the application of photograplry. If a gelatine picture is produced on glass by the transfer process, the surface of the glass at all the dark spots of the picture is protected b}T a layer of gelatine. If now a sand stream is allowed to operate upon it, it roughens the glass only at the uncovered places ; thus a picture is formed. If the gelatine picture is a negative, the shadows are dim. A plate ground in this manner may be used for printing with printer's ink. The metal plates of Talbot's heliographic process may be etched by the sand-blast instead of by