GLASS-MAKING. 169 



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 de- 

 pressed 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 natu- 

 rally 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 origi- 

 nal patent has expired, any one is at liberty to use the machine. 

 The inventor has since made a number of modifications and im- 

 provements, which are protected by subsequent patents. The 

 newer form is used, I believe, more in England than in this coun- 

 try — 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 es- 

 teemed, in both the household and the laboratory, is its almost 

 total indifference at ordinary temperatures to acids and other cor- 

 rosive chemicals. It is slightly acted upon by the strongest sul- 

 phuric 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 im- 

 portance. 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 



