VOL. LXXVI.] PHILOSOfHICAL TRANSACTIONS. 103 



the proportion that might naturally be expected, and consequently serves in 

 some measure to confirm the foregoing reasoning. 



After Mr. M*Nab had diluted these acids as above-mentioned, he divided each 

 of them into 2 parts, and tried what degree of cold could be produced by mixing 

 them with snow. On January 15th, one of these parts of the common spirit of 

 nitre was tried. It was fluid when the experiment began, though its tempera- 

 ture, as well as that of the snow, was — 21°4-; but on adding snow it immedi- 

 ately began to freeze, and grew thick, and its heat increased to — 2°-i- ; but by 

 the addition of more snow it quickly sunk again, and at last got to — 43°i. 

 During the addition of the snow, the mixture grew thinner, and by the lime it 

 arrived at nearly the greatest degree of cold, consisted visibly of 3 parts: the 

 lowest part, which consisted of frozen acid, was white and felt gritty ; the upper 

 part, which occupied about an equal space, was also white, but felt soft, and 

 must have consisted of unmelted snow ; the other part, which occupied by much 

 the smallest space, was clear and fluid. The quantity of snow added was about 

 -^^ of the weight of the acid, and consequently its strength was reduced 

 to .243. 



Though snow was added to the acid in this experiment as long as, and even 

 longer than, it produced any increase of cold, yet some days after, on adding 

 more snow to the mixture, while it was fluid, and of the temperature of — 40°4, 

 the cold was increased to — 44°4, or 1 degree lower than before. Mr. M'Nab 

 did not perceive the snow to melt, though in all probability some must have done 

 so, or no cold would have been produced. The cause of this seems to be, that 

 in the preceding experiment the congealed part of the acid was stronger than 

 the fluid part ; so that, though the fluid part was not strong enough to dissolve 

 snow in a cold greater than — 43°^-, yet the whole acid together was strong 

 enough to do it in a cold 1° greater. A circumstance occurred in the last ex- 

 periment which I cannot at all see the reason of; namely, a small part of the 

 acid being poured into a saucer, before the addition of the snow, it was in an 

 hour's time changed into solid ice, though the cold of the air, at the time the 

 acid was poured out, was only — 41°^, and does not seem to have increased 

 during the experiment. 



On December 30, the other half of the same acid had been tried in the same 

 manner : at the beginning of the experiment not more than -l part of the acid 

 was fluid, the rest solid clear ice ; its temperature was -- 34°-|-, and that of the 

 snow nearly the same ; the greatest degree of cold produced was — 42°|- ; and 

 the quantity of snow employed was about -^V of the weight of the acid ; so that 

 the strength of the mixture was .38. The freezing point of the acid thus di- 

 luted appears to be about — 45°^ ; for by the increase of warmth during the 

 day-time, most of the congealed matter dissolved ; but in the evening it began 



