Ee] 
free alkali, or at leaft of an alkali combined only with fixed air or 
fulphur, to whofe quantity it is always proportional. It is true 
quick lime will alfo decompofe allum, but the prefence of quick 
lime is eafily difcovered by the addition of a few drops of any 
mild alkaline folution, and by the fame means as eafily fe-. 
parated. 
_ gdly, That if the earth of allum takes up fixed air, (which would 
increafe its weight) this air will be feparated by the heat em- 
ployed in drying it, or at leaft by the fpirit of falt poured upon 
it, and fo may alfo another heterogeneity which will hereafter be 
mentioned. 
I can fee but one inaccuracy attending this teft, and that of 
little moment ; it.is this, if the alkali contains fulphur, this will 
alfo be precipitated with the earth of allum and _ increafe its 
weight. The limits of this inaccuracy, at leaft in common cafes, 
fcarcely reach 2 or 3 grains, as we fhall prefently find. 
SuLPuourR is eafily deteed in any alkaline folution by faturating 
it-with an acid; hepatic air is generally developed, and the liquor 
becomes troubled. 
Not only the proportion but alfo the ab/olute weight of alkali 
in different alkaline fubftances or afhes may be found by this teft, 
as will appear by the following experiments: 
BECTION 
