( 6 ) 
to use this residue as cattle food and to return it 
ultimately to the soil in the form of cattle manure. 
In the case of some seeds, for example, castor oil 
seed, this is impossible, since the meal or cake 
contains some poisonous substance, and can there- 
fore only be utilised directly as a fertiliser. 
Essential Occurrence and Extraction . — Unlike 
Oils. fixed oils, which occur almost entirely 
in the seed or fruit, essential oils may 
be found in various parts of the plant; the leaves, 
roots, flowers and bark; for example, in the case of 
the rose, the oil is obtained from the flower petals, 
while in lemon grass the oil is present in the leaf. 
Nutmegs contain both fixed and essential oils. 
Frequently, too, more than one essential oil occurs 
in a plant; thus in the case of the cinnamon tree, 
the bark, the leaves and the roots all yield different 
oils. 
Essential oils occur in plants generally in very 
small amounts; it is seldom that the yield of oil, 
calculated on the weight of fresh material, exceeds 
a few per cent., frequently it is less than one 
per cent. 
The methods employed for the production of essen- 
tial oils may be classed as follows: — 
(1) Steam distillation of the plant. 
(2) Mechanical processes. 
(3) Solvent extraction. 
With regard to the first method, which is by far 
the most important, the distillation from the plant 
material is carried out by means of steam, all 
essential oils being readily volatile in the presence 
of water vapour. 
