VOL. LXXVIirJ PHILOSOPHICAL TRANSACTIONS. 429 



part melted, both were again exposed to the cold. They both were made to con- 

 geal by agitation, and the temperature of the undecanted was then found to be 

 — 35°, and that of the decanted part — 37°: so that it should seem as if the 

 freezing point found by the hasty congelation was always lower than that found 

 the other way, which may perhaps proceed from this cause ; namely, that when 

 sufficient time is allowed, the watery part will separate from the rest, and freeze 

 in a degree of cold much less than what is required to produce that effect, when, 

 it is performed in a more rapid manner. 



These experiments confirm the truth of the conclusions I drew from Mr. 

 M'Nab's former experiments ; for, first, there is a certain degree of strength at 

 which spirit of nitre freezes with a less degree of cold than when it is either 

 stronger or weaker ; and when spirit of nitre, of a different strength from that, is 

 made to congeal, the frozen part approaches nearer to the foregoing degree of 

 strength than the unfrozen. Also this strength, as well as its corresponding 

 freezing point, and the freezing point answering to the strength of .54, come 

 out very nearly the same as I concluded from those experiments ; for by the pre- 

 sent experiments they come out .418, — 2-i\°, and — 31°, and by the former 

 .41 J, — H°, and — 31°. But the freezing point answering to the strength of 

 .38 is totally different from what I there supposed. This must have been owino- 

 to the strength of that acid having been very different from what I thought it ; 

 which is not improbable, as its strength was inferred only from the quantity of 

 snow which was added to it in finding the degree of cold produced by its mixture 

 with snow. 



On the Vitriolic j4cid. — An irregularity of a rertiarkable kind occurred in try- 

 ing 2 of these acids ; namely, when the undecanted part was melted and again 

 made to congeal, its freezing point was found to be much less cold than that of 

 the decanted part, and the difference was much greater than could be attributed 

 to the difference of strength. This seems to have happened only in the strongest 

 2 acids, namely, N° 1 and 2, and in great measure confirms the supposition 

 which I formed from Mr. M'Nab's former experiments, that the congealed part 

 of oil of vitriol differs from the rest, not merely in strength, but also in some 

 other respect, which I am not acquainted with. It should seem however that 

 this property does not extend to weak oil of vitriol. Some smaller irregularities 

 occurred in trying the vitriolic acid, the cause of which I believe was, that when 

 this acid has been cooled below the freezing point, and begins to freeze, the con- 

 gelation proceeds but slowly ; so that a considerable time elapses before it rises to 

 the true freezing point. Something of the same kind seems to take place in the 

 nitrous acid also, though in a less degree ; for the decanted liquors usually con- 

 tinued to freeze and deposit a small quantity of ice, for a few minutes after they 

 were poured off, though their cold, at least in some instances, was found rather to 



