ALUM. 



ble in warm water, and the solution red- 

 dens vegetable colours, which proves the 

 acid to be in excess. 3. When mixed 

 with a solution of carbonate of potash, 

 an effervesence is produced by the un- 

 combined acid, which prevents the first 

 portions of alkali that are added to the 

 solution of sulphate of alumina from occa- 

 sioning any precipitate. 4. When sulphate 

 of alumina is heated, it swells up, loses its 

 regular form, and becomes a dry spongy 

 mass ; but, according to the experiments 

 of Vauquelin, the whole of its acid cannot 

 be thus expelled. 5. The combination of 

 sulphuric acid with alumina is incapable 

 of crystallizing without an addition of 

 potash, which makes a constituent part of 

 all the alum of commerce. 6. It is de- 

 composed by charcoal, which combines 

 with the oxygen of the acid, and leaves 

 the sulphur attached t o the alumina. 



Dr. Thompson says there are four va- 

 rieties of alum, all of which are triple 

 salts ; two neutral, and two he calls su- 

 per-salts. These are thus denominated : 



1. Sulphate of alumina and potash. 



2. -Sulphate of alumina and ammonia. 



3. Super-sulphate of alumina and potash. 



4. Super-sulphate of alumina and am- 

 monia. 



The discovery of alum was made in 

 Asia, from whence it continued to be im- 

 ported till the end of the fifteenth centu- 

 ry, when a number of alum works were 

 established in Italy. In the sixteenth 

 century it was manufactured in Germany 

 and Spain; and during Queen Elizabeth's 

 reign, an alum manufactory was establish- 

 ed in England. The alum of commerce 

 is usually obtained from native mixtures 

 of pyrites and clay, or sulphuric acid and 

 clay." Bergman has published a very com- 

 plete dissertation on the process usually 

 followed. The earth from which it is pro- 

 cured is usually called aluminous schistus, 

 because it is slaty. Its colour is blackish, 

 because it contains some bitumen. In 

 most cases it is necessary to burn it be- 

 fore it can be employed : this is done by 

 means of a slow smothered fire. Some- 

 times long exposure to the weather is 

 sufficient to produce an efflorescence of 

 alum on the surface. It is then lixiviated, 

 and the water concentrated by evapora- 

 tion, and mixed with putrid urine, or mu- 

 riate of potash ; crystals of alum and of 

 sulphate of iron usually farm together. 

 The composition of alum has been but 

 lately understood with accuracy. It has 

 been long known, indeed, that one of its 

 ingredients is sulphuric acid ; and the ex- 

 periments of Pott and Margraft' proved in- 



contestibly that alumina is another ingre- 

 dient. Bui sulphuric acid and alumina 

 are incapable of forming alum. Manu- 

 facturers knew that the addition of a quan- 

 tity of potash or of ammonia, or of some 

 substance containing these alkalies, is al- 

 most always necessary, and it was proved, 

 that in every case in which such additions 

 are unnecessary, the earth from which the 

 alum is obtained contains already a quan- 

 tity of potash. Various conjectures were 

 made about the part which potash acts 

 in this case ; but Vauquelin and Chaptal 

 appear to have been the first chemists 

 that ascertained, by decisive experiments, 

 that alum is a triple salt, composed of sul- 

 phuric acid, alumina, and potash or ammo- 

 nia united together. Alum crystallizes in 

 regular octahedrons, consisting of two 

 four-sided pyramids applied base to base. 

 The sides are equilateral triangles. The 

 form of its integrant particles, according 

 to Hauy, is the regular tetrahedron. Its 

 taste is, as we have observed, astringent. 

 It always reddens vegetable blues. Its 

 specific gravity is 1.7109. At the tempe- 

 rature of 60 "it is soluble in from 15 to 

 20 parts of water, and in f ths of its weight 

 of boilling Water. When exposed to the 

 air it effloresces slightly. When exposed 

 to a gentle heat it undergoes the watery 

 fusion. A strong heat causes it to swell 

 and foam, and to lose about 44 per cent 

 of its weight, consisting chiefly of water 

 of crystallization. What remains is called 

 calcined or burnt alum, and is some- 

 times used as a corrosive. By a violent 

 heat the greater part of the acid may be 

 driven off. Though the properties of 

 alum are in all cases pretty nearly the 

 same, it has been demonstrated by Vau- 

 quelin that three varieties of it occur in 

 commerce. The first is super-sulphate of 

 alumina and potash ; the second, super- 

 sulphate of alumina and ammonia; the 

 third is a mixture or combination of these 

 two, and contains both potash and ammo- 

 nia. It is the most common of all ; doubt- 

 less, because the alum-makers use both 

 urine and the muriate of potash, to crys- 

 tallize their alum. Vauquelin has lately 

 analysed a number of specimens of alum 

 manufactured in different countries. The 

 result was, that they all contain very near- 

 ly the same proportion of ingredients. 

 The mean of all his trials was as follows : 



Acid 30.52 



Alumina . . . . 10.50 

 Potash . . . . 10.40 

 Water .... 48.58 



100.00 



