464 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1788. 



Rochelle Salt. 

 Of all the solutions submitted to these experiments, there were none more 

 transparent and elegant than those made with Rochelle salt. The water dissolved 

 a large proportion of this substance, and had its freezing point sunk according 



to the following table. 



The 3d column is calculated from the last ex- 

 periment but one, in which the freezing point 

 of a solution of 1 part of the Rochelle salt in 2 

 parts of water proved to be 21°. All the so- 

 lutions of Rochelle salt bore to be cooled re- 

 markably well. In one instance the liquor sunk 

 11°-|- below its freezing point; namely, the so- 

 lution of 1 part of the salt in 5 of water, whose 

 freezing point proved 27°4-, and which cooled to 

 l6° before the crystals of ice shot. In 2 other 

 instances it sunk fully 9° below its freezing point. In trying the greatest cold 

 to be obtained by mixing Rochelle salt with snow, the thermometer could not be 

 got lower than 24°. 



Glauber's salt was likewise subjected to the experiments; but its utmost effect 

 in producing cold with snow appearing to be only 2°, this was too small a scale 

 for settling any thing as to the ratio. A solution of it in water, in the proportion 

 of 1:5, cooled readily to 31°; but the salt was deposited in great quantities, and 

 often so fast as to stop the cooling of the bottom of the liquor entirely, though 

 the vessel was immersed in a strong frigorific mixture. This phaenomenon is ex- 

 actly the converse of the cold produced by dissolving salts in water; for as there 

 some heat is absorbed, and becomes latent, by the change of the salt from a 

 solid to a fluid state, so here some heat is evolved as the salt assumes the solid 

 crystalline form. The effect is so much more manifest with Glauber's salt only, 

 because the formation of the crystals proceeds so rapidly ; whence the quantity 

 of heat generated equals or exceeds the cold communicated by the freezing 

 mixture. Some odd appearances are produced by this sudden stop of the cooling, 

 and the rapid deposition of salt ; for instance, a particular ebullition in certain 

 parts of the liquor; but any intelligible description of them would be too 

 minute. 



These were all the salts with an alkaline basis that were tried. They all 

 agreed as to the chief object of these experiments, namely, to determine how 

 much the freezing point of water would be sunk by dissolving them in it in 

 various proportions; which by these experiments appears to be, as nearly as could 

 be determined, according to the simple ratio of the proportion each salt bears to 

 the water. Dr. B. now resolved to try a few salts with an earthy and metallic basis. 



