10 Prof. Buff on the Relation between the 



ment with the determinations of the older academicians, of even 

 greater value. 



If we were entitled to employ the equation 



«=0'06479 + 0-0001722T-0-0000001T 2 

 beyond the limits of the experiments from which it was derived, 

 it would follow that a has a maximum at 861° which corresponds 

 to the value a = 0*13892. This maximum, however, is not in 

 the least degree probable. The increase of temperature result- 

 ing from compression of a volume of aqueous vapour in 



Zio + 1 



a completely gaseous condition, measured at t°, amounts to 

 0*277°* ; and it is to be presumed that with increasing tempe- 

 ratures a approaches this limiting value in the same degree as 

 the influence of cohesion decreases. If observations were made 

 on the tension of aqueous vapour at temperatures far above 230°, 

 it would probably be found that a fourth positive term would 

 enter the above equation, wherein T would be raised to the third 

 power. 



The numbers calculated for a are the mean values between 100°, 

 and the chosen temperatures T above or below 100°. 



If the melting-point of ice instead of the boiling-point of water 

 were to be taken as the starting-point for comparison, it would 

 then be necessary to make t equal to 0°, and p = 4S millims. in 

 the equation 



log (273 + T) log (273 + Q 



l0Sa ~ log (273 + OP-log (273 + T>" 

 The values thus found can be represented between 0° and 100° 

 by the equation 



«'=0-05279 + 0-000131 T-0-00000011 T 2 . 

 If we neglect that term in the above equation which contains 

 the second power of T, as of trifling importance within the limits 

 indicated, the formula then signifies that aqueous vapour, satu- 

 rated at 0°, cannot be converted into such at higher tempera- 

 tures by compression, unless we impart heat to it from without 

 in the ratio 0-000131 T to 0-05279, or of 0*248 T to 100. In 

 order, for example, to produce saturated aqueous vapour at 100°, 

 besides the 100 parts of heat set free by compression, 24*8 parts 

 of heat must be supplied from without. 



This variation in the quantity of heat which it contains at 

 different temperatures, explains why the relations between the 

 temperature and the tension of aqueous vapour at different 

 degrees of maximum density cannot be represented in so simple 

 a manner as the same relations in the case of the permanent 



gases. 



* Ann. der Chem. und Pharm. vol. cxv. p. 312. 



