If these facts are to be depended on, they explain much of the influence which man exercises over the soil, and of its reaction on himself; the hay ague or fever is the produce of his own hands; when he leaves _on the surface_ millions of tons of fermentable and putrescible organic remains, he prepares for himself some at least of the diseases which are to follow. It is possible that epidemic influence, over which he neither has nor can have any control, might be greatly modified, and its evil effects abated by prudent action on his part. Typhus fever, the scourge of modern Europe, may not originate in any condition of the soil produced by man, but it sweeps thousands in the prime of life from the earth when placed in circumstances clearly dependent on man himself. Ten thousand young men are lodged in a barrack; speedily hundreds of these are swept off by typhus or consumption of the lungs; now something causes this, and the cause may rest with man himself. Pestilence and typhus follow in the train of famine; if they originate in fermentescible and putrescible substances, all these were present prior to the famine, and yet were not equal to the production of the maladies. Next comes famine, and prepares the way for malaria to do its work. The question, as may be already seen, is not so simple as chemists supposed it to be. The number of substances occurring in nature which are truly putrescible is singularly small;[39] but they are everywhere diffused, and form part of every organized being. To form an idea of what this amounts to, we have but to reflect on the life which naturally exists on the earth, and on that which is the result of man’s social condition. Let but the acre of heath or bog, even of pasture, which in its natural state supports so little of what lives, be converted into a garden, a wheat field, a nursery, and see what an amount of putrescible matter is the result. Let that spot on which nature has placed a single peasant’s family be converted into a city, and reflect on the influence man exerts on that soil. It is, I believe, a fact universally admitted, that all those substances which destroy the communicability or arrest the propagation of contagions and miasms, are likewise such as arrest all processes of putrefaction or fermentation; that under the influence of empyreumatic bodies, such as pyroligneous acid, which powerfully oppose putrefaction, the diseased action in malignant suppurating wounds is entirely changed; that in a number of contagious diseases, especially typhus, ammonia, free or combined, is found in the exposed air, in the liquid and solid excreta (in the latter as ammonio-phosphate of magnesia); such being the case, it seems impossible any longer to entertain a doubt as to the origin and propagation of many contagious diseases. [39] Liebig.