» La seconde confirmation de la théorie se tire de l’expérience directe pratiquée selon la méthode de différence. Nous pouvons, en refroidissant la súrface de n’importe quel corps, atteindre en tous les cas une température à laquelle la rosée commence à se déposer. Nous ne pouvons, à la vérité, faire cela que sur une petite
the laws of heat, that the contact of the air with a body colder than itself, will necessary lower the temperature of the stratum of air immediately applied to its surface ; and will therefore cause it to part with a portion of its water, which accordingly will, by the ordinary laws of gravitation or cohesion, attach itself to the surface of the body, thereby constituting dew. This deductive proof, it will have been seen, has the advantage of proving at once causation as well as coexistence ; and it has the additional advantage that it also accounts for the exceptions to the occurrence of the phenomenon, the cases in which, although the body is colder than the air, yet no dew is deposited ; by shewing that this will necessarily be the case when the air is so undersupplied with aqueous vapour, comparatively to its temperature, that even when somewhat cooled by the contact of the colder body, it can still continue to hold in suspension all the vapour which was previously suspended in it : thus in a very dry summer there are no dews, in a very dry winter no hoar frost.
