(J7B PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



when cooled considerably below its freezing point. I placed a phial, containing 

 some congelable vitriolic acid, in a mixture of salt, snow, and water ; and soon 

 afterwards, while the acid was yet fluid, I immersed in it a thermometer, the 

 mercury of which quickly sunk from 50° to 29°. While I was moving the ther- 

 mometer in the fluid, in order to make it acquire the exact temperature, I saw 

 the mercury suddenly rise, and on looking at the acid, I observed numberless 

 small crystals floating in it, which had been suddenly formed. The degree to 

 which the mercury then rose was 46^°. Another time, while the acid was freez- 

 ing, the thermometer placed in it stood at 45°. 



From the above observations, the following inferences may be drawn. 1st, That 

 the vitriolic acid has a point of easiest freezing; that is, there is a certain strength 

 or density, at which this acid freezes with considerably less cold than at any other 

 strength, greater or less ; and that this density is nearly to that of water as 178O 

 is to lOOO. 2dly, That the greater or less disposition of congelation of the 

 vitriolic acid, which is free from the smoking quality peculiar to the acid obtained 

 by distilling martial vitriol, does not depend on any other quality or circumstance 

 than its strength or density. 3dly, That the freezing and thawing degree of the 

 most congelable acid, is about 45° of Fahrenheit's scale. It is however to be ob- 

 served, that this degree is inferred from the temperature indicated by the ther- 

 mometers immersed in the freezing and thawing acids ; but that I never effected 

 the congelation of the fluid acid, without exposing it to a greater cold, namely, 

 either that of melting snow, or of the external air in frosty weather. Like 

 water, this acid possesses the property of retaining its fluidity when cooled 

 several degrees below its freezing point ; and of rising suddenly to this point, 

 when its congelation is promoted by agitation, or by contact with even a warmer 

 thermometer. 4thly, That, like water and other congelable fluids, the vitriolic 

 acid generates cold during its liquefaction, and heat during its congelation ; the 

 quantity of which heat and cold, so generated, remains to be determined by 

 future experiments. 5thly, That the acid, by congelation, when the circum- 

 stances for distinct crystallization are favourable, assumes a regular crystalline 

 form, a considerable solidity and hardness, and a density much greater than it 

 possessed in a fluid state. 



With respect to the first mentioned species of congelation, which is peculiar 

 to the smoking vitriolic acid that is procured from martial vitriol, though I have 

 had no opportunity of seeing it, as all the vitriolic acid, that is used in this coun- 

 try, is obtained by burning sulphur, yet I will beg leave to suggest, that it may 

 be worth the attention of those chemists to whom it occurs, to observe more ac- 

 curately than has been done, the freezing temperature and the density of the con- 

 gelable acids ; and to examine whether the density of this smoking acid also i* 

 connected with the glacial property. It seems further to be deserving of investi- 



