52 K. D. Kraevitch on an Approximate Law of the 



If x has no maximum or minimum, then the vapour does not 

 possess the properties of a perfect gas at any temperature, 

 within the limits of observation, and the theory is not applic- 

 able to it. Then y does not vary proportionally to the tem- 

 perature. 



To calculate the values of x and y it is sufficient to have two 

 equations and three observed tensions corresponding to three 

 temperatures. For the sake of simplicity let us take equi- 

 distant temperatures : — 



T-T =T -T 1 = A. 



We shall thus have two equations of the first degree with two 

 unknown quantities : — 



-(log|-m.^j}.+(^- jr)»*=]ogjJ 

 T,' T'-T„\ , / 1 1 \ , p' 



On solving them we obtain 



T.log^-T'log^ 

 J-i J-o 



X — m rrv> \P) 



± rn/i „-*- 



(log J. log f -log £ log f-)w 

 (Tilog^-T'log^j.A 



To obtain x to a sufficient degree of accuracy, the three 

 temperatures for which this value is calculated must be taken 

 as near as possible to one another, because the direction of 

 the curve of x's can only be looked upon as parallel to the 

 axis of abscissae, near the maximum and minimum, for an 

 inconsiderable distance, Moreover, this requires exceedingly 

 careful observations of the vapour-pressures, as otherwise the 

 series of values of x present such irregularities as to prevent 

 the possibility of distinguishing the maximum or minimum of 

 this quantity. These irregularities become less perceptible as 

 the difference of temperature increases and the order of the 

 variation of the #'s becomes clearer ; but in this case, accord- 

 ing to what has been said above, the maximum or minimum 

 value of x and the corresponding temperature may prove 



