VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. QQ 



some cases, where it froze slower, and where, in consequence, it shot into 

 larger solid masses, they were transparent, and of the same colour as the acid 

 itself. By the continuance of a sufficient cold, the acid, which by hasty freez- 

 ing put on the white appearance, would become hard solid ice, but still retained 

 its white appearance, owing perhaps to the filaments first shot, consisting of an 

 acid differing in strength from that which froze afterwards, and filled up the 

 interstices. In all these experiments, whether the ice was formed into minute 

 filaments or solid masses, still, whenever there was a sufficient quantity of 

 fluid matter to admit of it, they constantly subsided to the bottom ; a proof 

 that the frozen part was heavier than the unfrozen. The difference indeed 

 is so great, that in one case where it froze into solid crystals on the surface, 

 these crystals, when detached by agitation, fell with force enough to make 

 a tinkling noise against the bottom of the glass. 



These acids contract very much on freezing. Whenever the acid is frozen 

 solid, the surface, instead of being elevated in ridges, like frozen water, is de- 

 pressed and full of cracks. In one experiment Mr. M'Nab, after a glass almost 

 full of acid was nearly frozen, filled it to the brim with fresh acid ; and then, 

 after it was completely frozen, the surface was visibly depressed, with fissures -f 

 of an inch broad, extending from top to bottom. It is this contraction of the 

 acid in freezing which makes the frozen part subside in the fluid part ; as it was 

 found, in the undiluted acid, that the latter consisted of a stronger, and con- 

 sequently heavier acid than the former. But still the subsidence of the frozen 

 part shows that the ice is not mere water, or even a very dilute acid; which 

 indeed was proved by the examination of the liquors sent home. The experi- 

 ments show, that though the acids bear being cooled greatly below the freezing 

 point, without any congelation taking place, yet as soon as they begin to freeze 

 they immediately rise up to their freezing point ; and this point is always very 

 nearly, if not exactly, the same in the same acid ; for those acids were frozen 

 and melted again 3 or 4 times, and were cooled considerably more below the 

 freezing point in one trial than another, and yet as soon as they began to freeze 

 the thermometer immersed in them constantly rose nearly to the same point. 



The quantity which these acids will bear being cooled below the freezing point, 

 without freezing, is remarkable. The diluted spirit of nitre, whose freezing 

 point is — 1%, once bore being cooled to near — 39°, without freezing, that is, 

 near 37 degrees below its freezing point. The diluted dephlogisticated spirit of 

 nitre, whose freezing point is —• 5°, bore cooling to — 35° ; and the dephlogis- 

 ticated spirit of nitre (141) whose true freezing point is most likely — 1Q°, bore 

 being cooled to — 49°: perhaps too they might have borne to be cooled consider- 

 ably lower without freezing, but how much does not appear. It must be ob- 



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