You will not, therefore, so please you, expect me to explain anything to you,--I have come solely and simply to put before you a few facts, which you can't see by candlelight, or in railroad tunnels, but which are making themselves now so very distinctly felt as well as seen, that you may perhaps have to roof, if not wall, half London afresh before we are many years older.
I go back to my point--the way in which clouds, as a matter of fact, become visible. I have defined the floating or sky cloud, and defined the falling, or earth cloud. But there's a sort of thing between the two, which needs a third definition: namely, Mist. In the 22d page of his 'Glaciers of the Alps,' Professor Tyndall says that "the marvelous blueness of the sky in the earlier part of the day indicated that the air was charged, almost to saturation, with transparent aqueous vapor." Well, in certain weather that is true. You all know the peculiar clearness which precedes rain,--when the distant hills are looking nigh. I take it on trust from the scientific people that there is then a quantity--almost to saturation--of aqueous vapor in the air, but it is aqueous vapor in a state which makes the air more transparent than it would be without it. What state of aqueous molecule is that, absolutely unreflective[12] of light--perfectly transmissive of light, and showing at once the color of blue water and blue air on the distant hills?
I put the question--and pass round to the other side. Such a clearness, though a certain forerunner of rain, is not always its forerunner. Far the contrary. Thick air is a much more frequent forerunner of rain than clear air. In cool weather, you will often get the transparent prophecy: but in hot weather, or in certain not hitherto defined states of atmosphere, the forerunner of rain is mist. In a general way, after you have had two or three days of rain, the air and sky are healthily clear, and the sun bright. If it is hot also, the next day is a little mistier--the next misty and sultry,--and the next and the next, getting thicker and thicker--end in another storm, or period of rain.
I suppose the thick air, as well as the transparent, is in both cases saturated with aqueous vapor;--but also in both, observe, vapor that floats everywhere, as if you mixed mud with the sea; and it takes no shape anywhere: you may have it with calm, or with wind, it makes no difference to it. You have a nasty haze with a bitter east wind, or a nasty haze with not a leaf stirring, and you may have the clear blue vapor with a fresh rainy breeze, or the clear blue vapor as still as the sky above. What difference is there between _these_ aqueous molecules that are clear, and those that are muddy, _these_ that must sink or rise, and those that must stay where they are, _these_ that have form and stature, that are bellied like whales and backed like weasels, and those that have neither backs nor fronts, nor feet nor faces, but are a mist--and no more--over two or three thousand square miles?