510 CLARIFIED AND UNCLARIFIED SEWAGE. 



practical point of view, and from numerous experiments 

 have arrived at the conclusion that sewage when " clari- 

 fied " is a most valuable manure for porous or well-drained 

 soils, even when cultivated according to the recognised 

 systems. On the other hand, I have no faith in the value 

 of sewage of any kind, or for any land if used in an " un- 

 clarified" or sludgy state, unless accompanied with a 

 laborious and costly system of cultivation. In order to 

 show clearly the grounds of my preference for " clarified" 

 over "unclarified" sewage, it would seem necessary to 

 advert briefly to the sources whence plants derive their 

 food, and to the conditions favourable to the free use of 

 this food. Plants feed on the air through their leaves, 

 and on water through their roots. And here it is im- 

 portant to bear in mind that the roots of plants can no 

 more absorb solids from the soil than the leaves can 

 absorb solids from the air water and gases are their 

 food they cannot eat like animals, they live by breathing 

 and drinking. Whatever earthy or metallic compounds 

 may be found in the ashes of plants after incineration 

 must have been introduced there in a state of solution, 

 or have been manufactured within the plants themselves. 

 " The water absorbed by the roots contains matters held 

 in solution ; these are deposited in the plant, and remain 

 there with about a third part of the water, the rest escaping 

 almost as pure as distilled water/' Decandolle Physiologic 

 Vegetate, Tome /.,/. 113. Now, our best vegetable physio- 

 logists are of opinion that carbonic acid enters the plant 

 both in the water and the air ; the oxygen is afterwards 

 set free by the agency of solar light, the carbon remaining 

 behind in a solid state. I need not enlarge on the manner 

 in which plants feed through their leaves, because, practi- 

 cally speaking, we cannot influence them much through 

 this channel ; in out-of-door culture at least we influence 

 them chiefly through their roots. To this end 1st, we 

 put on or into the soil such manures as we judge likely 

 or have found by experience to promote the develop- 



