104 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1786. 



to freeze again, so as to become thicker, its temperature being then — 45°:i- 

 and the next morning it was frozen solid, its cold being 1° greater. 



On December 12, the diluted spirit of nitre N° 139, whose strength was 

 .175, was found frozen, its temperature being — 17. The fluid part, which 

 was full of thin flakes of clear ice, and was of the consistence of syrup, was 

 decanted into another bottle, and sent back. Its strength was .21, and was 

 greater than that of the undecanted part in the proportion of .21 to .16 ; so 

 that, as not much of the undecanted part was really congealed, the frozen part 

 of the acid must have been much weaker than the rest, if not mere water. 

 Accordingly, during the melting of the undecanted part, the frozen particles 



hquor. 



but it 



swam at top. Mr. M'Nab added snow to a little of the decanted 

 did not dissolve ; and no increase of cold was produced. 



From these experiments it appears, that spirit of nitre is subject to 2 kinds of 

 congelation, whic)i we may call the aqueous and spirituous ; as in the first it is 

 chiefly, if not entirely, the watery part which freezes, and in the latter the 

 spirit itself. Accordingly, when the spirit is cooled to the point of aqueous 

 congelation, it has no tendency to dissolve snow and so produce cold, but on the 

 contrary is disposed to part with its own water ; whereas its tendency to dissolve 

 snow and produce cold, is by no means destroyed by being cooled to the point 

 of spirituous congelation, or even by being actually congealed. When the acid 

 is excessively dilute, the point of aqueous congelation must necessarily be very 

 little below that of freezing water: when the strength is .21, it is at — 17°, 

 and at the strength of .243, it seems to be at — 44°^. Spirit of nitre, of the 

 foregoing degrees of strength, is liable only to the aqueous congelation, and it 

 is only in greater strengths that the spirituous congelation can take place. This 

 seems to be performed with the least degree of cold, when the strength is .411, 

 in which case the freezing point is at 1%. When the acid is either stronger or 

 weaker, it requires a greater degree of cold ; and in both cases the frozen part 

 seems to approach nearer to the strength of .411 than the unfrozen part ; it cer- 

 tainly does so when the strength is greater than .411, and there is little doubt 

 but that it does so in the other case. At the strength of .54 the point of 

 spirituous congelation is 31^4^, and at .33 pro- 

 bably — 45°4- ; at least one kind of congelation 

 takes place at that point, and there is little 

 doubt but that it is of the spirituous kind. In 

 order to present this matter more at one view, 

 I have given the annexed table of the freezing 

 point of common spirit of nitre answering to 

 different strengths. 



In trying the first half of the dephlogisticated spirit of nitre, the cold pro- 



1 spiri 



ipirituous con- 

 gelation. 



} 



aqueous con" 

 gelation. 



