VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 273 



the vitriolic acid, when strong, had frozen with less cold than when diluted ; but 

 these experiments did not enable Mr. Cavendish to determine, whether this acid 

 has any determinate strength or point of easiest freezing (such as he had dis- 

 covered to be possessed by spirit of nitre), or whether the cold requisite for con- 

 gelation does not continually diminish, as the strength of the acid increases, with- 

 out limitation. This latter opinion he thinks the most probable, from the cir- 

 cumstance of the Duke d'Ayen's and M. de Morveau's acids having frozen with 

 a considerably less intense cold than those of Mr. M'Nab, which, he supposes, 

 were weaker, as the former acids had been concentrated purposely. 



The observations which I have made, (says Mr. K.) and am going to relate, 

 apply solely to the latter kind of congelation of the vitriolic acid, as the acid which 

 I employed was of the kind that is prepared by burning sulphur, and is commonly 

 sold in England under the name of oil of vitriol, and was perfectly free from 

 colour, smell, or smoking quality. After a severe frost at the end of the year 

 1784, and beginning of 1785, I observed that some vitriolic acid contained in a 

 corked phial, had congealed ; while other parcels of the same acid, some stronger 

 and some weaker, equally exposed to the cold, had remained fluid. As I im- 

 puted the congelation to the great intensity of the cold, I was afterwards much 

 surprized, when the frost ceased, to find that the acid remained frozen during 

 many days, when the temperature of the air was sometimes above 40° of Fahren- 

 heit's scale ; and when the congealed acid was brought into a warm room, pur- 

 posely to thaw it, a thermometer, placed in contact with it during its thawing, 

 continued stationary at 45°. From these circumstances I concluded, that the 

 freezing and thawing point of this acid was very near the last mentioned degree ; 

 and accordingly, on exposing the liquor which had been thawed to the air, at the 

 temperature of 30°, the congelation again took place in a few hours. From the 

 circumstance of other parcels of the same acid, but of different strengths, re- 

 maining fluid, though they had been exposed to a much greater cold than was 

 necessary for the congelation of that acid liquor which had frozen, I was led to 

 believe, that there must be some certain strength at which the vitriolic acid was 

 more disposed to freeze than at any other, greater or less. I knew that the 

 specific gravity of the acid which had frozen was nearly to that of water as 1 800 

 to 1900, and that of the stronger acid, which had not frozen, was as 1846 to 

 1000 ; which last is the usual density of the oil of vitriol commonly sold in Eng- 

 land. I knew also, that the acid which had frozen was in no respect but in 

 strength different from the stronger acid which had retained its fluidity ; having 

 myself, some weeks before, taken the former acid from the bottle containing 

 the latter, and diluted it with water till it was reduced to the specific gravity 

 of 1800. 



Though from the above observations I was convinced of the proposition gene- 

 raHy, that the vitriolic acid is most disposed to freeze when at a certain strength, 



VOL. XVI. N if 



