331 
of the body with varnish or oil. Should this, how- 
ever, be done, the emission of the odorous particles 
cannot be said to be prevented by the absence of any 
cause required to attract them, but by the removal 
of those circumstances which were necessary to fa- 
vour their separation, or by the interposition of other 
bodies which prevent their escape, even although 
their separation be made. 
634. In like manner, we consider the separation of 
carbon from the other elements of seeds, so far to 
resemble these operations, that, at the commencement 
of germination, it is effected by the spontaneous 
changes which arise in the seed, and not by the at- 
tractive force of the oxygen of the air. Whjsn, 
however, oxygen gas is supplied, its superior affinity 
for the carbon of the seed leads to its combi- 
nation with it, to the exclusion of nitrogen gas ; and, 
by a subsequent agency, arising out of that combina- 
tion, the development of the seed is accomplished. 
Although, however, oxygen gas be necessary to 
this particular change in the seed, we are not from 
thence to conclude that it is necessary to every 
change ; for we have seen that the seed experiences 
other changes when no oxygen gas is present. 
Hence, therefore, it appears to us that the carbon 
yielded by seeds, both under decomposition and in 
germination, proceeds, primarily, from spontaneous 
changes excited in the seed itself, and that it is not 
removed by the operation of an attractive force, ex- 
erted by the surrounding oxygen. 
635. What, then, are the circumstances or agents 
which favour or promote that spontaneous change in 
