504 FLUO-S1LICIC ACID. 



the condensation ceases; but if the receiver be shaken, the crust 

 will break and fall, and the condensation will go on again. 



(i.) If the gas be let up through water standing over mercury, the 

 silica is deposited in the form of vertical tubes. 



(j.) When moist substances are placed in an atmosphere of silica- 

 ted fluoric acid gas, they become encrusted with it, so as to resemble 

 petrifactions ; moistened sponge, frogs, lizards, &c. may be envelop- 

 ed in this manner, and covered with a siliceous coat. 



(k.) Silicated fluoric acid gas condenses ammoniacal gas ; 1 vol. 

 of the former to 2 of the latter ; and it would seem that the combina- 

 tion takes place in no other proportion ; the product is a dry white 

 acidulous salt, from which water precipitates silica, and if the solution 

 be boiled in glass vessels, they are corroded with energy. Henry. 



(I.) By combining the silicated fluoric acid gas with liquid ammo- 

 nia, a pure fluate of ammonia is obtained, while the silica is all pre- 

 cipitated. This fluate of ammonia may then be decomposed by sul- 

 phuric acid, and fluoric acid obtained free from silica. 



(m.) This gas unites with other bases, and forms compounds that 

 have been called, as Berzelius thinks improperly, fluo-silicates.* 



3. ETCHING UPON GLASS. 



(a.) In consequence of the energy with which fluoric acid acts 

 upon glass, it is necessary only to protect it where we would not choose 

 to have it corroded, and to expose it in those places where we would 

 wish an indelible trace. 



(b.) Sees wax^ forms a good protection, but one stilt better is made 

 of this substance and turpentine melted together and spread over the 

 warm glass, until an even coating is obtained ; a rim or border of the 

 same substance is made to surround the glass, and then the pure fluid 

 acid, diluted to such a degree that it does not smoke, may be poured on, 

 and the glass should be carefully turned till the whole is thoroughly 

 moistened. Two or three minutes are ordinarily sufficient to com- 

 plete the etching, and the same portion of acid will etch a number of 

 plates successively.^ 



(c.) Those who have not a proper distilling apparatus may effect 

 the same object, but much more tardily and imperfectly, by allowing 

 the vapor of the fluoric acid, as it rises from an open vessel of tin or 

 lead to strike the glass plate, but there is danger of corroding it on 

 the wrong side, unless that too is protected ; and also of melting and 

 disfiguring the varnish by the contact of the hot acid. 



* See his memoir, Ann. de Chim. et de Phys. Tom. XXVII, and Ann. of Philos. 

 N. S. quoted by Henry. 



t Isinglass is also mentioned by Mr. Murray, as a protection, but this I have never 

 tried. 



+ Am. Jour. Vol. VI, p. 355. I find this process easy and always successful; an 

 engraver prepares the plates, and the etching is done in the laboratory. 



