1 94 Continuation of the Experiments and Qhfer nations 
OF EARTH OF ALLUM OR ARGILLACEOUS EARTH. 
This earth I found to contain about 26 per cent, of fixed air, 
though 1 had previoufly kept it red-hot for half an hour : this 
furprifed me much, as mold writers fay it contains fcarce any. 
It dftlolved in acids with a moderate effervefcence until the heat 
was railed to 220°, after which I found the folution lighter 
than the quantities employed in the proportion 1 mentioned. 
100 gr. of this earth (exclusive of the fixed air) require 133 
of the mere vitriolic acid to didolve them. This folution I 
made in a very dilute l'pirit of vitriol, whofe fpecific gravity 
was 1,093, which the proportion of acid to that of water 
was nearly as 1 tom 4. This folution contained a flight excefs 
of acid, turning vegetable blues into a brown ilh red ; but it 
cryflallized when cold, and the cryftals were of the form of 
allum ; fo that I believe this to be nearly the proper proportion- 
of its acid and eartli ; but there was not water enough to form 
large cryftals. As this folution contained an excefs of acid, I 
added more earth to it, but could not prevent its tinging blue 
paper red, until it formed an infoluble fait, that is, one that re- 
quired an exceeding large quantity of w'ater to dilfolve it,, and' 
while part was thus become infoluble, yet another part would 
ftill retain an excefs of acid ; fo that at the fame time part would, 
be fuperfaturated with earth, and another with acid, if tinging- 
vegetable blues be a mark of an excefs of acidity, which in- 
deed in this- cafe feems dubious. 
ico gr. of Allum, perfectly dried, contain 42,74 of acid*. 
32,14 of earth-, and 25,02 of water; but cryftallized allum 
lofes 44 per cent by defecation ; therefore 100 gr. of it contain' 
23,94 acid, 18 of earth,, and 58,06 of. water. 
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