466 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



experiments. The only numbers to be relied on are the following, which agree 

 sufficiently with the general result. 



„ . ^ . The 3d column is calculated from the last ex- 

 Freezing Freezing . . • i • i i r • 

 point by the point by cal- penmen t, m which the freezmg pomt of a solu- 

 experiment. culation. tion of J part of the white vitriol in 3 of water 



Proportion 



of water to 



the salt. 



10 : I 

 5 : 1 

 3 : 1 



31° 



28f 



31© proved to be 28f°. These solutions cooled very 

 30 ill, none of them having sunk much below the 



^ freezing pomt, and the strongest, which had a 

 copious sediment, forming a crust of ice at the bottom of the tumbler, before it 

 was reduced at all below the term of congelation. 



M. Achard, of Berlin, having alleged^, that borax, instead of raising the 

 boiling point of water, like other saline substances, very sensibly depresses it. 

 Dr. B. determined, however extraordinary the fact might appear, to try whether 

 it had any peculiar effect on the freezing point. But having made the experi- 

 ment with nearly a saturated solution of borax, the thermometer when it con- 

 gealed was evidently below 32^^: he believes about a degree. 



As a neutral or middle salt, which when crystallized is always nearly of the 

 same nature, and dissolves in a regular proportion in water, seemed likely to 

 afford the most simple case of the effect of extraneous admixtures, it was with 

 such that he began these experiments. But having found that with them the 

 simple ratio prevailed, he proceeded to try substances of a more variable nature, 

 and capable of being mixed with water in almost any proportion; such as acids, 

 alkalis, and ardent spirits. A material difference in the law, which seemed to 

 occur in these new experiments, renders it proper to defer the account of them 

 till some reflections on the preceding facts, with a few additional experiments to 

 which they gave rise, have been premised. 



It seems now universally allowed, that in frigorific mixtures the melting of the 

 snow or ice is the principal cause of the cold produced; all that heat which must 

 become latent in order to give water its fluent form being taken from the sensible 

 heat of the ingredients. But as, when crystallized salts are employed for the 

 purpose, these also are reduced to a liquid form, there must, from this circum- 

 stance, be some additional cold produced, such for instance a% would be occa- 

 sioned by dissolving the same salt in water. Suppose then that the latent heat 

 of water is 150°, and that sal ammoniac, in dissolving to saturation, produces so 

 much cold as sinks the whole solution about 20°; it is evident, that if this salt 

 and ice are mixed together in such proportions as just to melt each other, the 

 total cold generated in the operation must amount to 170°. And so much actu- 

 ally is produced before the whole liquefaction is effected; and yet a mixture of 

 these 2 substances will sink the thermometer no lower than to 4° of Fahrenheit's 

 * See Crell's Chem. Annalen, 1786, vol. 1^, p. 501. 



