the Congelation of Vitriolic Acid, 277 
difference arifes principally from the bubbles of elafbic fluid, 
which may be in greater quantity, and may add more to the 
bulk of the water than of the acid. 
Greater cold is produced by mixing fnow or pounded ice with 
the congealed than with the fluid acid, but the quantity I 
have not determined. There is reafon to believe it may be 
confiderable. In the experiments made at Hudfon’s Bay, by 
Mr. M c Nab, the greatefl: cold which he had produced by 
mixing acids with fnow, was effected by a vitriolic acid which 
had previoufly congealed; and to this circumftance of the 
congelation of the acid, Mr. Cavendish juflly imputes the 
intenfity of the cold, as the liquefaction of both the frozen 
acid and the fnow had concurred towards this efteCt ; whereas, 
in mixing fluid acids with fnow, the thawing of the fnow is 
probably the foie productive caufe. 
I was defirous of comparing the times required for the lique- 
faction of ice and of congealed acid, when both were expofed 
to the fame temperature. For this purpofe I filled two equal 
and fimilar cylindrical glafies; one with the congelable vitriolic 
acid, and the other with water ; and, after having immerfed 
them in a freezing mixture till both fluids were frozen, and 
reduced to the temperature of 28% I withdrew the glafles from 
the freezing mixture, wiped them dry, and placed them toge- 
ther in a room, where the thermometer flood at 62°. In 40 
minutes the ice w T as thawed, and in 95 minutes the acid was 
liquefied, at the end of which time the thermometer, which 
flood near the glafles, had rifen to 64°. It appears then, that 
the congealed acid requires more than twice the time for its 
liquefaClion, when expofed to that temperature, that ice does ; 
but I do not think that we can infer, that the heats abforbed 
and rendered latent , as fome late philofophers exprefs them- 
Vol. LXXVII. Sf felves ; 
