306 PHYSIOLOGY. BOOK II. 



water, atmospheric air, or other elements that it finds it- 

 self in contact with : whence proceed the gummy, amylaceous, 

 resinous, oily, and other products peculiar to the vegetable 

 kingdom. Upon this subject it has been observed by a modern 

 writer, " that if the roots of a plant are placed in a close vessel, in 

 distilled water, from which carbonic acid has been carefully ex- 

 pelled, the plant may increase a little in size, in consequence of 

 the decomposition of the water and the combination of its ele- 

 ments with the vegetable system; but it is only when carbonic 

 acid is added that tlie plant acquires its natural vigour and 

 rate of growth. But if a plant is placed in solid carbon, and 

 you water it with distilled water, it might as well be planted 

 in powdered glass, vmtil the carbon begins to combine with 

 the oxygen of the air, and to form carbonic acid. Sir Hum- 

 phrey Davy placed a plant of Mint in water mixed with carbon 

 in a state of impalpable powder, and he found that not a par- 

 ticle could enter the roots. If we look to the effects of 

 manui'es, we shall find that in most cases, except when their 

 object is to alter the state of the soil mechanically, or to act 

 as stimulants, as is probably the case with sulphate of iron, 

 their energy is in proportion to their capability of forming 

 carbonic acid. Yeast, for instance, which is one of the most 

 active manures we have, is so from possessing, beyond all 

 other substances, the power of exciting fei'mentation, and thus 

 of causing the formation of carbonic acid among the vegetable 

 matter which lies buried in the soil. 



" While, however, all experiments combine to prove that 

 carbonic acid is the most essential of the elements upon which 

 plants are nourished, it is necessary that the student should 

 be aware that other species of matter are constantly taken 

 into the system, and probably, therefore, contribute to their 

 nutrition. 



" Water is one of these. Although we know that a very 

 lai'ge proportion of all the water absorbed by a plant is lost 

 again by evaporation, yet the experiments of Theodore de 

 Saussure have shown that a portion of it is actually solidified. 

 He found that when plants are grown in a close vessel, in an 

 artificial atmosphere, containing a little carbonic acid, the 

 weight which the plant acquired in a given time was aug- 



