gQ PHILOSOPHICAL TRANSACTIONS. [aNNO 1780. 



Xlfl. Account of Experiments made by Mr. John M'Nab, at Henley House, 



Hudson s Bay, relating to freezing Mixtures. By Henry Cavendish, Esq., 



F.R.S., and J.S. p. 241. 



In my former paper, in the 73d vol. of the Phil. Trans., I said, that the cold 

 produced by mixing spirit of nitre with snow, is owing to the melting of the 

 snow; and that in all probability there is a certain degree of cold, in which spirit 

 of nitre is so far from dissolving snow, that it will yield out part of its own water, 

 and suffer that to freeze, as is the case with solutions of common salt; so that 

 if the cold of the materials, before mixing, be equal to this, no additional cold 

 can be produced. A circumstance however, which at first sight seems repugnant 

 to this opinion, occurred in an experiment of Fahrenheit's for producing cold by 

 a mixture of spirit of nitre and ice; namely, that the acid, which had been re- 

 peatedly cooled by different frigorific mixtures, was found frozen before it was 

 mixed with the ice ; yet cold was produced by the mixture. Professor Braun 

 also found, that cold was produced by mixing frozen spirit of nitre with snow. 

 On consideration however this appeared by no means inconsistent with the opi- 

 nion there laid down, as there was great reason to think, that the freezing of the 

 acid was of a different kind from that considered in the above-mentioned paper, 

 and that it did not proceed from the watery part separating from the rest and 

 freezing; but that the whole acid, or perhaps the more concentrated part, froze; 

 in which case it would not be extraordinary that the acid should dissolve more 

 snow, and produce cold. 



To clear up this point, I sent to Hudson's Bay a bottle of spirit of nitre, of 

 nearly the same strength as Fahrenheit's ; and desired Mr. M'Nab to expose it 

 to the cold, and, if it froze, to ascertain the temperature, and decant the fluid 

 part into another bottle, and send both home to be examined, as it would thus 

 be known, whether it was the whole acid, or only the watery part, which froze. 

 For the same purpose also I sent some strong oil of vitriol. I also sent some 

 spirit of nitre and spirit of wine, both diluted with so much water, that it was 

 expected, that with the cold of Hudson's Bay they would suffer the first kind of 

 congelation ; that is, their watery part would freeze, and so make the difference 

 between the two kinds of freezing more apparent. 



As it seemed likely that, by following the method, pointed out in my former 

 account, a greater degree of cold might be produced than had been done 

 hitherto, I sent 3 other bottles of spirit of nitre and oil of vitriol, all 3 diluted, 

 but not so much so, but that I thought they would require a little further dilu- 

 tion, to reduce them to their properest degree of strength. I also sent a bottle 

 of highly rectified spirit of wine, and a mixture of equal quantities of the above- 

 mentioned common spirit of nitre and oil of vitriol ; and desired Mr. M'Nab to 

 find what degree of cold, could be produced by mixing them with snow, after 



