Experiments on freezing Mixtures , 251 
— i°|. The latter was twice re melted and expofed to the 
cold, and both times the temperature of the frozen acid came 
out the fame as before. 
ir. The white colour of the ice in thefe experiments Teems 
owing only to its confiding of very {lender filaments ; for in 
fome cafes, where it froze (lower, and where, in confequence, 
it (hot into larger foil'd maffes, they were tranfparent, and of 
the fame colour as the acid itfelf. By the continuance of a 
fufficient cold, the acid, which by hafty freezing put on the 
white appearance, would become hard folid ice, but yet dill 
retained its white appearance, owing perhaps to the filaments 
firft (hot confiding of an acid differing in ftrength from that 
which froze afterwards, and filled up the interfaces. 
In all thefe experiments, whether the ice was formed into 
minute filaments or folid maffes, dill, whenever there was a 
fufficient quantity of fluid matter to admit of it, they con- 
ftantly fubfided to the bottom ; a proof that the frozen part 
was heavier than the unfrozen. The difference indeed is fo 
great, that in one cafe where it froze into folid cryftals on the 
furface, thefe cryftals, when detached by agitation, fell with 
force enough to make a tinkling noife againft the bottom of 
the glafs. 
Thefe acids contract very much on freezing. Whenever the 
acid is frozen folid, the furface, inftead of being elevated 
in ridges, like frozen water, is dep refled and full of cracks. 
In one experiment Mr. M c Nab, after a glafs almoft full of 
acid was nearly frozen, filled it to the brim with frefh acid; 
and then, after.it was completely frozen, the furface was vifi- 
bly deprefied, with fiffures one-eighth of an inch broad, ex- 
tending from top to bottom. It is this contraction of the acid 
in freezing which makes the frozen part fubfide in the fluid 
K k 2 part ; 
