EXPERIMENTAL KNOWLEDGE OF THE PROPERTIES OF MATTER. Ill 



From 0° to 100° the formula 



log F = a + So* — c/3*. 

 From 100° to 230° 



log F = a — ha^ — o/j'^ ; where x = t + 20°. 



The formulae used here are of the form proposed by Biot. 



The mere fact of choosing different formulae for different parts of the 

 curve of vapour -pressures, and of choosing these formulfe from amono- 

 ether exponential formulje, shows that these are empirical formulae ; and 

 from Regnault's experimental results different systems of formulfe and 

 interpolation give tables differing slightly from Regnault's. Compare, 

 e.g., the tables given by Landolt and Bornstein, pp. 40-49, with Reo-nault's. 



Rankine has since suggested the formula log F = a — —. 



Magnus ' made detei-minatipns up to 111° with results agreeing 

 closely with those of Regnault. 



In ' Phil. Trans.' for 1860, p. 220, are given the results of experiments 

 by Fairbairn and Tate, for the pressure and temperature of saturated 

 vapour of water, and for each tem^^erature the ratio of the volume of 

 steam to that of the water for temperatui'es ranging from about 58° to 144°. 



The Statical and Dynamical Methods of finding the Relations letween Tem- 

 perature and Vapour-pressure. 



Two distinct methods of finding the relation between pressure and 

 temperature of saturated vapours are used, one by readings of the pres- 

 sures of the vapour over the liquid in a mercury- vacuum at known 

 temperatures, and the other by the readings of a thermometer immersed 

 in the vapour of a liquid boiling at known artificial atmospheric pressures. 



The first of these methods is called the statical, and the other the 

 dynamical method ; ^ the former method is only applicable to moderate or 

 low temperatures, at which as at 50° the vapour-pressure of mercury is 

 inconsiderable ; the latter may be applied at high temperatures. Exam- 

 ples of both are given in the case of steam,^ and the two methods are 

 found, when both are employed, to yield identical results for water. 



In t. xxvi. p. 642, Regnault says : ' It is not evident a priori that for a 

 given substance the two methods (static and dynamic) give the same rela- 

 tion between elastic forces and temperatures. The boiling of a liquid is 

 in fact a very complex phenomenon. The vapour which escapes from a 

 boiling liquid has not only to contend against the elastic atmosphere 

 which pi-esses on the liquid, it has to overcome the attraction which the 

 liquid exerts on the molecules which have taken the gaseous state or which 

 tend to take it ; it has to overcome the capillary resistance of the liquid 

 walls, which form globules, more or less easily extensible, in which the 

 vapour is imprisoned while it traverses the liquid, &c., &c. These acces- 

 sory resistances can only be overcome by an excess of heat, and there is 

 the fear that the vapour may, on emerging from the liquid, possess at the 

 same time an excess of elastic force (pressure) and an excess of tempera- 

 ture. The two excesses may neutralise each other and disappear, more or 



' Poggendorff's Annalen, Ixi. p. 225. 

 * Memoir es, t. xxvi. p. 341. 

 ' Ibid., t. xxi. 



