PHYSICAL AND LITERARY. 157 



Art. Vm. 



Experiments upon Magneiia alba, ^icklime, 

 and fome other Ale aline Subjidmes ^ by 

 Joseph Black, M, D.* 



Ho FFMA N, in one ~oi^ his 6bferya-r 

 tions, gives the hiftory of a powder 

 called magnejia alba-, whicli.iiad been long 

 'ufed and efteemed as, a mild and taftelefs 

 purgative 5 but the method of.preparing.it 

 w^as not generally known before he made it 

 .public -f-. 



It was originally obtained, from a liquor 

 called the mother of 7iitre^ which is produced 

 in the following manner : 



Salt-petre is feparated from the brine 

 which firft affords it, or from the water 

 with which it is wafhed out of nitrous earths, 

 by the procefs commonly ufed in cryflallizing 

 falts. In this procefs the brine is gradually 

 ^iminifhed, and at length reduced to a fmall 

 quantity of an undluous bitter faline liquor, 



affordincr 



• June 5. 1755. 



I HofF. op. T. iv. p. 479. 



