120

ACTION OP PLANTS ON THE AIR.

in respect to temperature, moisture, and light. These,however,are practical matters into which we cannot enter.

370. In the last stage of the existence of a plant, orwhen, either from excessive cold, disease, or merely oldage, the vitality becomes extinct, all that curious seriesof changes by which organic matters are generated underthe influence of light in the cells of plants ceases ; decaycommences, the organic compounds of the plant begin todecompose, and their elements re-arrange themselvesinto simpler forms. The greater part of the hydrogencombines with oxygen, and is gradually given off in thestate of water ; the nitrogen and hydrogen combine,and form ammonia; whilst the carbon is slowly dissi-pated in the air in the state of carbonic acid, and at lastlittle remains besides mould, or charcoal in combinationwith a very little hydrogen (317), and the small quantityof insoluble earthy matters which the plant may havecontained.

371. As has already been observed, plants are con-stantly employed in preserving the purity of the atmo-sphere : they feed on the foul or vitiated air producedby respiration, combustion, and decay; and their vigourand luxuriance are always in proportion to the impurityof the air. It must be clearly understood, that there isa limit to the quantity of carbonic acid plants are ableto decompose: an excess would kill them; whilst if therewere none of it in the air, they could not live. Thevegetation of the whole globe is just enough to keep theair in a uniform state of purity.