Variation in the Pressure of Saturated Vapours. 53 



insufficiently accurate. Let us apply equation (8) to aqueous 

 vapour and take, at random, temperatures 40°, 45°, and 50°, 

 If we take the temperature of absolute zero as —273°, then 

 these temperatures counted from the absolute zero will be 

 313°, 318°, and 323°, and the corresponding pressures 

 calculated by Broch* according to Regnaulfs observations, 

 54-8651 miliim., 71-3619 millim., and 91*9780 millim. On 

 substituting these figures in equation (8) we find 



# 46 =:3-9542. 



In this, and in all further calculations, x refers to the inter- 

 mediate temperature, and we now mark # 45 by the number 

 45. In a like manner we find for temperatures 45°, 50°, and 

 55°, 



# 5 o=4-3293. 



Below is a table in which the upper line contains the inter- 

 mediate temperatures (t) and the lower the corresponding 

 values of x : — 



t 45° 50° 55° 60° 65° 70° 75° 80° 



x 3-9542 4-3293 4-6800 5-0031 5-2109 5-2948 5-1997 3-8844 



In this series of figures the greatest value of x is 5*2948, 

 corresponding to 70° ; x diminishes with a rise and fall of 

 temperature, at 15° it is 2-0640 and at 125° 3'4263. Hence 

 it is evident that near 70° aqueous vapour follows the laws of 

 Boyle and Gay-Lussac ; L e. satisfies the equation 



^=DT. 



If in the equation 



AD a 

 we replace x and AD by their values, we have 



c- Cl =x . AD = 5*2948 . | =5*2948 . ^ =0*58831. 



The specific heat c as determined by various experimenters 

 shows a great diversity. According to Eegnault c at 70° 

 equals 1*0072, while according to Hendrichsen it is 1*0419. 

 The value of c x was determined by Eegnault for temperatures 

 above 100°, at 70° it is probably less. Thus, according to 

 Eegnault c—c-l =0*5167, and according to Hendrichsen it is 

 0*5614. The latter more nearly approximates to that found 



* Pliysikalisch-chemische Tdbellen von Landolt mid Bornstein, § 18. 



