308 HEAT. 



The Volume of Saturated Steam. The latent heat equation at once 

 gives us the volume of a saturated vapour at any temperature at which 

 we know the latent heat, the liquid volume and the rate of change of 

 the vapour-pressure. Thus, for water and steam at 100 we have 



L = 537 x 4-19 xlO 7 

 6 = 373 

 v l =1-043 



Pim-5 77 '371 cm. of mercury 

 = 74-650 



whence -^ = 2'721 cm. of mercury 



(W 



= 2-721 x 13-596 x 981 dynes per sq. cm. 

 Substituting in equation (1), we have 



= 1 -043 4- 537 x 4-19 x 10 7 



373x~2T21 x 13-596 x 981 

 = 1663 c.c. 



Fairbairn and Tate (Phil. Tra?is., cl., 1860) found by direct experi- 

 ment that v 2 =161 6. While there is some uncertainty about the 

 thermodynamic result depending on the uncertainty in the values of the 

 constants, it is easily seen that we cannot accept so low a result as that 

 obtained by Fairbairn and Tate, which was probably vitiated by the 

 adherence of steam to the sides of the containing vessel. The thermo- 

 dynamic method is preferable here to direct methods.* 



It is interesting to compare the value 2 =1G63 with that which 

 steam would have if it followed the same law of expansion as air. We 

 know from Regnault's researches t that the density of steam at low tem- 

 peratures and pressures is 0*623 of the density of air almost exactly 

 the value obtained on the supposition that two volumes of hydrogen 

 unite with one volume of oxygen to form one volume of steam. If it 

 maintained the same relative density at 100 0. and 760 mm., its volume 

 would be 



GIG i rtf\n 



x ^- =1696 



0-001293x0-623 273 



which is certainly greater than the actual volume. Hence steam does 

 not expand according to the laws of Boyle and Charles at higher tem- 

 peratures. 



The Change of Melting-Point under Pressure. Taking the latent heat 

 equation we have, for water-ice 



L = 80x4-19xl0 7 

 0=273 

 0.-1-09 

 4-1 



whence |= - 8 -? 2 ***! - 136500000 dynes/cm. 



* llamsay and Young, " The Properties of Water and of Steam," Phil. Trans. 

 A., 1892, p. 107. 



+ Ann. de Chim. et de Phys., 3rd series, t. xv. p. 141. 



