These castings are full of microbes, and those who will take the trouble to scatter the smallest conceivable pinch of this impalpable dust upon a sterilised potato, after the manner and with all the precautions familiar to bacteriologists, will obtain an abundant and varied growth of bacteria and moulds, which will completely baffle their powers of enumeration and discrimination.
The greatest hindrance in the bacterial examination of the soil is this embarras de richesses, which makes the isolation of different species a matter of extreme difficulty.
The bacteria exist in the soil in countless millions, but it must be remembered that they get fewer as we go deeper. The first few inches of the soil are, in the matter of bacterial richness, worth all the rest, and at a depth of five or six feet they appear to be almost non-existent. The practical lesson which we have to lay to heart in applying this knowledge is that the upper layers of the soil are the potent layers in bringing about the circulation of organic matters, and that if we wish to hasten this process we must be careful to place our organic refuse near the surface, and not to bury it deeply, a process by which the circulation is inevitably delayed or practically prevented. If we bury it deeply we not only get no good, but we may get harm by poisoning our wells and springs.
It is the same with organic liquids. If these be poured on the surface, the 'living earth' (i.e. the humus stuffed with animal and microbial life) purges them of their organic matter, and transmits a relatively pure liquid to the deeper layers. If they be taken to the barren subsoil direct, as in underground sewers and cesspools, they escape the purifying action of air and aërobic organisms, and inevitably poison the water. Filthy liquids accumulating in cesspools and leaking under pressure to our wells have cost us health and money incalculable.
Liquids poured upon the surface cannot, owing to the crumby nature of the humus, exert any appreciable hydraulic pressure. This is a fact of huge importance in the practical management of organic refuse.
All effete organic matter instantly becomes the prey of animals and plants. The dead body of an animal teems with life—Le roi est mort, vive le roi. M. Mégnin, a skilled entomologist and a member of the French Academy of Medicine, has made a study, which is full of gruesome interest, of the living machinery which makes away with the bodies of animals not buried but exposed to the air and protected from beasts of prey.
M. Mégnin shows that the destruction of the animal is accomplished in no haphazard fashion, but that successive squadrons of insects are attracted by the successive stages of putrefaction.
The first squadron which arrives, sometimes before death and always before putrefaction, consists entirely of dipterous insects, house-flies and their relative, the blow-fly.
The next squadron are also diptera, and are said to be attracted by the commencing odour of decomposition. These squadrons use the carcase as a procreant cradle, and thus ensure the nourishment of the larvæ so soon as they are hatched. Amongst these flesh-seeking flies there are said to be specialists which prefer the flesh of particular animals.