VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 277 



by Mr. M*Nab, the greatest cold which he had produced by mixing acids with 

 snow, was eftected by a vitriolic acid which had previously congealed ; and to this 

 circumstance of the congelation of the acid, Mr. Cavendish justly imputes the 

 intensity of the cold, as the liquefaction of both the frozen acid and the snow had 

 concurred towards this effect; whereas, in mixing fluid acids with snow, the 

 thawing of the snow is probably the sole productive cause. 



I was desirous of comparing the times required for the liquefaction of ice and 

 of congealed acid, when both were exposed to the same temperature. For this 

 purpose I filled 2 equal and similar cylindrical glasses ; one with the congelable 

 vitriolic acid, and the other with water ; and, after having immersed them in a 

 freezing mixture till both fluids were frozen, and reduced to the temperature of 

 28°, I withdrew the glasses from the freezing mixture, wiped them dry, and 

 placed them together in a room, where the thermometer stood at 62°. In 40 

 minutes the ice was thawed, and in 93 minutes the acid was liquefied, at the end 

 of which time the thermometer, which stood near the glasses, had risen to 64°. 

 It appears then, that the congealed acid requires more than twice the time for its 

 liquefaction, when exposed to that temperature, that ice does ; but I do not 

 think that we can infer, that the heats absorbed and rendered latent, as some 

 late philosophers express themselves ; or, in other words, that the cold generated 

 by the liquefaction of ice and of congealed acid, are in the above proportion of 

 the times, from the following consideration ; that, as during the liquefaction of 

 the ice, its temperature remains stationary at 32°, and during the liquefaction of 

 the acid, its temperature remains about 44° or 45°, the ice, being considerably 

 colder than the acid, will take the heat from the contiguous air much faster. 



The experiment does however show, that a considerable quantity of cold is 

 generated by the liquefaction of this acid ; and hence it appears probable, that in 

 making experiments of producing cold artificially, by mixing snow with acids in 

 very cold temperatures, it would probably be useful to employ a vitriolic acid of 

 the proper density for congelation, and to freeze it previously to its mixture with 

 snow. It must not however be imagined, that the cold generated by the mixture 

 of these 2 frozen substances, is nearly equal to the sums of the colds generated 

 by the separate liquefactions of the congealed acid and ice, when singly exposed to 

 a thawing temperature : for the mixture resulting from the liquefaction, consist- 

 ing of the vitriolic acid and the water of the snow, appears, from the generation 

 of heat which occurs in the mixture of these ingredients in a fluid state, to be 

 subject to different laws relatively to heat, than either of the ingredients separately. 

 And the heat thus generated, as soon as the congealed acid and ice are brought to 

 a fluid state, must counteract in some measure, the cold produced by the lique- 

 faction. 



The vitriolic acid, like water and other fluids, is capable of retaining its fluidity 



