1386. Rain. When the vapour which exists in the atmosphere has been precipitated by a sudden reduction of temperature or other causes, it forms a number of 'hollow vesicles or bladders, the coatings of which are inconceivably thin and similar in structure to those usually blown from soap-suds.' These vesicles seem to be all charged with the same kind of electricity, which causes them to repel each other, and as long as this continues to be the case they continue to float in the atmosphere. When the air in which these vesicles float is dry, the vesicles are so much lighter than the air that they rise to a great height, and are dispersed. 'Accordingly,' as Dr. Thomson observes, 'we find that when clouds rise in the atmosphere, they speedily diminish in size, and at last vanish away; being gradually converted again into vapour. If the air within the vesicles were in the same state with respect to moisture as the air in which the cloud floats, the vesicles should be heavier than air, and constitute what we distinguish by the name of fogs.' (Heat and Electricity, p. 274.) If, however, the atmosphere becomes saturated with moisture, the vesicles which form the clouds are forced closer together by the compression, of the air, and additional vesicles are formed, till at last the atmosphere becomes so loaded with vesicles that, as Hutchison observes, any further precipitation of moisture will cause the vesicles to run together, and thus the vesicular form will be destroyed, and the increased gravity which each integrant particle of moisture acquires will cause it to descend rapidly in the form of a drop of rain. When the air is still and the precipitation of humidity into the vesicular form continues to go on slowly and regularly, what is called a drizzling rain will be produced; but 'every increase in the rapidity with which the precipitation of moisture into the vesicular form goes on, by correspondingly accelerating the running together of the vesicles, will augment the size of the drops of rain, and the amount that falls in a given time. During windy weather, rain can never assume the drizzling form. The agitation of the atmosphere in such circumstances favours the uniting of the integrant particles of moisture so much, that they never can reach the surface of the earth except in drops of considerable size.' (Meteorological Phenomena, p. 176.)
