12. Mist

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 22nd page of his Glaciers of the Alps, Professor Tyndall says that “the marvellous blueness of the sky in the earlier part of the day indicated that the air was charged, almost to saturation, with transparent aqueous vapour.” 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 vapour in the air, but it is aqueous vapour in a state which makes the air more transparent than it would be without it. What state of aqueous molecule is that, absolutely unreflective of light—perfectly transmissive of light, and showing at once the colour of blue water and blue air on the distant hills?