ACTION OF LEAVES. 43 



is in proportion to the intensity of the light which 

 strikes a leaf, the smallest amount being in shady 

 places ; and the healthiness of a plant is, cceteris pan- 

 bus, in proportion to the quantity of carbonic acid 

 decomposed; therefore, the healthiness of a plant 



during the day and eyening, and are changed, by the separation of 

 a part of their oxygen, into compounds containing oxygen and 

 hydrogen either in the same proportions as in water, or even with 

 an excess of hydrogen, which is the composition of all tasteless and 

 bitter substances. .... Most vegetable physiologists have 

 connected the emission of carbonic acid during the night with the 

 absorption of oxygen from the atmosphere ; and have considered 

 these actions as a true process of respiration in plants, similar to that 

 of animals, and, like it, having for its result the separation of carbon 

 from some of their constituents. This opinion has a very weak and 

 unstable foundation. The carbonic acid, which has been absorbed 

 by the leaves and by the roots, together with water, ceases to be 

 decomposed on the departure of daylight. It is dissolved in the 

 juices which pervade all parts of the plant, and escapes every mo- 

 ment through the leaves, in quantity corresponding to the water 



which evaporates Plants during their life constantly 



possess the power of absorbing by their roots, moisture, and, along 

 with it, air and carbonic acid. Is it, therefore, surprising that the 

 carbonic acid should be returned unchanged to the atmosphere, 

 along with water, when light (the cause of the fixation of its carbon) 

 is absent ? Neither this emission of carbonic acid nor the absorption 

 of oxygen has any connexion with the process of assimilation ; nor 

 have they the slightest relation to one another ; the one is a purely 

 mechanical, the other a purely chemical process. A cotton wick, 

 enclosed in a lamp which contains a liquid saturated with carbonic 

 acid, acts exactly in the same manner as a living plant in the night. 

 Water and carbonic acid are sucked up by capillary attraction, and 

 both evaporate from the exterior part of the wick." Liebig, Organic 

 Chemistry in its applications to Agriculture and Physiology, (London, 

 1840,) pp. i1 — 33, passim; a work which comprises a masterly view 

 of the chemical phenomena of vegetation. G.j 



