336 SALTS SULPHATES. 



(p.) But they become cubical, by letting a solution of common alum 

 stand for some time upon either alumina or potassa ; still, with a great 

 excess of potassa, alum does not crystallize. 



(q.) Saturate alum, with alumina, by boiling a solution of common 

 alum upon it ; it becomes a tasteless insoluble powder. 



(r.) Digest natural clays in sulphuric acid; they dissolve only 

 partially, and scarcely saturate the acid ; dissolve newly prepared 

 alumina (added in excess) in sulphuric acid, and a neutral sulphate 

 is formed, which crystallizes in thin flakes, and becomes alum by 

 adding potassa or its sulphate. 



2. COMPOSITION AND VARIETIES. 



(a.) The most common variety of alum is that which contains po- 

 tassa, but there has been considerable diversity in the statements 

 made of its constitution : the following is the average of six analyses.* 

 Sulphuric acid, 33.22 5 aluminous earth, 1 1 .07 ; potassa, 9.88 ; water, 

 45.92 = 100. 



(b.) According to Mr. R. Phillips, alum consists of 1 proportion of 

 bi-sulphate of potassa, 128; 2 of sulphate of alumina, (67 X2) =134; 

 25 of water, (9x25) =225=487,f its equivalent number. 



Dr. Thomson supposes alum to be composed of 1 proportion of 

 sulphate of potassa, 88; 3 of sulphate of alumina,{ (58x3) 174; 

 25 of water, 225=487. 



The difference between these two views is, that in the former the 

 equivalent of alumina is taken at 27, and in the latter at 18; and 

 adding 40 in each case for the sulphuric acid, we have 67 and 58 

 for the equivalent of sulphate of alumina, of which 2 proportions are 

 taken in Mr. Phillips' statement, and 2 in that of Dr. Thomson. 



(c.) Alum with basis of ammonia,^ consists of 1 proportion of sul- 

 phate of ammonia, 57 ; 3 of sulphate of alumina, 58 X 3 = 174 ; 24 of 

 water, 9 X24=216 ; and of the acid, 26.979 are united to 11.906 of 

 the earth, and 9.063 are united to 3.898 of ammonia. 



(d.) Jllum with basis ofsoda.\\ Its composition is stated as being 

 water, 51.21; acid, 32.14; earth, 10.; soda, 6.32: or, 2 propor- 

 tions of sulphate of alumina, 1 of bi-sulphate of soda, 28 of water. 

 A native soda alum is found in the isle of Milo, Greece, and in 

 South Am erica. TT 



* As given by Dr. Henry, Vol. I, p. 632, 10th ed. 



t In this statement, the experimental results are slightly changed, to accommo- 

 date them to definite proportions, and the equivalent of alumina is taken at 27. 



t The equivalent of alumina being taken at 18. The chemical equivalent of alu- 

 mina is not yet ascertained with certainty, but Mr. Murray remarks, (II. 182,) that 

 from the analysis of salts and minerals containing alumina, it is more probable that 

 18 is the true number. 



According to Riffault, Ann. de Chim. et de Phys. IX, 106. 



II Quarterly Jour. VIII- 386, and XIII. 276. I have prepared a lithia alum, in 

 large quantities, from the Sterling spodumene, in following Berzelius' process for 

 extracting lithia. It is deliquescent, hut in other respects resembles the potassa 

 alum, J. T. IT Am. Jour. Vol. XVI. p. 203. 



