546 CARBONIC ACID. 



feed plants, and most abundantly, is proved by evidence that cannot be 

 controverted. 



" The great utility of charcoal and wood ashes," writes a correspondent 

 oi ^6 Gardeners^ Chronicle, "being admitted on all hands for gardening 

 purposes, I would direct attention to the necessity of a somewhat syste- 

 matic course of procedure in the mode by which it is made. Whilst 

 the felling of trees, the ' stocking' of hedges, or thinning of woods are 

 proceeding, is the time to lay in a considerable stock for the year. The 

 process of burning is most simple. I begin by burning all the largest 

 of the brush as a centre of operations ; following up with the smaller 

 wood ; and when in a due state of combustion, covering the whole with 

 a rough refuse of the kitchen garden, which has been twelve months in 

 collecting. Finally, a coating of turves or soil — double if turves ; the 

 latter being reserved for prime potting purposes. The material thus 

 managed will furnish large masses of charcoal for Orchids, &e. ; smaller 

 lumps for drainage to pots ; and wood-ash in abundance for dressing 

 seed-beds, for any plants which require fresh material." 



Being heavier than atmospheric air, carbonic acid has a 

 constant tendency to fall to the earth and to settle down among 

 its crevices, even if it is not carried thither in water. Hence 

 we find it abundantly in wells, drains, old sewers, and similar 

 places, in which, if moisture be present, roots develope with 

 prodigious rapidity. (See page 20.) 



Boussingault and L6wy have ascertained experimentally that the 

 quantity of carbonic acid in the soil is very much beyond what has 

 been supposed, especially if it has been recently manured, as appears 

 from the following statement : — 



In its normal condition atmospheric air contains ,0004 in volume of 

 carbonic acid. In soil, on the contrary, twelve months after the appli- 

 cation of manure, from 22 to 23 times as much were found, and in land 

 recently manured as much as 245 times in weight. If, however, the 

 object is to ascertain the quantity of carbonic acid that is placed at the 

 disposal of the plants growing in the soil, the proportion contained in 

 the air confined in its pores wiU not suffice. It is necessary, then, to 

 know the quantity of air in a given volume of earth. This volume may 

 be easily estimated by saturating the soil with water, as the volume of 

 air displaced will exactly equal the volume of water introduced. Some 

 of the main results of the experiments, instituted for this second object, 

 are stated by the authors as follows : — 



1. The air inclosed in a hectare (10,000 square metres, 11960,33 

 square yards) of arable land, one year after being manured, contains as 

 much caibonio acid as 18,000 cubic metres of atmospheric air. That is 

 to say, inasmuch as taking the average depth at 35 centimetres (about 



