274 PHILOSOPHICAL TKANSACTIONS. [anNO 1787. 



and then it is susceptible of congelation by means of much less cold than has been 

 hitherto imagined ; yet, as only part of my acid had frozen, I could not with 

 certainty know the strength of the frozen part, and I therefore was not able to 

 state, with any accuracy, the degree of strength most favourable to congelation, 

 nor the limits of strength within which the acid may be congealed by such mode- 

 rate cold. In the following winter I had not leisure to pursue the subject ; but 

 since the commencement of the present year, I have verified my former observa- 

 tion with more attention to the exact densities of the acids ; and I have found, 

 that the point of strength most favourable to congelation is very determinate, and 

 that a very small variation above or below that point renders the acid incapable of 

 freezing without a considerable augmentation of cold. As the acid, when brought 

 to the proper strength, was capable of freezing with less cold than water does, I 

 immersed several acids of different strengths in melting snow, instead of exposing 

 them to the air, the temperature of which was variable, whereas that of melting 

 snow was constant and determinate. Those acids which would not freeze in 

 melting snow, were afterwards immersed in a mixture of snow, water, and com- 

 mon salt, the temperature of which was not so constant and determinate as that 

 of melting snow ; but it generally remained for several hours at about 18°, and 

 was sometimes several degrees lower. The intention of adding water to the snow 

 and salt was to lessen the intensity of the cold of this mixture, and to render it 

 more permanent than if the snow and salt alone were mixed. 

 The acids which had frozen in melting snow, and which were ^^^^ 



1784 



five in number, having been thawed and brought to the tempera- 1730 



ture of 6o^ were found on examination to have the annexed ^778 



•^ .,. 1775 



specific gravities. 



Those acids which would not freeze in melting snow, but which 1814 



froze when immersed in snow, water, and salt, having been ex- jgQ^ 



posed to a greater cold, were of a greater latitude of density. 1794 



Their specific gravities, when brought to the temperature of 60°, JIS^ 



were found to be expressed by the annexed numbers. 1759 



The acids which remained, and which would not freeze either ^^^^ 



in melting snow, or in the mixture of snow, salt, and water, 1345 



were found on examination to have the annexed specific era- 1^39 



^ ^ 1815 



vities. 1J.45 



It appears, from the 1st table of specific gravities, that the ^720 



medium density of the acids which did freeze with the cold of iq^q 



melting snow was 178O ; and from the 2d table it appears that, at 1551 



the densities of 1790 and 17 70, the acid had been incapable of freezing with that 

 degree of cold. Hence it follows, that 178O is nearly the strength or density of 

 easiest freezing; and that an increase or diminution of that density, equal to 

 T-L-g-th part, renders the acid incapable of freezing with the cold of melting snow. 



