We have, in several articles, already recorded our belief that the globules constituting the clouds are not hollow, and our disbelief of the existence of what has been termed vesicular vapour,' that is, of vesicles of water (whether containing vapour or not), resulting from the condensation of invisible vapour or water in its pure gaseous con dition. Sir J. F. W. Herschel (' Meteorology,' par. 92,) after describing the production of mist or fog [MisT] by the precipitation of moisture from the atmosphere, offers the following important observation on the subject in question.
" It is a favourite dogma with many meteorologists, that the particles so precipitated assume the form, not of drops, but of hollow spheres or bubbles. De Saussure states that he has seen suck floating before his eyes in clouds and fogs on the Alps ; and the dusty appearance of the vapour floating over a cup of coffee in the sunshine is adduced in proof. The strongest argument in their favour, however (for there is great room for optical illusion in such matters), is that adduced by Kratzenstein, that the sun striking on a fog or cloud [t] produces no rainbow, which it ought to do were the water collected in spherical drops. This argument does not admit of a ready answer ; but the difficulty, on the other hand, of conceiving any possible mode in which such bubbles can be formed, disposes us to believe that the extreme minuteness of the globules may perhaps he found to afford one, their diameters being probably of an order comparable with the breadths of the luminiferous undulations."* In the steam-cloud issuing from the boiler of a locomotive engine, whether immediately above the funnel, when blowing off, or at a dis tance from it, or in the puffs of steam given off during the course of the engine, no rainbow is ever visible, as the present writer can testify from observation : they appear to be physically identical with the clouds of the sky. With this agrees another point, the production in all these cases of opalescent vapour [VAPOUR, OPALESCENT; WEATHER], which also is always caused in the clouds of nature ; intervening, as a middle term, in the conversion of invisible vapour into cloud, and in the resolution of cloud into invisible vapour. Its occurrence in both processes may be observed in and about the clouds at almost all times during the presence of the sun, or just before or after it; but less fre quently, and only under favourable circumstances, in moonlight. The
solar radiation, and the capacity of the air for more vapour on and above the upper surfaces of clouds, often render the intervention of opalescent vapour invisible in that situation, but it may always be seen in the reverse process, in the vapour-plane or vapour-zone [VAPOUR PLANE] at the base of masses of clouds, especially of cumuli. But the production of opalescent vapour does not occur with the spray of water falls or waves. The reason seems obvious. The clouds of the atmo sphere and of the steam-engine consist of globules so minute that very slight local changes of temperature will effect their resolution, and so near to each other that the amount of opalescent vapour produced by every one, individually, coalesces into an aggregate mass for all, so that, though only momentary for each globule, it is persistent for the mass, and therefore becomes visible. But the particles of the spray are, comparatively, so large, that much greater differences of tempera ture are required fur their conversion into invisible vapour, and also so distant, comparatively, from each other, that the local momentary opalescence ceases, before, by the coalescence of that produced at many points, it can become visible.
We may safely conclude, therefore, it would appear, from all these facia and arguments, and in corroboration of opinions generally but not universally held, that the clouds are really composed of excessively minute globules of liquid water, not hollow, and not consisting in any respect of vapour (though necessarily intermingled with it), which, having resulted from the condensation of volumes of continuous vapour, or of vapour no otherwise discontinuous than as occasioned by the uniformly interspersed particles of the air, are formed at insensible distances from each other ; and that these at length (under circum stances not altogether understood, and whether related primarily to heat or to electricity is also uncertain) coalesce into much larger glo bules, or drops, exceeding in diameter the breadths of the luminiferous undulations, and therefore capable, like the drops of waterfall- and wave-spray, of exhibiting the rainbow.