EFFECTS OF LIGHT. 61 



"Most physiologists have conuected the exhalation of carbonic acid 

 during night -with the absorption of oxygen from the atmosphere, and 

 consider this function as the real respiration of plants, which (as we 

 know) produces in animals a decarbonisation of the blood. There is 

 scarcely an opinion which rests on such a feeble basis. The water 

 received by roots contains carbonic acid, which is not decomposed in 

 the absence of light, but remains dissolved in the sap which pervades 

 aU parts of a plant ; and every moment, along with the water evapo- 

 rating through the leaves is a proportionate amount of carbonic acid 

 expelled. Soil in which plants vegetate luxuriantly contains a certain 

 quantity of moisture (an indispensable condition of their life), and such 

 a sou is never deficient in carbonic acid, either derived from the atmo- 

 sphere or from the putrefaction of vegetable matter. No water, either 

 rain or that of springs, is free from carbonic acid ; and at no period of 

 the life of a plant does the capability of its roots to absorb moisture, 

 and consequently air and carbonic acid, altogether cease. Can it 

 therefore surprise us that carbonic acid, conjoiutly with the evaporating 

 water of the plant, is returned to the atmosphere, when the cause of 

 the fixation of carbon, viz. light, is deficient? That exhalation of 

 carbonic acid is as unconnected with the process of assimilation and 

 with the life of a plant as the absorption of oxygen. They do not bear 

 the least relation to each other ; the one is a purely mechanical, the 

 other a chemical process. A wick of cotton shut up iu a lamp which 

 contains a fluid impregnated with carbonic acid will be in just the 

 same position as a living plant in darkness. Water and carbonic acid 

 are absorbed by the power of capUlaiy attraction, and both evaporate 

 again on the surface of the wick." — lAehig^s Organic Chemutry, 1840. 

 The foregoing passage is objected to by some physiologists who do not 

 believe that carbonic acid can pass through a, plant without being 

 decomposed. And Mr. Haseldine Pepys, in his careful experiments on 

 Vegetable Respiration, arrived at the conclusion that no carbonic acid 

 whatever is parted with either by night or day. 



Whether plants do or do not give oflf some small quantity of carbonic 

 acid, this at least is certain, that they do not in this way deteriorate 

 the atmosphere in any appreciable degree. If there is one absurdity 

 among popular prejudices greater than another, or leading more to 

 privation of comfort when most wanted, it is that of fancying that 

 growing plants vitiate the air of an apartment by the carbonic acid they 

 emit. The reasoning on which this is founded is as follows: — 

 1. Growing plants form carbonic acid in their interior, by absorbing 

 oxygen from the atmosphere ; they thus rob the air of that which is 

 most necessary to animal Ufe, and therefore they are prejudicial, espe- 

 cially to sick persons. 2. Growing plants also give out carbonic acid 

 or fixed air, a pernicious gas, in which animal life cannot be main- 

 tained ; therefore, they should be expelled from all apartments, espe- 



