107 
duce the entrance of air into the vegetable body, is 
exhibited in the production of many other effects. 
Thus water is capable of receiving and containing 
within its pores different gaseous fluids, in very dif- 
ferent proportions. Nitrogen gas enters in small 
quantity, oxygen in a larger proportion, and carbo- 
nic gas in a much larger still. Now the mechanical 
operation of pressure acts alike on all these gp,ses, 
and, to a certain extent, causes their entrance into 
the water ; and hence, if the pressure be artificially 
increased, the quantity of gas that enters is increased, 
or, if it be diminished, or entirely removed, the gaseS 
in part escape, or are no longer retained. But the 
reason why oxygen is received in larger quantity 
than nitrogen, and carbonic gas than oxygen, can 
only be explained on the supposition of a difference 
in the affinity subsisting between water and those 
.gases respectively. In the same manner, the en-, 
trance of all elastic fluids into vegetables, may, to a 
certain extent, be deemed mechanical ; but, after 
their admission, a chemical cause may be considered 
to operate. Hence it is, that, in the foregoing expe- 
riments of M. de Saussure, the oxygen gas which SQ 
largely enters the leaves of fleshy plants, cannot be 
abstracted by the air pump, nor, while they are kept 
in darkness, by placing them in temperatures equal 
to 40 Reaumur * ; but it is entirely expelled (342.) 
in its primitive form and quantity, by the chemical 
agency of the solar rays. 
349. From the facts which have now been stated 
* Recherches Chim, p. 67. 6p. y 
