GILDING. 



271 



If such a furnace is made of strong forged (not 

 milled) iron plate, it will be sufficiently durable. 

 The upper end of the chimney may reach above a 

 foot and a half higher than the level of the fire ; 

 over this is to be placed a larger tube, leaving an 

 interval of an inch, or more, all round between it 

 and the chimney, and reaching to the height of ten 

 or twelve feet ; the higher the better. The exter- 

 nal air, passing up between the chimney and the 

 outer pipe, prevents the latter from being much 

 heated, so that the mercurial fumes will condense 

 against its sides into running quicksilver, which 

 falling down to the bottom, is there catched in a 

 hollow rim, formed by turning inwards a portion 

 of the lower part, and conveyed by a pipe at one 

 side into a proper receiver. 



Gilding Iron or Steel. In gilding iron or steel 

 by means of an amalgam, as the metal has no 

 affinity for the mercury, an agent must be em- 

 ployed to dispose the surface to receive the gilding. 

 For this purpose, a solution of mercury in nitrous 

 acid (aqua fortis,) or what the workmen call quick- 

 silver water, is applied to the parts intended to 

 be gilded ; the acid, by a stronger affinity, seizes 

 on a portion of the iron, and deposits in the place 

 of it a thin coating of mercury, which will not 

 refuse a union afterwards with the gold amalgam 

 that may be applied ; but, by this process, the 

 surface of the metal is injured by the nitrous acid, 

 and the union of the mercury is very slight, so 

 that a bright and durable gilding cannot be 

 obtained. 



Another method. Sometimes a solution of blue 

 vitriolis applied, with a camel's hair pencil, to the 

 parts of the steel intended to be gilt. By a che- 

 mical action, exactly similar to what we have 



