468 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1788. 



common salt depressed the freezing point of the solution of nitre something less 

 than it would have depressed the freezing point of water, if added to it in the same 

 proportion. To show this more evidently, he added a 4th and a 5th column to 

 the table: the 4th column is formed by taking the freezing point of the saturated 

 solution of nitre as 26^°, and then finding how many degrees the quantity of 

 common salt added would have depressed the freezing point of water; this num- 

 ber of degrees subtracted from the constant number 26-i-°, gives the freezing 

 point by calculation, namely, what it should have been if the salt had produced 

 the same effect on the solution of nitre as it would on pure water ; and the differ- 

 ence between this and the freezing point found by the experiment gives the num- 

 bers in the 5th column. From the table it is apparent, that the deficiency of 

 effect from the salt goes on increasing to the 3d experiment, after which it de- 

 creases. Probably some particular law takes place, which it would require a great 

 number of experiments to develope ; but the decrease toward the last may in part 

 be owing to the greater quantity of nitre which the water, when it began to be 

 loaded with common salt, retained at the time of congelation, and which must 

 have its effect in depressing the freezing point. The above-mentioned circum- 

 stance seems rather contradictory to an opinion which has been entertained, that 

 when one salt, added to a saturated solution of another salt, enables it to take up 

 more of the former salt, it is only because the water of crystallization of the 2d 

 salt really adds to the quantity of the dissolving fluid. 



Dr. B. next proceeded to try a similar experiment with sal ammoniac and the 

 purified common salt, but with this difference, that neither salt should be added 

 to the water in such quantity as to come near the point of saturation, suspecting 

 that the diminution of effect observed in the foregoing experiments might depend, 

 in part at least, on this circumstance. The sal ammoniac therefore was dissolved 

 in water in the proportion of 1 : 10, and the corresponding point of congelation 

 appeared by experiment to be 20-^°, agreeing very well with the table of sal am- 

 moniac formerly given. To this solution was added the purified common salt, in 

 proportion to the water as 1 : 15, and then as 1 : 10; the resulting points of 

 congelation were as shown in the following table, constructed in all respects as 

 the immediately preceding. 



Proportion of 



water to ihe sal 



ammoniac. 



10 : 1 

 10 : 1 



Compound solution of sal ammoniac and common salt. 



Proportion of 



water to the 



common salt. 



15 : 1 

 10 : 1 



Freezing point 



by the 



experiment. 



121° 

 91 



Freezing point 



by 



calculation. 



13 ° 

 9\ 



Difference. 



+ i 



Hence it appears, that in this compound solution both salts produced, as 

 exactly as the experiments can be expected to show, their full effect in depressing 



