Freezing points. 



VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 425 



ones it lasts several days; and this may possibly, combined with other observa- 

 tions, lead to a discovery of the real use of the corolla of plants, and the share it 

 has in the impregnation, about which there has yet been no probable conjecture. 



XIII. An Account of Experiments made by Mr. John M'Nab, at Albany Fort, 

 Hudson s Bay, relative to the Freezing of Nitrous and Fitriolic Acids. By 

 Henry Cavendish, Esq. F. R. S., and A. S. p. 1 66. 



From the experiments made by Mr. M'Nab, of which I gave an account in 

 the 76th volume of the Philos. Trans, p. 241, it appeared that spirit of nitre was 

 subject, not only to what I call the aqueous congelation, namely, that in which 

 it is chiefly, and perhaps entirely, the watery part which freezes, but also to an- 

 other kind, in which the acid itself freezes, and which I call the spirituous con- 

 gelation. When its strength is such as not to dissolve so much as -rVoV o^ its 

 weight of marble, or when its strength is less than .243, as I call it for short- 

 ness, it is liable to the aqueous congelation solely ; and it is only in greater 

 strengths that the spirituous congelation can take place. This seems to be per- 

 formed with the least degree of cold when the strength is .411, in which case 

 the freezing point is at — 14^°. 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 '4,^^ 



strength of .4 1 1 than the unfrozen part. The .38 



freezing points answering to diflcrent degrees '^-. 



of strength, seemed to be as annexed. 



As some of these properties however were deduced from reasoning not suffici- 

 ently easy to strike the generality of readers with much conviction, Mr. M'Nab 

 was desired to try some more experiments to ascertain the truth of it ; which he 

 has executed with the same care and accuracy as the former. For this purpose, 

 I sent him some bottles of spirit of nitre of different strengths, and he was desired 

 to expose each of these liquors to the cold till they froze ; then to try their tem- 

 perature by a thermometer; afterwards to keep them in a warm room till the ice 

 was almost melted, and then again expose them to the cold, and when a con- 

 siderable part of the acid had frozen to try the temperature a 2d time ; then to 

 decant the unfrozen part into another bottle, and send both parts back to Eng- 

 land, that their strength might be examined. The intent of this 2d exposure to 

 the cold was as follows : spirit of nitre bears, like other liquors, to be cooled 

 greatly below its freezing point without freezing : then the congelation begins 

 suddenly ; the liquor is filled with fine spicula of frozen matter, and the ice be- 

 comes so loose and porous, that if the process be continued long enough for a 

 considerable portion of the acid to congeal, scarcely any of the fluid part can be 



VOL. XVI. 3 I 



— 3U 



— 1 1 ^ spirit, congel. 



44 1 -J 



— 17 J ^^^' '^^"S®^' 



} 



