SALTS OF ALUMINA. 517 



sulphate of alumina is formed. Alum-stone appears to be con- 

 tinually produced at the Solfatara near Naples, and other vol- 

 canic districts, by the joint action of sulphurous acid and oxygen 

 upon trachyte, a volcanic rock composed almost entirely of fel- 

 spar. 



Alum requires 18.4 parts of cold and only 0.75 parts of boil- 

 ing water to dissolve it, arid crystallizes very readily in regular 

 octohedrons, of which the apices are always more or less trun- 

 cated, from the appearance of faces of the cube. The taste of 

 alum is sweet and astringent, and its action decidedly acid, and 

 it dissolves metals, with evolution of hydrogen, as readily as 

 free sulphuric acid. The crystals effloresce slightly in air, and 

 when heated melt in their water of crystallization, which 

 amounts to 45.5 per cent of their weight or 24 atoms. The 

 fused salt in losing this water, becomes viscid, froths greatly, 

 and forms a light porous mass known as burnt alum. 



A pyrophorus is formed from an intimate mixture of 3 parts 

 of alum and 1 of sugar, which are first evaporated to dryness 

 together, and then introduced into a small stoneware bottle, 

 and this placed in a crucible and surrounded with sand. The 

 whole is heated to redness till a blue flame appears at the mouth 

 of the bottle, which is allowed to burn for a few minutes, and 

 the mouth then closed by a stopper of chalk. After cooling, 

 the bottle is found to contain a black powder, which becomes 

 red hot when exposed to air, and catches fire also and burns 

 with peculiar vivacity in oxygen gas. This property appears 

 to depend upon the highly divided state of sulphuret of potas- 

 sium, which is intermixt with charcoal and sulphate of alumina. 

 A pyrophorus can be produced from the sulphate of potash 

 alone, without the sulphate of alumina, but it does not so cer- 

 tainly succeed. 



If the quantity of carbonate of soda, necessary to neutralise 

 a portion of alum, be divided into three equal portions, and 

 added in a gradual manner to the aluminous solution, it will be 

 found that the alumina at first precipitated, is redissolved upon 

 stirring, and that no permanent precipitate is produced till nearly 

 two parts of alkaline carbonate are added. It is in the condition 

 of this partially neutralised solution that alum is generally ap- 

 plied as a mordant to cloth. Animal charcoal readily withdraws 

 the excess of alumina from this solution, and so does vegetable 



