CHEMISTRY, METEOROLOGY, &C. 239 
logous compound ?) till the blood reaches the ultimate termina¬ 
tion of the arteries. In these minute tubes the oxygen changes 
its mode of union ; it combines with a portion of carbon, and is 
converted into carbonic acid, which carbon must be derived 
from the albuminous principles of the blood. Two distinct 
alterations take place during the union of the carbon with the 
oxygen; a portion of the albumen contained in the blood is 
supposed to be reduced to the state of gelatine, which gelatine 
is appropriated to the production and renovation of those tex¬ 
tures whose composition is chiefly gelatinous. At the same 
time, the carbonic acid which had been formed from the reduced 
albumen unites with the blood, communicates to that fluid its 
dark venous colour, and is transferred to the lungs, where it is 
expelled from the system along with a portion of aqueous vapour, 
derived principally from the weak albumen of the chyle, as 
formerly explained. 
The blood is the source, not only of all the constituent prin¬ 
ciples of animal bodies, but likewise of all the various secretions; 
many of which differ altogether in their properties from those of 
the primary fluids, and perform secondary offices of great im¬ 
portance in the animal economy. Other products separated 
from the blood are purely excretions; as, for instance, the car¬ 
bonic acid gas from the lungs, which could not be retained in 
the animal system without destroying life. 
Such is a summary of those operations of living bodies which 
the author has presented in the 3d Book. We have been brief, 
but we hope intelligible. 
Most of the facts on which the author has dwelt are of a 
character so obvious, that they require only to be understood, in 
order to be admitted among the proofs of benevolent design. 
In considering the economy of organized being, one of the 
circumstances most calculated to arrest our attention, is the ex¬ 
traordinary skill manifested in the disposal of the various parts 
of the organized system with regard to each other. As an 
instance of this, the mutual relation and dependence of plants 
and animals may be noticed. Thus, as we once before had the 
pleasure of pointing out in The Veterinarian, carbonic acid 
gas constitutes the chief food of plants; and nearly the whole 
of the superfluous carbon produced by the operations of the 
animal system is actually thrown off in the form of carbonic 
acid. Plants, therefore, on the one hand, supply the chief 
nourishment to animals; while that gaseous matter which is 
separated by the animal economy, and which, if retained within 
animals, would to them be fatal, constitutes, on the other hand, 
the chief food of plants. 
