Mr. J. H. Alexander on the Tension of Vapour of Water. 3 



gested too, even unconsciously, from the recollection that it 

 had before (though with different factors) served both Young 

 and Tredgold, and perhaps others, in approximating the re- 

 sults of experiments. Moreover, it was apparent that the 

 numerical results in English inches from the formula were 

 altogether an accidental coincidence, which, dependent upon 

 the properties of numbers, could not be expected to occur 

 upon the use of any other scale than that of Fahrenheit; un- 

 less, indeed, one should adopt the fancy of Mr. Woolf, who 

 supposed that he had discovered an immediate numerical re- 

 lation between the atmospheric pressure and the pound avoir- 

 dupois, in which case the English inch might also claim to 

 be among natural dimensions. It is, of course, quite possible, 

 by a little artifice among the terms of a formula of this shape 

 and retaining the same index, to produce a series of numbers 

 corresponding to any linear scale. Thus, for instance, sub- 

 stituting Centigrade degrees for those of Fahrenheit, but car- 

 rying the denominator of the first term down to the degree at 

 which the pressure becomes by the formula zero (which may 

 be presumed to correspond to an absolute negation of heat, 

 and which in fact has to be used with the present formula 

 when it is intended to give the pressure in atmospheres), we 

 obtain pressures expressed in French metres, and through a 

 range of several atmospheres, but little discordant with the 

 results of experiment. In using Centigrade degrees, how- 

 ever, and transforming the equation so as to express atmo- 

 spheres, the index (6*) gradually diverges from regular mul- 

 tiples ; serving to show what we might otherwise conjecture, 

 that such index is not based upon any general relation in 

 nature. It was, therefore, of less interest for me to weary 

 myself with comparisons of other thermometric and linear 

 scales; it is enough that the formula affords a remarkable 

 coincidence in its own terms with the measures recognized 

 among ourselves. 



It is readily seen that the first term is positive as far as 0° 

 of Fahrenheit ; for temperatures lower than that it becomes 

 negative; and there is a point, of course, where the negative 

 value of the first term equals the constant positive value of the 

 second, and the pressure, therefore, as was said just now, be- 

 comes itself zero. This point occurs at —105°'l 3; of which 

 it is enough to say, that it is not very far from the lowest de- 

 gree of heat yet produced, and that long before it is reached 

 the mercurial thermometer becomes useless. Whether there 

 is in the theory of nature (for it is admitted that there is not 

 in fact) such a point as that of the absolute privation of heat, 

 and if so, how it should be reckoned and where placed, — are 

 questions which, although kindred to the matter in hand, are 



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