VOL. LXXVl.] PHILOSOPHICAL TRANSACTIONS. ^ 105 



duced was — 44°^. The acid was fluid before the addition of the snow, and of 

 the temperature of — 30°, but froze on putting in the thermometer, and rose 

 to — 5°, as related before. In trying the 2d part, the acid was about 0° be- 

 fore the addition of the snow, and therefore had no disposition to freeze. The 

 cold produced was — 42*^-^. As the quantity of snow added in these experiments 

 was not observed, they do not determine any points of aqueous or spirituous 

 congelation in this acid ; but there is reason to think, that these points are 

 nearly the same as those of common spirit of nitre of the same strength, as the 

 cold produced in these experiments was nearly the same as that obtained by the 

 common spirit of nitre. 



On the Vitriolic Acid. — On Dec. 12, the strong oil of vitriol N° 151 was 

 found frozen, and was nearly of the colour and consistence of hogs-lard. Its 

 temperature, found by pressing the ball of a thermometer into it, was — 15°, 

 and that of the air nearly the same ; but in the night it had been exposed to a 

 cold of — 33°. It dissolved but slowly on being brought into a warm room, 

 and was not completely melted before it had risen to + 20°, and even then was 

 not very fluid, but of a syrupy consistence. During the progress of the melt- 

 ing, the congealed part sunk to the bottom, as in spirit of nitre : and many air 

 bubbles separated from the acid, which, when it was completely melted, formed 

 a little froth on the surface. As soon as it was sufficiently melted to admit of it, 

 which was not till it had risen to the temperature of -\- 1 0°, the fluid part was 

 decanted, and both were sent home to be examined. It appeared by another 

 bottle of oil of vitriol, which also froze by the natural cold of the air, that this 

 acid, as well as the nitrous, contracts in freezing, 



Dec. 21, when the weather was at — 30°, the vitriolic acid N° 103 was di- 

 luted with snow, as before directed. The snow dissolved immediately, and no 

 signs of congelation appeared during any part of the process. The temperature 

 of the acid rose only 1*^ before it began to sink, and the weight of the snow 

 added was only VtV of that of the acid, so that its strength was thus reduced to 

 .605 ; which is therefore the best degree of strength for producing cold by the 

 addition of snow, when the degree of cold set out with is — 30°. 



The acid thus diluted was divided into 2 parts, and the next day Mr. M'Nab 

 tried what degree of cold could be produced by adding snow to one of them. 

 The temperature of the air at the time was — 39°, and the mixture sunk by the 

 process to — 55°4-. The snow dissolved readily, and the mixture did not lose 

 much of its fluidity till it had acquired nearly its greatest degree of cold, nor 

 did any congealed matter sink to the bottom in any part of the process. The 

 quantity of snow added was about -^^ of the weight of the acid, so that the 

 strength of the mixture was about .325. 



On Jan. I, thin crystals of ice were found diffused all through this mixture, 



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