74 

 TABLE V. 



WEIGHT OF VAPOR, IN GRAMMES, 



CONTAINED IN A CUBIC METRE OP SATURATED AIR UNDER A BAROMETRIC PRESSURE OP 

 760 MILLIMETRES, AND AT TEMPERATURES BETWEEN 20 AND -j-40 CENTIGRADE. 



THE theoretic density of aqueous vapor is very nearly 0.622, or f , of the density of 

 the air at the same temperature and pressure. Regnault's experiments gave similar 

 results. From this ratio the weight of the vapor contained in a given volume of air, 

 the temperature and humidity of which are known, can be computed. 



If we call 



t = the temperature of the air ; 



/= the elastic force of the vapor contained in the air at the time of the observation ; 

 F the maximum elastic force of vapor due to the temperature Z, as given in the 



table ; 

 p the weight of the vapor contained in a litre of air at the temperature Z, and with 



a force of vapor fj 

 P = the weight of vapor in a litre of air at the temperature t, and at full saturation, 



or F. 

 rrn, * * 1.293223 r - 



Then, p = 0.622 



In which 1.293223 grammes is the weight of a litre of dry air, at the temperature 

 of zero Centigrade, and under a barometric pressure of 760 millimetres, according to 

 the determination of Regnault ; 0.00367, the coefficient of the expansion of the air 

 as found by the same ; 760 millimetres, the assumed normal barometric pressure. 



The weight of a litre of air given by Regnault in the Memoires de Plnstitut, Tom. 

 XXI. p. 157, is 1.293187 grammes ; but by correcting a slight error of computation 

 (see E. Ritter, Memoires de la Societe Physique de Geneve, Tom. XIII. p. 361), it be- 

 comes, as given above, 1.293223 grammes. 



In order to obtain the weight of vapor in a cubic metre, or 1000 litres, of saturated 

 air, the formula becomes, 



P = 622 - 293 : 223 - F 



l_j_ 0.0036 7 t ' 760"""-' 



From this formula Table V. has been computed. The tensions due to the tem- 

 peratures in the first column are placed opposite the weights of vapor ; they are 

 taken from Table I. It will be seen that, throughout the table, the number of 

 grammes of vapor nearly corresponds to the number of millimetres of pressure ex- 

 pressing the tension. 



The table of the weights of vapor given in Pouillet's Elements des Physique, Tom. 

 II. p. 707, being based on older values, gives results somewhat different. In that pub- 

 lished by Becquerel, Elements de Physique Terrestre, p. 354, Regnauh's tensions and 

 coefficient of expansion of the air have been used, but the value of the weight of 

 vapor in a litre of air formerly determined by Biot and Arago, viz. 1.29954 grammes, 

 has been retained. 



B 38 



