When the evaporation of sea water is quickened by an elevation of temperature, as in the South of France, noxious and unpleasant odours, injurious to vegetable life, are distinctly perceptible. The putrescence and fermentation caused by heat acting on the remains of life in sea water left to evaporate, as between Rio and Cape Frio, in the Brazils, seem to be the cause of, or at least to give terrible effect to, yellow fever. Vegetable life is equally abundant, and it may be as injurious when decomposing in its effects on human life. Lichens speedily cover the walls of neglected houses, and cause sickness by their decomposition. The spore or sporule, which in flowerless plants performs the office of seeds, floats in the atmosphere, and seems to be the cause of the hay-fever so frequent in fertile lowlands. Nor need we quote the recent drainage of the Lake of Haarlem in proof of the sure results of exposing masses of dead animal and vegetable substances to putrefaction--namely, ague, various fevers, and other ailments indicative of a poison or malaria affecting the general mass of the blood. Of the minuteness of animal life, it is only necessary to remark that we are acquainted with animals possessing teeth and organs of motion, which are wholly invisible to the naked eye. Other animals exist which, when measured, are found to be many thousand times smaller, and which nevertheless possess the same apparatus. Their ova must be many hundreds of times still smaller. It is to this invisible world in all probability, and to its decomposition and putrefaction, or at least to influences arising therefrom, that the essential cause of ague, and other febrile diseases of an intermittent and remittent character may be referred, aggravated, no doubt, by insalubrious atmospheric constitutions of which we know nothing. These from time to time affect and lower human vitality--a fact admitted by all physicians. * * * * * NOTE ON THE QUESTION OF QUARANTINE. (See Chapter IV.)