Makgill. — Nature's Efforts at Sanitation. 145 



products, while water-animals act as scavengers. So that 

 we find an organically polluted water will in course of time 

 get purified — unless the pollution be excessive. The putrefac- 

 tion and disease-producing organisms die out under the an- 

 tagonism of the normal water-living organisms, just as happens 

 in the earth. The danger is lest we use the water for domestic 

 purposes before the elimination process is complete. Typhoid 

 germs die out of ordinary water standing in flasks in from 

 three days to a fortnight. Much depends en the other organ- 

 isms present. 



Light, also, is an agent of power in Nature's disinfecting 

 process. It has been shown that the numbers of organisms 

 in a stream or lake diminish in the upper layers of water, 

 where light penetrates freely. But the earth is the only re- 

 liable purifier for our water. It acts like the most perfect 

 filter; but it is not a mechanical filtration. The mere strain- 

 ing of impure water through a gravel subsoil does not purify 

 it to any practical extent. It is only in the upper layers — 

 perhaps to a depth of 6 in. to 8 in. — that the work is done by 

 the agency of the bacteria in the humus. Below a depth of 

 5 ft. to 6 ft. the earth is practically sterile, being devoid of 

 these valuable nitrifying organisms. It has been repeatedly 

 shown that outbreaks of typhoid and cholera have been due 

 to sewage percolating through subsoil to sometimes great dis- 

 tances without purification. Some years ago at Worthing, in 

 England, wells 80 ft. deep were polluted by leakage from a 

 cesspit travelling a great distance in a fissure in the subsoil. 

 This resulted in an outbreak of typhoid. This constitutes the 

 danger iu leaking sewers, deep cesspits, and so on. The filth 

 is not subjected to the beneficial action of the humus filter on 

 the surface. The manurial value of decaying organic matter 

 is not to be judged by chemical analysis only. Certain arti- 

 ficial manures may be richer in nitrates than are decaying 

 leaves ; but the latter contain the nitrification agents which 

 enable plants to use the chemical matters present, and, further, 

 there are present certain fungoid growths which live among 

 the roots of green-leaved plants to their mutual advantage, 

 since these fungi can take up chemical matters and oxygen 

 and give off carbonic acid, which the green pigment in the 

 leaves of the larger plants absorb. 



To sum up now, we see that Nature disposes of refuse in a 

 manner both economical and efficient. The highly organized 

 products of animal and vegetable life when dead lie on the 

 surface of the ground, where they are attacked by forces 

 making for disintegration — (1) Animal life, insects, &c, feed- 

 ing on them ; (2) bacteria, which dissolve them and decom- 

 pose them into simpler bodies, and in the process kill out the 

 disease - producing germs ; (3) the humus, acting by means 

 10 



