The Food of Plants. 337 
If we now turn our attention more particnlarly to the 
elements of the first group, or those which disappear in the 
process of combustion, we find them to be carbon, hydro- 
gen, sulphur, nitrogen, phosphorus, oxygen and chlorine. 
In the process of rapid combustion, the hydrogen is convert- 
ed into water and passes off as aqueous vapor. The carbon 
becomes changed into carbon dioxide—a gas prejudicial 
to animal life—and disappears in part into the surrounding 
atmosphere, the remainder being fixed in the ash residue, 
where we also find the acids of sulphur, nitrogen and 
phosphorus combined with the mineral constituents to 
form the corresponding salts. In decay or slow combus- 
tion, the same changes are finally accomplished, with the 
additional formation of volatile sulphur and ammonia com- 
pounds. The loss or diminution in volume which a plant 
‘suffers in the process of combustion, will thus be seen to cor- 
respond, in general terms, to the elimination of the organic 
matter, which consists almost wholly of carbon, hydrogen 
and oxygen, with very small quantities of the other ele- 
ments mentioned. 
If we next inquire into the composition of the second 
or incombustible group, we find it to contain potassium, 
sodium, calcium, magnesium, iron and silicon. These 
elements, as already stated, are found in combination with 
the acids derived from combustion of the elements of the 
first group. In exceptional cases, manganese, bromine 
and iodine, as well as arsenicum, copper and other metals 
may be found in the ash, but for various reasons which 
need not be dealt with at the present time, they are usually 
not regarded as constituting elements of plant food. It thus 
appears that of the sixty-seven chemical elements known to 
science, only thirteen are to be regarded as of importance 
in the economy of the plant. 
With these general facts before us, we are now pre- 
pared to inquire into the sources whence they are derived ; 
and in this respect we may again divide them into two 
groups, those derived from—l1st, the air, and 2nd, the soil. 
