194 FKAGMENTS OF SCIENCE. 



ful friend, Mr. Josiah Quincey, to see the action of the 

 sand-blast. A kind of hopper containing fine silicious 

 sand was connected with a reservoir of compressed air, 

 the pressure being variable at pleasure. The hopper 

 ended in a long slit, from which the sand was blown. 

 A plate of glass was placed beneath this slit, and caused 

 to pass slowly under it; it came out perfectly depol- 

 ished, with a bright opalescent glimmer, such as could 

 only be produced by the most careful grinding. Every 

 little particle of sand urged against the glass, having 

 all its energy concentrated on the point of impact, 

 formed there a little pit, the depolished surface consist- 

 ing of innumerable hollows of this description. 



But this was not all. By protecting certain por- 

 tions of the surface, and exposing others, figures and 

 tracery of any required form could be etched upon the 

 glass. The figures of open iron-work could be thus 

 copied; while wire-gauze placed over the glass pro- 

 duced a reticulated pattern. But it required no such 

 resisting substance as iron to shelter the glass. The 

 patterns of the finest lace could be thus reproduced; 

 the delicate filaments of the lace itself offering a suf- 

 ficient protection. All these effects have been obtained 

 with a simple model of the sand-blast devised by my 

 assistant. A fraction of a minute suffices to etch upon 

 glass a rich and beautiful lace pattern. Any yielding 

 substance may be employed to protect the glass. By 

 diffusing the shock of the particle, such substances 

 practically destroy the local erosive power. The hand 

 can bear, without inconvenience, a sand-shower which 

 would pulverise glass. Etchings executed on glass with 

 suitable kinds of ink are accurately worked out by the 

 sand-blast. In fact, within certain limits, the harder 

 the surface, the greater is the concentration of the 

 shock, and the more effectual is the erosion. It is not 



