There is an atmospherical appearance which is not explained by the known laws of heat and cold. It is the breath of animals becoming visible, in being expired into an atmosphere which is cold or moist; and the transformation of transparent steam into the state of mist, when mixed with air which is of a colder temperature. Natural philosophers have certainly considered these appearances as being explained in the general law by which heat and cold are communicated among contiguous bodies, otherwise they would have endeavoured to point out this particular law, which seems to depart from a more general rule, or does not follow the natural course of things observed on other occasions. The subject of this paper is to investigate a certain rule which, in the case now mentioned, may be discovered as directing the action and effects of heat and cold; and to form a theory of rain upon that investigated rule, concerning the evaporation and condensation of water.