626 ON A CERTAIN EXCRETION OF CARBONIC ACID BY LIVING PLANTS. 
varying from 17° to 60° C. considerable quantities of carbonic acid gas were evolved, 
showing that the rise of temperature markedly increased the excretion of this gas. But 
as soon as the temperature rose above the coagulating-point of albumen, gas quite ceased 
to be evolved, although the pump was kept at work and the temperature raised to 160° C. 
The experiment hence gave no support to the idea that the oxygen was occluded. The 
constituent in plants which acts in relation to oxygen as the blood-corpuscles in animals 
is quite unknown. 
It is a rather remarkable fact that growing cinchona bark, which contains a large 
amount of a tannin greedy of oxygen, continually absorbs oxygen, retains it for days, 
and excretes carbonic acid, without in the least affecting the easily alterable tannin. 
The main conclusions which are arrived at by the foregoing investigation are : — 
1st. That nearly all parts of growing plants evolve carbonic acid in considerable 
quantities, quite independently of direct oxidation. 
2nd. That this evolution is connected with the life of the plant. 
3rd. That it is due to two causes — namely, to previous oxidation, resulting after a lapse 
of time in the production of carbonic acid, and to the separation of carbonic acid from 
the proximate principles of the plant while undergoing the chemical changes incident to 
plant-growth. 
