VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 2/5 



notwithstanding this cold is some degrees above the freezing point of the most 

 congelable acid. From the 2d table of specific gravities it appears, that by ap- 

 plying a more intense cold, namely, that produced by a mixture of snow, salt, and 

 water, the limits of the density of the acids capable of congelation were extended 

 to about -j-fr above or below the point of easiest freezing : and there seems little 

 reason to doubt that, by greater augmentations of cold, these limits may be 

 further extended ; but in what ratio these augmentations and extensions proceed 

 cannot be determined without many observations made in different temperatures. 



Though it is probable that the most concentrated acids may be frozen, pro- 

 vided the cold be sufficiently intense, yet there seems reason to believe, that some 

 of the congelations which have been observed in highly concentrated acids have 

 been effected in consequence of the density of these acids having been reduced 

 nearly to the point of easy freezing by their having absorbed moisture from the 

 air : for the Duke d'Ayen and M. de Morveau exposed their acids to the air, in 

 cups or open vessels; and the latter author even acquaints us, that on examining 

 the specific gravity of the acid which had frozen, he found it to be to that of 

 water as 1 ig to 74 ; which density being less than the point of easiest freezing, 

 proves that the acid which he employed, and which he had previously concen- 

 trated, had actually been weakened during the experiment. I have several times 

 exposed concentrated oil of vitriol in open vessels in frosty weather ; and I have 

 sometimes, but not always, observed a congelation take place. On separating 

 the fluid from the congealed part, and on examining the specific gravity of the 

 latter, after it had thawed, I found that it had been reduced to the standard of 

 easiest freezing. When the congealed acid was kept longer exposed, it gradually 

 thawed, even when the cold of the air increased ; the reason of which is not to 

 be imputed to the heat produced by the moisture of the air mixing with the acid, 

 for this cause operated during the congelation, but principally to the diminution 

 of density below the point of easy freezing, which was occasioned by the con- 

 tinued absorption of moisture from the air, and which rendered the acid incapa- 

 ble of continuing frozen without a great increase of cold. 



It appears then, that the concentration of M. de Morveau's acid, at the time 

 of its congelation, from which circumstance Mr. Cavendish infers generally, that 

 the vitriolic acid freezes more easily as it is more dense, is not a true premise ; 

 and that therefore the inference, though justly deduced, is invalid. On the con- 

 trary, there seems every reason to believe, from the analogy of my experiments 

 abovementioned, that as the density of the acid increases beyond the point of 

 easiest freezing, the facility of the congelation diminishes ; at least to as great 

 density as we have been ever able to obtain the vitriolic acid ; for if it were possible 

 to divest it entirely of water, it would probably assume a solid state in any tem- 

 perature of the air. 



NN 2 



