INFLUENCE OF VEGETATION ON THE ATMOSPHERE. 199 



to reproduce a little carbonic acid, and thus demolish a portion of 

 the rising vegetable structure which the setting sun left, as it were, 

 in an unfinished or unstable state. This is what actually takes place 

 in a dead plant at all times, and whenever an herb is kept in pro- 

 longed darkness ; chemical forces, exerting their power uncontrolled, 

 demolish the whole vegetable fabric, beginning with the chlorophyll 

 (as we observe in blanching Celery), and at length resolve it into 

 the carbonic acid and water from which it was formed. But this 

 must all be placed to the account of decomposing, not of growing 

 vegetation ; and even if it were a universal phenomenon, which is 

 by no means the case,* would not affect the general statement, that, 

 hy so much as plants grow, they decompose carbonic acid and give 

 its oxygen to the air ; or, in other words, purify the air. 



359. Every six pounds of carbon in existing plants have withdrawn 

 twenty-two pounds of carbonic acid gas from the atmosjjhere, and 

 replaced it Avith sixteen pounds of oxygen gas, occupying the same 

 bulk. To form some general conception of the extent of the influ- 

 ence of vegetation upon the air we breathe, therefore, we should 

 compute the quantity of carbon, or charcoal, that is contained in the 



* It is stated that many ordinary plants, wlicn in full health and vigorous 

 vegetation, impart no carbonic acid to the air during the night. — See Pepys, in 

 Philosophical Transactions, for 1843. — Plants deteriorate the air only in their 

 decay, and in peculiar processes, distinct from vegetation and directly the re- 

 verse of assimilation ; as in gennination, for instance, where the proteinc in- 

 duces the decomposition of a portion of the store of assimilated matter, in order 

 that the rest may be brought into a serviceable condition. The evolution of 

 carbonic acid by plants, therefore, when it occurs, is no part of vegetation. And 

 it is by a false analogy that this loss which plants sustain in the night has been 

 dignified with the name of vegetable respiration, and vegetables said to vitiate the 

 atmosphere, just like animals, by their respiration, while they purify it by their 

 digestion. If, indeed, this were a constant function, in any way contributing to 

 maintain the life and health of the plant, it might be properly enough compared 

 with the respiration of animals, which is itself a decomposing operation. But 

 this is not the case. And herein is a characteristic difference between vegetables 

 and animals : the tissues of the latter require constant interstitial renewal by 

 nutrition, new particles replacing the old, which are removed and restored to the 

 mineral world by respiration : while in plants there is no such renewal, but the 

 fabric, once completed, remains unchanged, ceases to be nourished, and conse- 

 quently soon loses its vitality ; while new parts are continually formed farther 

 on to take their places, to be jn turn abandoned. Plants, therefore, having no 

 decomposition and rccomposition of any completed fabric, cannot properly be 

 said to have the function of respiration. 



