Manchester Mejiioirs, Vol. Iv. (191 1), No. 5. 5 



^general, though not the universal opinion was, that this 

 vapour was present in a state of combination with the air. 

 The evaporation of water was thought to be an act of 

 ■chemical combination between air and water, whilst 

 boiling was a physical action. For since the atmospheric 

 pressure prevents water from boiling at ordinary tem- 

 peratures, it was thought that boiling was something 

 ■quite distinct from evaporation, which takes place at all 

 temperatures and pressures of the air. This distinction 

 had received the sanction even of Lavoisier." 



Dalton's theory had a special bearing on this subject. 

 For the theory meant that the pressure of a mixture of 

 gases is the sum of the respective pressures of the gases 

 in the mixture. Dalton saw that the water vapour in the 

 atmosphere had to be considered in terms of the pressure 

 ■of the vapour. Experimentally he showed that the 

 evaporation of water is proportional to the pressure of 

 the vapour which the water gives off. At any given 

 temperature there is a maximum which this pressure can 

 reach, and water, whether in contact with the air or not, 

 •can evaporate till the pressure of its vapour reaches this 

 maximum and no further. On the other hand, air in 

 which the water vapour is not at this maximum pressure 

 can be cooled till the maximum is reached, and then, on 

 further cooling, the water is deposited as dew. This led 

 to observations of the " dew-point," which Dalton was the 

 iirst to institute. 



It was thus in the direction of Meteorology that 

 Dalton's theory first bore fruit. In this science, as Play- 

 fair has pointed out, it is easier than in any other to 

 ^' accumulate observations, and more difficult to ascertain 

 principles." At the beginning of the nineteenth century, 

 hy pointing out the significance of the dew-point, Dalton 

 * " Traite Elementaire de Chimie," 3rd ed., pp. 7-1 1, 39, 



