100 
now described ? That an affinity subsists between the 
carbon of vegetables, and the oxygen of the air, the 
phenomena of vegetation abundantly testify j and 
some experiments of M. de Saussure seem, also, to 
shew, that, beside converting oxygen gas into carbo- 
nic acid, certain vegetables possess an attractive 
power for that gas, under which it enters, unchanged^ 
into the vegetable body. Thus, he remarks, that if 
the leaves of the cactus opuntia be confined, through 
the night, in atmospheric air inverted over mercury, 
they will remove a portion of its oxygen without 
producing an atom of carbonic acid, or, in the small- 
est degree, affecting its nitrogenous portion*. When 
the leaves have thus acquired a full dose of oxygen, 
they beguvsays M. de Saussure^.to produce carbonic 
acid, by yielding their carbon to unite with the sur- 
rounding, oxygen,"*--a combination which does not, in 
the least, affect the volume of air. The greater num- 
ber of leaves, and; in particular, those which are not 
fleshy, thus form carbonic acid, at the same time that 
they receive, or, asM. de Saussure says, inspire oxy- 
gen gas 1v 
34fl . But though the entrance of oxygen gas, to 
the exclusion of nitrogen, seems, in the foregoing ex- 
amples, to favour the notion of the operation of an 
elective affinity ; yet M. de Saussure observes, that 
when the cactm has been confined through the night 
in pure hydrogen gas, it has afterwards afforded, by 
the action of the air-pump^ small quantities both of 
hydrogen and nitrogen gases. He also found, that 
* Recherches Chirr', p. 64. f lb ' id - P- 67.- 
