DAVIS. — CERTAIN THERMAL PROPERTIES OF STEAM. 305 



The necessary values of the specific heat of water, c, are taken from 

 Marks and Davis' Steam Tables. Above 100° they are based on the 

 experiments of Dieterici which run to 303°. The Table shows that 

 C S3t . passes its maximum below 250° without becoming zero or positive, 

 and that at 300° it is already well on its way toward the value minus 

 infinity which it has at the critical point. The temperature-entropy 

 diagram for steam (see Figure la) is, therefore, fundamentally differ- 

 ent in shape from that of ether or chloroform. 



6. The Critical Volume of Water. 



The extrapolation formula for L also makes possible a computation 

 of the critical volume of water by the method of Cailletet and Mathias. 

 These investigators announced in 1886 50 their well-known "law of 

 the straight diameter," according to which, if the densities of a liquid 

 and its saturated vapor are plotted against the corresponding tem- 

 peratures to form a steam dome, the mid-points of the horizontal 

 chords of the dome lie in a straight line. This law has been tested by 

 a number of observers, 51 but particularly by Young, who proved that 

 the diameter is accurately straight only in the case of a few " normal " 

 substances of which normal pentane is the best known example, but 

 that it is always nearly straight and can almost always be represented 

 within the limit of error of the observations by a second degree 

 equation in t. . In certain cases, notably acetic acid and the alcohols, 

 a third degree equation is necessary. All departures of the diameter 

 from perfect straightness are commonl^ attributed to association in 

 the liquid. 



If the equation of the diameter is known, the substitution in it of the 

 critical temperature gives the critical density with an accuracy far 

 surpassing that of any known method of direct measurement. This 

 accuracy is greatly increased by the fact that the diameter is always 

 so nearly parallel to the t axis that even a considerable error in the 

 critical temperature makes very little difference in the critical volume. 



In applying this method to the determination of the critical density 

 of water, one finds available in the third (1905) edition of Landolt 

 and Bornstein's " Physikalische Tabellen " a satisfactory set of values 



60 C. R., 18S6, 102, 1202. 



61 Mathias, Ann. de la Fac. des Sci. Toulouse, 1892, 6, Ml ; C. R., 1892, 

 115, 35; Mem. Soc. Roy. des Sci., Liege, 1899, 2; Journ. de Phys., 1899, 8, 407; 

 and 1905, 4, 77 ; Young, Journ. Chem. Soc., trans., 1893, 63, 1237 ; Phil. Mag., 

 1892, 34, 503; and 1900, 50, 291 ; Guye, Archives des Sci. Phys. et Nat., 1894, 

 31, 43; Tsuruta, Phys. Rev., 1900, 10, 116. See also Young's " Stoichiome- 

 try," 1908, 165. 



VOL. XLV. — 20 



