426 PHILOSOPHICAL TRANSACTIONS. . [anNO 1788. 



decanted: whereas, if it be heated in this state till the frozen part is almost, but 

 not entirely, melted, and be again exposed to the cold, as the liquor is then in 

 contact with the congealed matter, it begins to freeze as soon as it arrives at the 

 freezing point, and the ice becomes much more solid and compact. 



The intent of decanting the fluid part, and sending both parts back, that their 

 strength might be determined, was partly to examine the truth of the supposition 

 laid down in my former paper, that the strength of the frozen part approaches 

 nearer to .411 than that of the unfrozen ; but it is also a necessary step towards 

 determining the freezing point answering to a given strength of the acid ; for as 

 the frozen part is commonly of a different strength from the unfrozen, the 

 strength of the fluid part, and the cold necessary to make it freeze, is continually 

 altering during the progress of the congelation. In consequence of this, the tem- 

 perature of the liquor is not that with which the frozen part congealed ; but it is 

 that necessary to make the remainder, or the fluid part, begin to freeze, or in 

 other words it is the freezing point of the fluid part. This is the reason that a 

 thermometer, placed in spirit of nitre, continually sinks during the progress of 

 congelation; which is contrary to what is observed in pure water, and other fluids 

 in which no separation of parts is produced by freezing. 



Further, from the above-mentioned experiments of Mr. M'Nab it appeared, 

 that oil of vitriol, as well as spirit of nitre, is subject to the spirituous congela- 

 tion ; but it seemed uncertain whether, like the latter, it had any point of easiest 

 freezing, or whether it did not uniformly freeze with less cold as the strength in- 

 creased. For this reason, some bottles of oil of vitriol, of different strengths, 

 were sent, which he was desired to try in the same manner as the former. This 

 point indeed has since been determined by Mr. Keir, who has shown that oil of 

 vitriol has strength of easiest freezing; and that at that point a remarkably slight 

 degree of cold is suflEicient for its congelation. The result of Mr. M'Nab's ex- 

 periments on the nitrous acid is given in the following table. 



The first column contains the numbers by which Mr. M'Nab has distinguished 

 the different bottles. The 2d and 3d columns contain the quantity and strength 



