470 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1788. 



sages to be found in authors on this subject, will be sensible how vaguely they are 

 expressed, leaving it doubtful whether he used common salt alone, sal ammoniac 

 alone, or both mixed together. There is no method of sinking a thermometer 

 exactly to O^ with these salts and snow, but by means of a certain smaller propor- 

 tion of common salt added to the sal ammoniac ; and it would have been an extra- 

 ordinary chance, that Fahrenheit should hit precisely this proportion often 

 enough to make him rely on the point so found as the commencement of his 

 scale ; more especially as the proportion is probably no greater than a 7th or a 6th 

 of common salt to the sal ammoniac. Is it possible that Fahrenheit, finding a 

 considerable difference in his experiments, took the mean between them for his 

 zero, without any respect to the different nature of the salts with which he 

 operated ? It appears that he was at this time so little acquainted with the subject, 

 as to consider his zero as the utmost limit of cold. 



Vitriolic acid. 



Proportion 



of water to 



acid. 



10 

 5 



4 



Freezing 

 point by the 

 experiment 



24| 

 12i 



Freezing 



point by 



calculation. 



22i 

 12f 



Dr. B. comes now to certain substances which, 

 by equal additions, seem to depress the freezing 

 point of water in an increasing ratio. These, as 

 was mentioned before, are acids, alkalis, and 

 spirit of wine. The specific gravity of the vi- 

 triolic acid employed was 1.837 at 62° of heat ; 

 its effect on the freezing point is shown by the 

 annexed table. 



This table is constructed in the same manner as those formerly given of the 

 solution of simple salts ; the last experiment, where the proportion is 4 : 1, being 

 taken as the standard for computation ; and the extreme difference between the 

 calculation and experiment is no less than 2^°, on a reduction of the freezing 

 point from 24-^° to 7-i-°. The freezing point, set down in the table, is that to 

 which the liquor rose on congealing, after having been cooled several degrees 

 lower ; which it is proper to remark, because the ice rose 2 or 3 degrees in 

 thawing. 



The nitrous acid employed was smoking, and had its specific gravity 1.454. It 

 acted on the freezing point according to the following table, which is constructed 

 in all respects like the preceding. 



The greatest difference between the calcula- 

 tion and experiment appears here to be only ■§- of 

 a degree; but that is more than can well be at- 

 tributed to inaccuracy. These mixtures cooled 

 remarkably well ; that in which the water was to 

 the acid as 7 -64 : 1 sunk down to 6° before it 

 froze. The ice heated about a degree before it 

 was melted. 



Freezing 



point by 



calculation. 



25f 

 2l| 

 18 

 11 

 7 



