172 ESSAYS anp OBSERVATIONS 
AR Tas, Wikis 
Experiments upon Magnefia alba, Quicklime, 
and fome other Alcaline Subftances 3 by 
Joseru Brack, M. D.* 
PART I, 
Tt OFF MAN, in one of his obferva- 
tions, gives the hiftory of a powder 
called magnefia alla. which had been long 
ufed and efteemed as a mild and taftelefs 
purgative; but the method of preparing 
it was not generally known before he made 
it public fT. 
Ir was originally obtained from a li- 
quor called the mother of nitre; 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 ni~ 
trous 
* June 5.1755, 
+ Hoff. op. T. 4. p. 479- 
