The two rival theories of evaporation, that of chemical solution and that of independent vapor, were, in various forms, advocated by the next generation of philosophers. De Saussure may be considered as the leader on one side, and De Luc on the other. The former maintained the solution theory, with some modifications of his own. De [168] 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 thermometrical 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[46] 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 capable of supporting a greater and greater pressure, and at the boiling heat, it can support the pressure of the atmosphere.
[46] Fischer, vol. vii. p. 453. Nouvelles Idées sur la Météorologie, 1787.
De Luc also marked very precisely (as Wallerius had done) the difference 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 hygrometrical 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 essay, 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. [169] Girtanner,[47] in his Grounds of the Antiphlogistic Theory, may be considered as one of the principal expounders of this view of the matter. Hube, of Warsaw, was, however, the strongest of the defenders of the theory of solution, and published upon it repeatedly about 1790. Yet he appears to have been somewhat embarrassed with the increase of the air’s elasticity by vapor. Parrot, in 1801, proposed another theory, maintaining that De Luc had by no means successfully attacked that of solution, but only De Saussure’s superfluous additions to it.
[47] Fischer, vol. vii. 473.
It is difficult to see what prevented the general reception of the doctrine of independent vapor; since it explained all the facts very simply, and the agency of air was shown over and over again to be unnecessary. Yet, even now, the solution of water in air is hardly exploded. M. Gay Lussac,[48] in 1800, talks of the quantity of water “held in solution” by the air; which, he says, varies according to its temperature and density by a law which has not yet been discovered. And Professor Robison, in the article “Steam,” in the Encyclopædia Britannica (published about 1800), says,[49] “Many philosophers imagine that spontaneous evaporation, at low temperatures, is produced in this way (by elasticity alone). But we cannot be of this opinion; and must still think that this kind of evaporation is produced by the dissolving power of the air.” He then gives some reasons for his opinion. “When moist air is suddenly rarefied, there is always a precipitation of water. But by this new doctrine the very contrary should happen, because the tendency of water to appear in the elastic form is promoted by removing the external pressure.” Another main difficulty in the way of the doctrine of the mere mixture of vapor and air was supposed to be this; that if they were so mixed, the heavier fluid would take the lower part, and the lighter the higher part, of the space which they occupied.
[48] Ann. Chim. tom. xliii.
[49] Robison’s Works, ii. 37.