502 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



would be much reduced, and then adding the salts, a much greater degree of 

 cold might be produced; but, by various diversified trials, but little advantage 

 was gained. In the course of this winter, some diluted nitrous acid, in a wide 

 mouthed phial, was immersed in a freezing mixture; when cooled to about — 32'^, 

 it froze intirely to the consistence of an ointment, when the thermometer sud- 

 denly rose to — 2°; on adding some snow that lay by, it became again liquid, 

 and the mercury sunk into the bulb of a thermometer graduated to — 76°; he 

 knew not its exact strength; but by the effect imagined it might correspond 

 nearly with that which is capable of the easiest point of spirituous congelation. 

 Cold, he found, may be produced by the union of such salts as on mixing 

 are decomposed, and become liquid or partially so. The mineral alkali produces 

 this effect with all the ammoniacal salts; but with nitrated ammonia to a con- 

 siderable degree. The mineral alkali added in powder to nitrous acid, diluted as 

 above, sunk the thermometer 22° only, from 53°, temperature of air and ma- 

 terials, to 31°. This salt contains nearly as much water of crystallization as 

 vitriolated natron, and produces more cold during solution in water than that 

 salt. The reason why it produces less when added to acid than the neutral salt 

 does, is perhaps sufficiently evident. He has observed the thermometer to be 

 stationary, or even to rise, during the violent efFervesence produced on mixing 

 those materials, and to sink as soon as that ceased. 



Vitriolated natron dissolved indifferently in rectified spirit of wine, and pro- 

 duced neither heat nor cold; the disposition to produce cold, during its solu- 

 tion, being perhaps exactly counteracted by the tendency which the dissolved 

 salt hath in uniting with the spirit to produce heat. Vitriolated magnesia, a 

 salt very similar to vitriolated natron, during solution in the diluted nitrous acid, 

 produced nearly as much cold as that salt: the small difference there is between 

 them, as to this effect, may be owing to the former containing rather less water 

 in its crystals. 



Vitriolated natron, liquified by heat, was set to cool: when its temperature 

 was reduced to 70°, it became solid, and the thermometer immediately rose to 

 88°, its freezing point. Does not the quantity of sensible heat evolved by this salt, 

 in becoming solid, indicate its great capacity for heat, in returning to a liquid 

 state, and consequently account in a great measure for its producing such intense 

 cold during solution in the diluted mineral acids? Two salts, vitriolated argillace- 

 ous earth (alum) and tartarized natron (Rochelle salt,) each contain nearly as 

 much water of crystallization as vitriolated natron ; but produced neither of them 

 any considerable effect during solution in the diluted nitrous acid; the latter made 

 the thermometer rise: neither did their temperatures increase, like that salt, in 

 changing from a liquid to a solid state. 



From the obvious application of artificial frigorific mixtures to useful purposes, 



