92 OXYGEN IS EMITTED DURING THE DAY, 



in the atmosphere is exceedingly small, [about l-2500tli of this bulk- 

 see Lecture If., p. 30 ;1 but if for tlie purj)ose of experiment we increase 

 this proportion in a gallon of air to five or ten per cent., introduce a liv- 

 ing plant into it, and expose it to the sunshine, the carbonic acid will 

 gradually disappear as before, while the oxygen will increase. And if 

 we analyse the air and estimate the exact bulk of each of these gases 

 present in it at the closeof our experiment, we shall find iliat the oxygen 

 has increased generally by as much as the carbonic acid has diminished. 

 That is to say, if five cubic inches of the latter have disappeared, five 

 cubic inches will have been added to the bulk of the oxygen. The 

 above general conclusion, therefore, is rendered more precise by this ex- 

 periment, which appears to show that under the injluence of the sun's 

 rays the leaves of plants absorb carbonic acid from the air, and at the same 

 time give off ais equal bulk of oxygen gas. 



And as carbonic acid (CO2) contains its own bulk of oxygen gas* 

 combined with a certain known weight of carbon, it is further inferred 

 that the oxygen given off by the leaves is the same which has been pre- 

 viously absorbed in the form of carbonic acid, and therefore it is usually 

 stated as a function of the leaves — that in the sunshine they absorb car- 

 bonic acid from the air, decompose it in the interior of the leaf rctainits 

 carbon, and again reject or emit the oxygen it contained. 



This conclusion presents a very simple viev7 of the relations of oxygen 

 and carbonic acid respectively to the living leaf in the presence of the 

 sun, and it appears to be fairly deduced from the facts above stated. 

 It has occasionally been observed, however, that the bulk of oxygen 

 given off by the leaf has not been precisely equal to that of the carbonic 

 acid absorbed, [see Persoz, Chimie Moleculaire, p. 54,] and hence it is 

 also fairly concluded that a portion of the oxygen of the carbonic acid 

 which enters the leaf is retained, and made available in the production 

 of the various substances which are formed in the vascular system of 

 different plants. On the other hand it is stated by Sprengel, that if com- 

 pounds containing much oxygen be presented to the roots of plants, and 

 thus introduced into the circulation, they are also decomposed, and the 

 oxygen they contain in part or in whole given off by the leaves, so that, 

 under certain circumstances, the bulk of the oxygen which escapes is 

 actually greater than that of the carbonic acid which is absorbed by the 

 leaves. Such is the case, for example, when the roots are moistened 

 with water containing carbonic, sulphuric, or nitric acids. — [Sprengel 

 Chemie, 11., p. 344.] 



It is of importance to note these deviations from apparent simplicity 

 in the relative bulks of the two gases which are respectively given off 

 and absorbed by all living vegetables. There are numerous cases of the 

 formation of substances in the interior of plants which theory would fail 

 to account for with any degree of ease, were these apparent anomalies 

 to be neglected. This will more distinctly appear when in a subsequent 

 lecture we shall inquire hoiv or by what chemical changes the substan- 

 ces which plants contain, or of which they consist, are produced from 

 the food which they draw from the air and from the soil. 



* This the reader will recftUectis proved by burning charcoal in a bottle of oxygen gas till 

 combustion ceases, wlien neaiiy the whole of the oxygen is converted into carbonic acid, but 

 without change of bulk.— See Lecture III., p. i5. 



