AGRICULTURAL CHEMISTRY. 135 



very variable, as might be anticipated. It takes place most rapidly in 

 a dry, warm air, but is not absolutely cheeked when the atmosphere 

 is saturated with moisture. Transpiration is remarkably diminished 

 by the presence of many soluble salts, and of the alkalies, in the water 

 of the soil; while free acids increase its rapidity and amount. 



As is well known, water is a compound of hydrogen and oxygen; 

 although we have no direct evidence, the inference is fully warranted 

 that a portion of the water which enters the plant by the roots is 

 arrested in its upward path, to become itself a part of the tissues. 

 It is either held in the form of hygroscopic moisture, or is united 

 chemically to carbon; or, finally, it is decomposed, its hydrogen being- 

 retained, and its oxygen eliminated wholly or in part. In fact, wo 

 must regard water as the chief source of the hydrogen which is a com- 

 ponent of almost every vegetable principle. 



Carbonic acid is a compound of carbon and oxygen. It exists in 

 immense quantities in solid combination with lime in the various mar- 

 bles, limestones and marls, and in chalk. Separated from these bodies 

 by pouring on them sulphuric or nitric acid, it may be collected as a 

 gas, which, unrecognizable by the other senses, is agreeably sour to 

 the taste; is two and a. half times heavier than common air, and con- 

 siderably soluble in water. This gas is never absent from the air, and 

 although it occurs there in relatively small quantity, its absolute 

 amount is so great that, taking the atmosphere up to its entire height, 

 we have no less than seven tons of carbonic acid over every acre of 

 surface. 



A plant confined in an atmosphere free from this gas cannot enlarge 

 itself.* Some plants will live and grow in a confined space, as for 

 example, sealed up in a bottle ; but in this case the carbonic acid con-^ 

 sumed by the growing parts of the plant is supplied by the decay of 

 the lower leaves. 



Priestly and Saussure long ago furnished experimental evidence 

 that carbonic acid is absorbed by growing plants, and Boussingault 

 has described the following illustration of the rapidity with which the 

 gas is imbibed by the foliage of vegetation. Into one of the orifices 

 in a three-necked glass globe he introduced the branch of a living- 

 vine bearing twenty leaves ; with another opening he connected an 

 apparatus by means of which a slow current of air, containing a small, 

 accurately known proportion of carbonic acid could be passed into the 

 globe. This air after streaming over the vine leaves, escaped by the 

 third neck into an arrangement for collecting and weighing the car- 

 bonic acid that remained in it. The experiment being set in process 

 in the sun-light, it was found that the enclosed foliage removed from 

 the current of air three-fourths of the carbonic acid it at first con- 

 tained. 



The absorption of the gas in question by the leaves is found to take 

 place only under the influence of the light of the sun, or of the accom- 

 panying chemical rays. Through the roots, carbonic acid, when held 

 in solution of water, may be absorbed at all times. 



8 Unless, indeed, as is probable, carburetted hydrogen may, to a small extent, be an actual 

 source of carbon to plants, a point not yet satisfactorily determined. 



