4 Mr. J. H. Alexander on the Tension of Vapour of Water. 



not necessary to the elucidation purposed. Whatever the 

 answers might be, they would not affect the working of a rule 

 which is intended for practicable temperatures; and if it 

 should be objected to the present formula (as it was to the 

 method of interpolation of Dr. Dalton) that it determines a 

 limit which does not exist in nature, or places it where it 

 should not stand, it may be very well replied, that the objec- 

 tion may hold good against the factors without prejudice to 

 the form. It is quite likely that, below 32° F., it is theoreti- 

 cally no longer proper to use in behalf of chilled and freezing 

 vapour, the number (990°) which belongs to boiling steam. But 

 as even here and for nearly 60^ below the melting-point of 

 ice, the actual formula was not so very discordant from the 

 results of experiment, 1 had the less motive for modifying or 

 transforming it into a nearer agreement at this unusual tem- 

 perature. The determination of a limit of this sort, whether 

 real or assumed, is necessary in converting easily a formula 

 like the present, irsto one which will show the pressure in at- 

 mospheres; — a method wliich, as it extricates the results from 

 any dependence on a particular system of weights and mea- 

 sures and thus makes them generally applicable, is of course 

 in any purely scientific investigation much to be preferred. 

 It is obvious that such a scale of atmospheres or volumes must 

 start from the degree where the expansions and the tendency 

 to expand (which is elasticity), so far as they are due to tem- 

 perature, are null. The limit that we have found, then, of 

 105°-13 below 0° Fahr., is such a term ; and the distance be- 

 tween that and the other extremity of the scale, or 317°*13 

 (which is the measure on the thermometer of one atmosphere), 

 becomes the new denominator to replace the 180° actually 

 used for the pressure in inches of mercury. 



In fact, I had expected, in advance, to find the present ma- 

 nometric formula (as it may be termed) becoming a barometric 

 one, by putting it into this shape; 



^ \317-13^Vl695/ / 



But this did not hold good. Applying it numerically, it re- 

 sults in giving, 



For 212°, a pressure of 1-059 atmospheres, equal to 31 "68 inches. 

 And for 322°-38, ... 6263 ... ... 187-37 ... 



In both these instances, to agree with the original formula, 

 the numbers representing the atmospheres should have been 

 without fractions. The excess, however (as is visible), goes 

 on in a converging series, and by and by disappears altogether, 

 the difference then changing its sign. Even then it is not 

 much; and at high temperatures the equation corresponds very 



