VOL. LXXVIII.] 



PHILOSOPHICAL TRANSACTIONS. 



467 



scale. The consideration of this apparent difficulty has led to the supposition, 

 that a certain quantity of fire is contained in the crystals of the salt, which being 

 disengaged in the solution keeps up the mixture to a certain temperature. But 

 Dr. B. conceives that the phenomenon depends simply on the gradual liquefac- 

 tion of the ingredients, a necessary consequence of the cold produced. A satu- 

 rated solution of sal ammoniac freezes itself at 4°; therefore, when the mixture 

 is reduced by the liquefaction of the ingredients, to that temperature, no more 

 of them can melt, because any addition of cold would freeze what is already 

 melted; and if the mixture, under such circumstances, were placed in an atmo- 

 sphere of its own temperature, the ingredients would remain for ever in that same 

 state, without any further liquefaction. But in an atmosphere warmer than 4°, 

 they continue to melt, more or less slowly, as the heat which is gradually com- 

 municated .furnishes what is necessary to become latent. This communicated 

 sensible heat being immediately converted into latent, the mixture will always be 

 kept down to the same temperature as long as there is a sufficient mass of un- 

 melted materials ; and it can sink no lower, because then the liquefaction would 

 be stopped; consequently such mixtures must preserve, as they have been found 

 to do, a pretty uniform temperature, so as to have been formerly used for gradu- 

 ating thermometers. And the whole cold produced, or, to speak properly, the 

 whole of the heat made to disappear, he presumes to be ultimately equal to the 

 full quantity of latent heat belonging to the dissolved ice and salt. 



As it is well known that water, after it has been saturated with one salt, will 

 take up a certain portion of another salt without depositing any of the former. 

 Dr. B. was curious to try what effiict the addition of this 2d salt would produce 

 on the freezing point; and particularly whether it would depress the freezing 

 point of the saturated solution the same number of degrees that an equal pro- 

 portion of the same salt would depress the freezing point of water; and whether 

 the same simple ratio would hold good, or any new law take place. To bring 

 this to the test of experiment, he took a saturated solution of nitre, whose 

 freezing point of course was between 1&^ and 27°; and adding to it the purified 

 common salt in various proportions, he obtained the following results. 



Compound solution of nitre and common salt. 



Proportion of 



water to the 



nitre. 



A saturated 

 solution. 



Proportion of 

 water to the 

 common salt. 



30,2 : 1 

 15 : 1 

 10 : 1 



7.4 : 1 



5 : 1 



Freezing point 



by the 



experiment. 



23^° 

 20| 



m 



I3i 



51 



Freezing point 



by 



calculation. 



221" 



19 



i5i 



4 



Difference. 



1| 

 2| 



2 



It is evident, from the freezing points of this compound solution, that the com- 



3 02 



