ETCHING GLASS. 225 



illustrated in the above experiments. Thus certain ores of zinc 

 are first roasted in order to convert the zinc compounds present 

 into oxide ; this oxide is then heated with carbonaceous matter 

 (small coal, coke dust, &c.), when the carbon unites with the 

 oxygen and sets free the zinc ; the heat employed is sufficient to 

 volatilise the zinc, so that the metal is obtained by carrying out 

 the process in a large distilling vessel or retort, in the cooler part of 

 which the zinc condenses much as the mercury does in Expt. 267. 

 The metal sodium is obtained somewhat similarly from the alkali 

 soda, a still higher temperature being employed to carry on the 

 action and distil the sodium produced. The metals aluminium and 

 magnesium cannot be easily extracted from their oxides by this kind 

 of treatment; but by heating their chlorides with sodium, a parallel 

 chemical change takes place, the sodium uniting with the chlorine 

 present and setting free metallic aluminium or magnesium. 



VARIOUS ACTIONS OF DOUBLE DECOMPOSITION. 



Expt. 270. To prepare concentrated Nitric Acid. Grind to 

 powder in a pestle and mortar an ounce of saltpetre (nitrate of 

 potassium) and place the powder in a glass stoppered retort. Now 

 pour in carefully through the stopper hole half a wineglassful of 

 oil of vitriol (sulphuric acid), and gently shake the retort so that 

 all the powder may be thoroughly wetted with the liquid, taking 

 care not to splash the mass into the upper portion of the retort. 

 The effect of this is that a chemical action takes place between the 

 sulphuric acid and the saltpetre, resulting in the formation of 

 liquid nitric acid, which is volatile and can be slowly distilled 

 by carefully heating the retort, receiving the strong acid which 

 condenses and drops down in a stoppered glass bottle (cork would 

 be quickly corroded by the acid). A nonvolatile substance called 

 acid sulphate of potassium is also formed and is left in the retort, 

 from which it can be dissolved out after cooling by pouring in 

 water and allowing to stand. 



Expt. 271. To etch Glass. Glass is a substance capable of resist- 

 ing the solvent and decomposing action of most kinds of corrosive 

 chemicals, so that they can be retained in glass bottles ; but one 

 substance is known which attacks and dissolves glass just as solu- 

 tion of hydrochloric acid would dissolve a zinc vessel (by the 

 evolution of hydrogen and formation of chloride of zinc as in 

 Expt. 10), or as nitric acid would similarly attack a copper vessel 

 (producing copper nitrate and evolving nitric oxide, as in Expt. 

 116). This substance is termed hydrofluoric acid, and is a gas 

 somewhat resembling hydrochloric acid gas, being also excessively 



P 



