*i1^ PHILOSOPHICAL TRANSACTIONS. [anNO 1787, 



The crystallization of the frozen vitriolic acid is more or less distinct, according 

 to the slowness of its formation, and other favourable circumstances. Sometimes 

 the crystals are very distinctly shaped, large, and very hard. Their form is the 

 same as the common form of mineral alkali and of selenitic spar, but with angles 

 different in dimensions from either of these They are solids consisting of 10 

 faces, of which the 2 largest are equal, parallel, and opposite to each other, and 

 are oblique-angled parallelograms or rhomboids, whose angles are, as near as I 

 could measure them, of 105 and Tb degrees. Between these 2 rhomboidal faces 

 are placed 8 faces of the form of trapeziums. Thus each crystal may be supposed 

 to be composed of 2 equal and similar frustums of pyramids joined together by 

 • their rhomboidal bases. I observed, that the crystals always sunk in the fluid 

 acid to the bottom of the vessel, which showed that their density was increased by 

 congelation. I thought of ascertaining their specific gravity by adding gradually 

 to this fluid part some concentrated vitriolic acid, till the crystals should float in 

 the liquor, the examination of whose specific gravity would determine that of the 

 floating crystals. But I was surprized to find that the crystals sunk even in the 

 concentrated acid, and consequently were denser. I then poured some of the 

 congelable acid, previously brought to the freezing temperature, into a graduated 

 narrow cylindrical glass, up to a certain mark, which indicated a space equal to 

 that occupied by 200 grains of water. The glass was placed in a mixture of snow, 

 salt, and water, and when the acid was frozen, a mark was made on the part of 

 the glass to which the acid had sunk. Having thawed the acid, and emptied the 

 glass, I filled it with water to the mark to which the acid had sunk by freezing, 

 and I found, that 15 grains more of water were required to raise it to the mark 

 expressing 200 grains ; which shows that the diminution of bulk, sustained by 

 the acid in freezing, had been equal to ^-i-.-a- of the whole. 



Computing from this datum, we should estimate the specific gravity of the con- 

 gealed acid to have been 1924 ; but as it contained evidently a great number of 

 bubbles, its real specific gravity must be considerably greater than the above 

 determination, and cannot easily be ascertained on account of these bubbles." By 

 way of comparison, I observed the alteration of bulk which water contained in 

 the same cylindrical glass would suffer by freezing ; and I found that its expansion 

 was equal to about -^Vth part of its bulk. The water had been previously boiled; 

 but yet it contained numberless bubbles. In this respect then there is a remarka- 

 ble difference between the congelations of water and of vitriolic acid ; but perhaps 

 the difference arises principally from the bubbles of elastic fluid, which may be in 

 greater quantity, and may add more to the bulk of the water than of the acid. 



Greater cold is produced by mixing snow or pounded ice with the congealed 

 than with the fluid acid, but the quantity I have not determined. There is rea- 

 son to believe it may be considerable. In the experiments made at Hudson's Bay, 



