272 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



kind of vitriolic acid is subject, whether it smokes or not, and whether it has been 

 prepared from martial vitriol, or from sulphur, provided the cold to which it is 

 exposed be sufficiently intense : for the cold requisite for this species of con- 

 gelation, is considerably greater than what is sufficient for the former. 



Mr. Macquer relates, in the 2d edition of his Diet, of Chemistry (article, 

 Vitriolic Acid), that the Duke d'Ayen had observed the congelation of concen- 

 trated vitriolic acid, which had been exposed to a cold expressed by 13 or ] 4 de- 

 grees below O of Reaumur's scale ; but that mixtures, consisting of 1 part of the 

 above-mentioned concentrated acid, with 2 or mqre parts of water, could not be 

 frozen by the cold to which he exposed them, till he had diluted the acid so much, 

 that its density was to that of water as 1044- ^o 96 ; in which latter case of con- 

 gelation, it is probable, that the water only did freeze, as it does in dilute solu- 

 tions of neutral salts. M. de Morveau (Mem. de I'Acad. de Dijon, pour 

 1782) made similar experiments, with a view to verify those of tlie Duke 

 d'Ayen, and with similar success. By means of an intense cold, produced by 

 adding spirit of nitre to pounded ice, he congealed a part of some vitriolic acid, 

 which he had previously concentrated. He observed, that though a very intense 

 cold had been employed to freeze the concentrated acid, it nevertheless remained 

 congealed in much less degrees of cold, and that it" thawed very slowly. Lastly, 

 some experiments have lately been made by Mr. M'Nab, at Hudson's Bay, on 

 the congelation of acids by intense cold ; an account of which experiments is given 

 in the Phil. Trans, for 1/86, by Mr, Cavendish, at whose desire they had been 

 made. These experiments are the more valuable, as the density of the acids em- 

 ployed, and the temperature, and other concomitant circumstances, have been 

 distinctly noted; and they are rendered still more interesting, by the very judi- 

 cious remarks made on them by Mr. Cavendish. It is there related, that a 

 vitriolic acid, whose specific gravity was to that of water as 1843.7 to 1000, froze 

 when exposed to a cold of — 15° of Fahrenheit's scale ; that another more dilute 

 vitriolic acid, consisting of 629 parts of the former concentrated acid, and 351 

 parts of water, congealed in a temperature of — 36° ; and that when the acid 

 was further diluted, it was found capable of sustaining a much greater cold with- 

 out freezing. In these experiments, as also in those of M. de Morveau, it ap- 

 peared that the whole of the acid did not congeal, but that part of it retained its 

 fluidity. Mr. Cavendish found, on examining the part which had congealed, and 

 that which had remained fluid, that they were nearly of the same strength ; and 

 he is thence led to think, that the difference between them, by which the one is 

 more disposed to congeal than the other, does not depend on their different 

 strengths, but on some quality less obvious, and the same which constitutes the 

 dift'erence between glacial and common oil of vitriol. In all the experiments 

 which had been made by the Duke d'Ayen, M. de Morveau, and Mr. M'Nab, 



