ALUM 



107 



One part of crystallised potash alum is soluble 



At 54 degrees Fahrenheit in 13 -3 parts of water. 

 70 8-2 



77 

 100 

 122 

 145 

 167 

 189-5 



4-5 

 2-2 

 2-0 

 0-4 

 0-1 

 0-06 



A solution saturated at 46 is 1-045 specific gravity. This difference in the rate 

 of solubility in hot and cold water renders it easily separated from many other salts. 

 The crystals are permanent in the air, or nearly so, unless the air bo very dry ; if 

 kept at 180 they lose 18 atoms of water, but alum deprived of its water, and exposed 

 to the air of summer, took up 18 atoms in 47 days. It melts at a low temperature in 

 its water of crystallisation. At 356 it loses 43-5 per cent, of water, or 23 atoms ; the 

 last atom is only lost when approaching rod heat. At a red heat the sulphate of 

 alumina loses its acid, and the alumina seems then able to remove some acid from the 

 potash, losing it again by heat. Alum, when heated with common salt, acts like 

 sulphuric acid, and gives off muriatic acid ; the same with chlorides of potassium and 

 ammonium. If boiled with a saturated solution of chloride of potassium, hydrochloric 

 acid is formed and a subsulphate of alumina falls down ; this occurs only to a small 

 extent with chloride of sodium, and still less with sal-ammoniac. 



Ammonia-Alum, which is now very extensively prepared, contains : 



Ammonia 3-75 per cent. 



Alumina . i . . . . 11-34 



Sulphuric acid 35-29 



Water , . 49-62 



100-00 



This salt also occurs in octahedrons, and can only be known from potash-alum by 

 trial. The addition of caustic lime, soda, or potash gives out the ammonia, easily 

 distinguished by the smell. Ammonia-alum readily loses all its ammonia when 

 heated, and the sulphuric acid may be driven off from the remaining sulphate of 

 alumina, so that the pure earth, alumina, will remain. 



The greater proportion of the alum at present used in this country is ammonia-alum 

 an abundant and convenient source of ammonia being furnished by the ammoniacal 

 liquor obtained in the manufacture of coal-gas. In commerce ammonia and potash- 

 alums are sometimes found mixed. 



Soda-alum is not an article of commerce, nor is it used in the arts. Nevertheless, 

 the great commercial value of compounds of potash or of ammonia renders it ob- 

 viously desirable to replace them, if possible, by the cheaper compounds of soda. Tho 

 cost of sulphate of soda, for example, is trifling compared with that of sulphate of 

 potash or of ammonia the latter salts being especially in demand by the agriculturist 

 as fertilising agents. Some experiments on the formation and crystallisation of soda- 

 alum were undertaken a few years ago by Mr. J. Carter Bell. Up to the present 

 time, however, there appear to have been groat difficulties in the manufacture of 

 this kind of alum, especially in respect to its crystallisation, but these difficulties 

 may not be altogether insuperable. Mr. J. Berger Spence, who has studied the 

 preparation of soda-alum, remarks that ' it may ultimately, now that the practica- 

 bility of producing soda-alum on the commercial scale has been demonstrated, even 

 with all the difficulty of crystallisation, be a more economical way of producing this 

 double salt.' ' 



For the composition of potash-, soda- and ammonia-alums found ready formed in 

 nature, see ATOM, NATIVE. 



Applications of Alum. Alum is an astringent. Its immediate effect on man is to 

 corrugate the fibres and contract the small vessels. It precipitates albuminous liquids 

 and combines with gelatine. It causes dryness of the mouth and throat, and checks 

 the secretions of the alimentary canal, producing constipation in largo quantities, 

 nausea, vomiting, and purging. It is given in lead colic, to convert the lead into 

 sulphate of lead, and is used externally. Its principal use is in dyeing ; calico-printers 

 print it as a mordant ; the cloth is then put into the dye, and the printed parts 

 absorb the colour. 



It is largely employed by the calico-printer in the preparation of acetate of alumina 



1 On the Phenomena of the Crystallisation of a Double Salt, by J. Berger Spcncc, F.C.S. ' Chem- 

 ical News, 1 vol. jodi. 1870, p. 181. 



