806 
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 the 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, until 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 
manures, we shall find that in most cases, except when their 
object is to alter the £tate 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 fermentation, 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 
large 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- 
