564 HISTORY OF THERMOTICS. 



plains the fact that pressure will condense vapour, 

 by supposing that it brings the particles within 

 the distance at which the repulsion arising from 

 fire ceases. In this way, he also explains the fact, 

 that though external pressure does thus condense 

 steam, the mixture of a body of air, by which the 

 pressure is equally increased, will not produce the 

 same effect ; and therefore, vapours can exist in 

 the atmosphere. They make no fixed proportion 

 of it ; but at the same temperature we have the 

 same pressure arising from them, whether they are 

 in air or not. As the heat increases, vapour be- 

 comes capable of supporting a greater and greater 

 pressure, and at the boiling heat, it can support 

 the pressure of the atmosphere. 



De Luc also marked very precisely (as Wallerius 

 had done) the difference between vapour and air: 

 the former being capable of change of consistence 

 by cold or pressure, the latter not so. Pictet, in 

 1786, made a hygrometrical experiment, which ap- 

 peared to him to confirm De Luc's views ; and De 

 Luc, in 1792, published a concluding essay on the 

 subject in the Philosophical Transactions. Pictet's 

 Essay on Fire, in 1791, also demonstrated that "all 

 the train of hygrometrical phenomena takes place 

 just as well, indeed rather quicker, in a vacuum 

 than in air, provided the same quantity of moisture 

 is present." This essay, and De Luc's paper, gave 

 the death-blow to the theory of the solution of 

 water in air. 



