Vol. lxxviii.] 



PHILOSOPHICAL TRANSACTIONS. 



469 



the point of congelation. When the solutions at length froze, after cooling 

 many degrees below the freezing point, the crystals shot in a very beautiful man- 

 ner round the bulb and up the stem of the thermometer. 



In a compound solution of Rochelle and common salt there was however a 

 deficiency of effect. For the solution of Rochelle salt in the proportion of 1 part 

 to 4 of water, having its freezing point at 264-°; when common salt was dissolved 

 in it, in the proportion of Vo-thj the freezing point appeared by experiment to 

 be 164-°, whereas by calculation it should have been depressed nearly to 15°. 



A composition of 3 salts was affected as follows ; 



Compound solution of Rochelle salt, common salt, and sal ammoniac. 



Proportion of Proportion of Proportion of Freezing point Freezing point 

 water to the water to the water to the by the by Difference. 



Rochelle salt, common salt, sal ammoniac, experiment. calculation. 



9.8 : 1 



10 : 1 



17 : 1 



13° — 



Hi 



— W 



The computation is made thus : Rochelle salt, in the proportion of 1 : 9.8, 

 depresses the freezing point 1\° ; common salt, in the proportion of 1 : 10, sinks 

 it IH ; and sal ammoniac, in the proportion of 1 : 17, sinks it 7° ; now 24- + 

 1 14 + 7 = 20-1- ; which subtracted from 32, leaves 1 14-° for the computed freez- 

 ing point of this mixture. The moment Dr. B. had found by experiment, that 

 the addition of a different salt to the saturated solution of any salt, would still 

 further depress its freezing point, it was obvious to conclude, that greater cold 

 could be produced with snow by a mixture of salts than by means of either taken- 

 separately. He made several experiments with this view, and found it uniformly 

 the fact, that by adding a certain proportion of a salt which had less power of pro- 

 ducing cold with snow, to one which had a greater power, the frigorific effect of 

 the latter was sensibly increased. Passing over examples of less consequence, it 

 may be sufHcient to instance common salt and sal ammoniac. The ordinary 

 common salt he used to mix with snow, sunk the thermometer to — 5°; the sal 

 ammoniac to -\- 4°; but when some of the latter salt was mixed with the former, 

 the composition produced with snow, a cold of — 12°. On several occasions he 

 made use of this composition to obtain a greater degree of cold than common salt 

 alone would produce, and found it a very convenient method. On this principle 

 it is that impure common salt always makes a stronger freezing mixture than the 

 pure ; it being, in fact, a composition of salts. Three salts have produced a 

 greater cold than 2, but he had not carried the experiments far enough to ascer- 

 tain the limits of this effect. 



As the cold produced by common salt with snow is — 4° or more, and by sal 

 aifimoniac -}- 4^ it is difficult to conceive in what manner Fahrenheit fixed the 

 zero of his thermometer. All those who have examined the few authentic pas- 



