268 Mr. Cavendish’s Account of feme 
M. de'Morveau’s acids, which, as they were concentrated on 
purpofe, were moil: likely ftronger than .Mr. M c Nab’s, froze 
with a cold lefs than zero of Fahrenheit ; whereas the 
freezing point of Mr. M c Nab’s undiluted acid, whofe ftrength 
was ,98, was — I5°,and that of the diluted acid, whofe ftrength 
was ,629, was — 36°; and when the acid was more diluted, it 
was found to bear a much greater cold without freezing. It 
appears alfo, both from Art. 21. and from M. de Morveau’s 
experiment, that during the ^congelation of the oil of vitriol, 
fome feparation of its parts takes place, fo that the congealed 
part differs in fome refpeft from the reft, in confequence of 
which it freezes with a lefs degree of cold ; and as there is 
reafon to think from Art. 21. thatthefe two parts do not differ 
much in ftrength, it feems as if the difference between them 
depended on fome lefs obvious quality, and probably on that, 
whatever it is, which forms the difference between glacial 
and common oil of vitriol. The oil of vitriol prepared from 
green vitriol, has fometimes been obtained in fuch a ftate as to 
remain conftantly congealed, except when expofed to a heat 
confiderably greater than that of the atmofphere, whence it 
acquired its name of glacial *. It is not known indeed upon 
what this property depends, but it is certainly fomething elfe 
than its ftrength ; for oil of vitriol of this kind is always 
fmoking, and the fumes it emits are particularly oppreffive 
and fuffocating, though very different from thofe of the vola- 
tile fulphureous acid. On redtification likewife it yields, with 
the gentled: heat, a peculiar concrete fubftance, in the form of 
feline cryftals ; and after this volatile part has been driven off, 
* Mem. de i’Academ, des Sc. 1738, p. 288® 
the 
