168 HISTORY OF THERMOTICS. 



Luc denied all solution, and held vapor to be a combination of the 

 particles of -water with fire, by which they became lighter than air. 

 According to him, there is always fire enough present to produce this 

 combination, so that evaporation goes on at all temperatures 



This mode of considering independent vapor as a combination of 

 fire with water, led the attention of those who adopted that opinion to 

 the therrnometrical changes which take place when vapor is formed 

 and condensed. These changes are important, and their laws curious. 

 The laws belong to the induction of latent heat, of which we have 

 just spoken ; but a knowledge of them is not absolutely necessary in 

 order to enable us to understand the manner in which steam exists in 

 air. 



De Luc's views led him 8 also to the consideration of the effect of 

 pressure on vapor. He explains the fact that pressure will condense 

 vapor, 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, vapors 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, vapor becomes 

 i.-apable 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 dif- 

 ference between vapor and air ; the former being capable of change of 

 consistence by cold or pressure, the latter not so. Pictet, in 1786. 

 made a hygrornetrical experiment, which appeared 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 

 <>say, and De Luc's paper, gave the death-blow to the theory of the 

 solution of water in air. 



Yet this theory did not fall without an obstinate struggle. It was 

 taken up by the new school of French chemists, and connected with 

 their views of heat. Indeed, it long appears as the prevalent opinion 



8 Fischer, vol. vii. p. 453. Nouvelles Idles sur la 3fcteorologif, 1*787. 



