HYDROFLUORIC ACID. 397 



but that acid mixture has no action upon the noble metals, such 

 as gold and platinum, which are dissolved by aqua regia. Se- 

 veral insoluble acid bodies, which are not acted on by sulphu- 

 ric, nitric or hydrochloric acid, are dissolved with facility by 

 hydrofluoric acid ; such as silica, titanic, tantalic, molybdic and 

 tungstic acids. Water is then formed from the oxygen of these 

 acids and the hydrogen of hydrofluoric acid, and fluorides of 

 silicon or of the metals of the acids enumerated are likewise pro- 

 duced ; which fluorides appear to combine with undecomposed 

 hydrofluoric acid, when water is present. This acid destroys 

 glass by acting upon its silica. If a drop of the concentrated 

 acid be allowed to fall upon a glass plate, it becomes hot, enters 

 into ebullition and volatilizes in a thick smoke, leaving the spot 

 with which it was in contact deeply corroded, and covered by a 

 white powder composed of the elements of the glass excepting a 

 portion of the silica, which has passed off as gaseous fluoride of 

 silicon. 



The diluted solution, or the vapour of hydrofluoric acid is 

 sometimes used to etch upon glass. The purity of the acid being 

 of little moment in this application of it, the sulphuric acid and 

 fluor spar may be mixed in a stone-ware evaporating bason. The 

 glass is warmed sufficiently to melt bees' wax rubbed upon it, 

 and thereby covered with a coating of that substance, which is 

 afterwards removed from the parts to be etched, by a pointed 

 rod of lead or tin, employed as a graver. A gentle heat being 

 applied to the bason, acid fumes are evolved to which the 

 etched surface of the glass is exposed for a minute or two, care 

 being taken not to melt the wax. The wax is afterwards re- 

 moved by warming the glass, and wiping it with tow and a little 

 oil of turpentine, when the exposed lines are found engraved to 

 a depth proportional to the time they have been exposed to the 

 acid fumes. But in taking impressions upon paper from glass 

 plates engraved in this way, as from a copper-plate, they are too 

 apt to be broken from the pressure applied in printing. 



To detect the minute quantity of hydrofluoric acid, which 

 exists in many minerals, Berzelius recommends that the sub- 

 stance to be examined be reduced to fine powder and mixed 

 with concentrated sulphuric acid, in a platinum crucible covered 

 by a small plate of glass, waxed and engraved as described. 

 The crucible is then exposed to a gentle heat, insufficient to 

 melt the wax, and in half an hour, the glass plate may be removed 



