200 THEORY OF THE CELLS. 
more clear, remark that the chemical changes produced by a 
galvanic pile are accompanied by corresponding changes in the 
pile itself. 
The more obscure the cause of the metabolic phenomena in 
the cells is, the more accurately we must mark the circum- 
stances and phenomena under which they occur, One condi- 
tion to them is a certain temperature, which has a maximum 
and a minimum. The phenomena are not produced in a 
temperature below 0° or above 80° R.; boiling heat destroys 
this faculty of the cells permanently ; but the most favorable 
temperature is one between 10° and 32° R. Heat is evolved 
by the process itself. 
Oxygen, or carbonic acid, in a gaseous form or lightly con- 
fined, is essentially necessary to the metabolic phenomena of 
the cells. The oxygen disappears and carbonic acid is formed, 
or vice versa, carbonic acid disappears, and oxygen is formed. 
The universality of respiration is based entirely upon this 
fundamental condition to the metabolic phenomena of the 
cells. It is so important that, as we shall see further on, 
even the principal varieties of form in organized bodies are 
occasioned by this peculiarity of the metabolic process in the 
cells. 
Each cell is not capable of producing chemical changes in 
every organic substance contained in solution, but only in par- 
ticular ones. The fungi of fermentation, for mstance, effect 
no changes in any other solutions than sugar; and the spores 
of certain plants do not become developed in all substances. 
In the same manner it is probable that each cell in the animal 
body converts only particular constituents of the blood. 
The metabolic power of the cells is arrested not only by 
powerful chemical actions, such as destroy organic substances 
in general, but also by matters which chemically are less un- 
congenial ; for instance, concentrated solutions of neutral salts. 
Other substances, as arsenic, do so in less quantity. The meta- 
bolic phenomena may be altered in quality by other substances, 
both organic and inorganic, and a change of this kind may re- 
sult even from mechanical impressions on the cells. 
Such are the most essential characteristics of the funda- 
mental powers of the cells, so far as they can as yet be deduced 
from the phenomena. And now, in order to comprehend dis- 
