FOOD OF PLANTS. 
655 
decompose carbonic acid at the same time with water, and 
absorbs its carbon; but this supposition has not been proved, 
though it has gained strength since the decomposition of 
carbonic acid combined with soda by means of phosphorus 
has been discovered. Others are of opinion that vegetables, 
earths, moulds, and water of dunghills furnish the carbon at¬ 
tenuated and even dissolved in water; that plants absorb it, 
and do not extract it from carbonic acid. According to this 
hypothesis, manures afford only carbon, and the water is 
nothing more than a saturated solution of this principle. To 
these data the theory of vegetation is at present restricted. 
Four earths have been found in plants—lime and silica 
most abundantly, and magnesia and alumina in much less 
quantities. All the alkalies have been found in the fluids of 
animals; potash not very common ; soda in all fluids combined 
with albumen ; ammonia in urine, and during its decomposition. 
And of the earths are found lime, magnesia, and silica; and 
of the metals, iron in blood, and manganese in air. None of 
the earths are simple substances, and animals have a power 
of forming them by the process of digestion. And it seems 
very probable that plants have a power of forming earths and 
other substances from their principles absorbed by the roots 
from the earth or from the atmosphere. Hence solution by 
chemical analysis of the constituent parts of the organisation 
of animals, and our knowledge of the nature of the substances 
that compose it, have not given any information in respect of 
the food which should be supplied to them ; they possess the 
instinctive power of choosing and rejecting by smell and 
taste, and thereby teach what articles are most agreeable to 
them, and consequently most suitable for promoting their 
growth. Any food they eat contains few or none of the sub- 
stances found in their constituent parts: they must be formed 
during the process of digestion by many unknown agencies 
and combinations; and if an analogy be drawn between 
animal and vegetable life, it may be supposed that the food of 
plants differs as much as that of animals from the constituent 
parts, and that the latter are derived from the former by 
similar processes and combinations. If an animal be re¬ 
stricted to one kind of food, or to two, the growth of the solid 
parts of the body and the secretion of the fluids will go on 
uninterrupted, and the animal will thrive and fatten. The 
kinds of food contain none of the substances found in the 
animal organization ; and though some of the minor parts may 
differ something in quantity than if fed on a greater variety of 
food, yet all the principal parts are unaffected. Charcoal has been 
found the only fixed ingredient in plants, and that substance 
