108 ALUM 



by precipitating a solution of alum with sugar of load. Paper-makers use alum in 

 their size, and bookbinders in their paste. It is used in tanning leather, and some- 

 times, both in Asia and Europe, it is used for precipitating rapidly the impurities of 

 water. This is a dangerous process, unless there be a great amount of alkaline salts, 

 such as carbonate of lime or soda, to neutralise the acid. It is extensively used in 

 correcting the baking qualities of bad flour, for -which the experience of many has 

 decided that it is a valuable remedy ; unfortunately, it is also used to make excellent 

 flour whiter, when there is no need of its presence. Liebig says that lime is equally 

 good, and of course much safer. It is also used in the adulteration of beer. From 

 time immemorial it has been used to prevent the combustibility of wood and cloth. 

 Milnor's fire-proof safes are said to be lined with a mixture of alum and sulphate of lime. 



Alum heated with charcoal or carbonaceous substances forms Homberg's phos- 

 phorus, which inflames spontaneously. It is composed of alumina, sulphide of 

 potassium, and charcoal. 



Burnt Alum, or dried alum, is made by gently heating alum till the water is driven 

 off. The alum first melts in its water of crystallisation, and is then dried. It has a 

 stronger action than the hydrated crystals, and is a mild escharotic. It roabsorbs water. 



Neutral Alum is a name sometimes given erroneously to alum which has had some 

 of its acid neutralised by an alkali. It is in fact a basic salt of alumina, which may 

 also be made by dissolving alumina in ordinary alum. It deposits a basic salt more 

 readily than ordinary alum, and may be of service in some cases of printing. 

 Properly speaking, the common alum is the neutral salt. 



Testing of Alum. Alum being generally in large crystals, any impurity is more- 

 readily seen ; this is said to be the reason for keeping up the practice of making this 

 substance instead of the sulphate of alumina alone, which is less bulky and fitted for 

 nearly every purpose for which alum is used. But probably the ancient accidental 

 discovery of the potash form has determined its use to the present day. Iron is readily 

 found in it, by adding to a dilute solution ferrocyanide of potassjum or yellow prussiato 

 of potash, which throws down Prussian-blue. A very delicate test is sulphide of ammo- 

 nium, which throws down both the alumina and iron, but the blacking of the precipitate 

 depends on the amount of iron. The total amount of iron is got by adding pure 

 caustic potash or soda till the solution is strongly alkaline, washing and filtering off 

 the oxide. To look for lime, precipitate the alumina and iron by ammonia, boil 

 and filter the lime and magnesia are in the solution add oxalate of ammonia ; 

 add tartaric acid to keep up the iron and alumina, make alkaline by ammonia, then 

 precipitate the lime by oxalate of ammonia, filter, and precipitate the magnesia by a 

 phosphate. Silica and insoluble basic sulphates are obtained by simply dissolving the 

 alum in water and filtering. If silica, it is insoluble in acids ; if a basic sulphate, it 

 will dissolve in sulphuric acid, and the addition of sulphate of potash or ammonia will 

 convert it into potash- or ammonia-alum. 



Pure alum gives a white precipitate with ammonia, no precipitate with sulphuretted 

 hydrogen gas, and no precipitate with oxalate of ammonia and ammonia, if tartaric 

 acid be previously added. 



The addition of ammonia to a solution of alum, or the addition of any other alkali, 

 in insufficient quantity, causes a precipitate, not of pure alumina, as one might 

 suppose, but of a subsulphate of alumina. Even an excess of alkali will not remove 

 all the sulphuric acid without heat being applied ; an excess, on the other hand, is 

 apt to dissolve some of the alumina, especially if few salts are present, and the 

 solution not much boiled. Sulphide of ammonium precipitates it thoroughly. 



In a saturated solution of tersulphate of alumina, the crystals of alum are almost 

 insoluble. 



If we dissolve alum in 20 parts of water, and drop this solution slowly into water of 

 caustic ammonia till this be nearly, but not entirely, saturated, a bulky white preci- 

 pitate will fall down, which, when properly washed with water, is pure aluminous 

 earth or hydrate of alumina ; and, dried, forms 10'94 per cent, of the weight of the 

 alum. If this earth, while still moist, bo dissolved in dilute sulphuric acid, it will 

 constitute, when as neutral as possible, simple sulphate of alumina, which requires 

 only two parts of cold water for its solution. If wo now decompose this solution, by 

 pouring into it water of ammonia, there appears an insoluble white powder, which is 

 subsulphato of alumina, or basic alum, and contains three times as much earth as 

 exists in the neutral sulphate. If, however, we pour into the solution of the neutral 

 sulphate of alumina a solution of sulphate of potash, a white powder will fall if the 

 solutions bo concentrated, which is true alum if the solutions be dilute, by evapora- 

 ting their mixture, and cooling it, crystals of alum will be obtained. 



When newly precipitated alumina is boiled in a solution of alum, a portion of the 

 earth enters into combination with the salt, constituting an insoluble compound which 

 falls in the form of a white powder. The same combination takes place, if we decbm-- 



