the Congelation of Vitriolic Acid. 275 
rendered the acid incapable of continuing frozen without a 
great increafe of cold. 
It appears then, that the concentration of M. de Mor- 
veau’s acid, at the time of its congelation, from which cir- 
cumdance Mr. Cavendish infers generally, that the vitriolic 
acid freezes more ealily as it is more denfe, is not a true pre- 
mile ; and that therefore the inference, though judly deduced, 
is invalid. On the contrary, there feems every real'011 to be- 
lieve, from the analogy of my experiments above mentioned, 
that as the denfity of the acid increafes beyond the point of 
ealieft freezing, the facility of the congelation diminifhes ; at 
lead, to as great denlity as we have been ever able to obtain 
the vitriolic acid ; for if it were pollible to dived: it intirely of 
water, it would probably affume a folid date in any tempera- 
ture of the air. 
The crydallization of the frozen vitriolic acid is more or lefs 
didiind, according to the downefs of its formation, and other 
favourable circumdances. Sometimes the crydals are very 
didinTly (haped, large, and very hard. Their form is the 
fame as the common form of mineral alkali and of felenitic 
fpar, but with angles different in dimenfions from either of 
thefe. They are folids confiding of ten faces, of which the 
two larged are equal, parallel, and oppolite to each other, and 
are oblique-angled parallelograms or rhomboids, whole angles 
are, as near as I could meafure them, of 105 and 75 degrees. 
Between thefe two rhomboidal faces are placed eight faces of 
the form of trapeziums. Thus each crydal may be fuppofed 
to be compofed of two equal and limilar frudums of pyramids 
joined together by their rhomboidal bales. I obferved, that the 
crydals always funk in the fluid acid to the bottom of the vedel, 
which (hewed that their denfity was increafed by congelation. 
I thought 
