that the little particle of moving sand can not be brought to rest immediately. However quickly its flight is arrested, there is an appreciable interval of time during which its motion must be parted with. Striking against the soft and flexible rubber, the sand is brought to rest gradually, for the rubber is sufficiently depressed by the Lilliputian blow to dispose of the motion stored up in the particle. When, however, the sand strikes against the hard and rigid glass, there is no giving way possible. The grains must either stop instantly or else they must penetrate between the molecules of the glass. In the latter case they would naturally detach little fragments in sufficient number to roughen the surface of the glass and make it translucent. Experience shows that this is precisely what happens. If the naked hand be held over the blast, a pricking sensation is felt, but the skin is not broken; it is too pliable. Thin sheet-iron stencils are sometimes substituted for those of rubber; their elasticity makes them fairly durable.
The sand-blast was invented by an American, but, as the original patent has expired, any one is at liberty to use the machine. The inventor has since made a number of modifications and improvements, which are protected by subsequent patents. The newer form is used, I believe, more in England than in this country—not so much from a failure on our part to appreciate its merits, as from a dislike of the peculiar royalty arrangements. The machines are sold, and a certain royalty charged each week, whether the works are running or not. As such an arrangement makes the expense a constant quantity, while the income is a variable, it is not acceptable to the majority of American glass workers.
Other agents besides mechanical find employment in the atelier. One of the properties of glass which makes it most highly esteemed, in both the household and the laboratory, is its almost total indifference at ordinary temperatures to acids and other corrosive chemicals. It is slightly acted upon by the strongest sulphuric acid and by steam under great pressure, but only after the lapse of considerable time. There are few substances, however, which are not, Achilles-like, vulnerable in some one particular. In the case of glass, the effective solvent is the comparatively rare compound, hydrofluoric acid. It is not strange, therefore, that in the numerous manipulations to which glass is subjected this fact should be utilized. It forms the basis of the one chemical process of the atelier, that of etching. It is a process readily and cheaply carried out, and from its effectiveness it is one of increasing importance. The piece of glassware to be treated is protected, in those parts which it is desired shall not be acted upon by the acid, by some substance indifferent to it, such as wax, paraffin, or a
