358 Apparatus for preparing Fluoric Acid, &C' 



. common method of preparing it is by employing a leaden 

 retort into which is put a quantity of Huor spar in powder, 

 and an equal or a greater quantity of sulphuric acid: a 

 gentle heat is then applied, and a gas comes over, which 

 may be received in the usual manner into jars standing over 

 mercury. This gas is the fluoric acid. Or the gas may be 

 Blade to pass into a receiver containing water ; in which 

 case the acid is obtained dissolved in the water. 



Mr. Knight's apparatus consists of a leaden body A, 

 fig. 1. (Plate VIII.) furnished with a wide groove, aa, round 

 its mouth. The body A is fitted into the mouth of an- 

 other vessel B, into which is put water, the medium through 

 yhich the heat is applied to the contents of A. To the 

 body A is fitted a cover C, which rests on the outer edge 

 of the groove a a, and also enters into the groove^ which is 

 filled with water when the apparatus is to be used. 



If flnpr spar in powder be put into A, sulphuric acid be 

 poured over it, and heat be applied to the vessel B, the 

 water therein will transmit heat to the materials and tl«e 

 gas will ascend into the head C, where it will be absorbed 

 by the water in the groove a a, and form liquid fluoric acid. 

 If a piece of glass, covered with wax, or any proper etching- 

 ground on which a design has been traced with a sharp 

 point, be exposed y\\ the head C to the action of the acid 

 gas, the parts of the glass that have been laid bare will b« 

 corroded in the same way as copper is by aquafortis when 

 etched. In this case the fluoric acid always leaves the sur- 

 face it has acted upon dull and opake. When liquid fluoric 

 acid is employed instead of the gas, the bottoms of the lines 

 arc transparent. 



For etching on glass, however, with the gas, the follow- 

 ing modification of Mr. Knight's apparatus seems preferable 

 to the former : — The materials are to be put into the leaden 

 vessel A 5 fig- 2, which has a cover fitted into a groove 

 . (filled with water when made use of), and a pipe In the 

 coyer by which it is connected with another leaden vessel D 

 (furnished with a cover), into which the glass Intended to 

 be acted upon by the acid is put resting on a strip of lead 

 bent into any form that may not interfere with the design 

 on the glass. In this case, as in the former apparatus, t^e 

 vessel A is to be heated by w<'\ter in an exterior vessel. 



Instead of drawing with a sharp point through an etch- 

 ing ground when the gas is to be employed, another pro- 

 cess may be followed : — With a hair pencil apply a proper 

 varnish to those parts of the glass which are wished to be 

 defended from the action of the acid : in a few moments 

 the; 



