THE VOLATILE PART OF PLANTS. 69 
lime, in tobacco leaves, in the tubers of the Jerusalem 
artichoke, in the bulbs‘of onions, in beet roots, in coffee- 
berries, and i in the needles of the fir tree. 
In the pure: state, citric acid forms large transparent 
or white crystals, very sour to the taste. 
Relations of the Vegetable Acids to each other and to the Amyloids.—The 
four acids above noticed usually occur together in our ordinary fruits, 
and it appears that some of them undergo mutual conversion in the liy- 
ing plant. 
According to Liebig, the unripe berries of the mountain ash contain 
much tartaric acid, which, as the fruit ripens, is converted into malic 
acid. Schmidt, (Ann. Chem. u. Pharm., 114, 109,) first showed that tar- 
taric acid can be artificially transformed into malic acid. The chemical 
change consists merely in the removal of one atom of oxygen. 
Tartaric acid. Malic acid. 
‘Cy, He Os — O = Cy He Os 
When citric, malic, and tartaric acids are boiled with nitric acid, or 
heated with caustic potash, they all yield oxalic acid. 
Cellulose, starch, dextrin, the sugars, and, according to some, pectic 
acid, yield oxalic acid, when heated with potash or nitric acid. Com- 
mercial oxalic acid is thus made from starch and from saw-dust. 
Gum (Arabic,) sugar, starch, and, according to some, pectin, yield tar-- 
taric acid by the action of nitric acid. 
5. Fars anp Ons (Wax).—We have only space here 
to notice this important class of bodies in a very general 
manner. In all plants and nearly all parts of plants 
we find some representatives of this group; but it is 
chiefly in certain seeds that they occur most abundantly. 
Thus the seeds of hemp, flax, colza, cotton, bayberry, 
pea-nut, butternut, beech, Hokory. almond, sunflower, 
etc., contain 10 to 70 per cent of oil, which may be in 
great part removed by pressure. In some plants, as the 
common bayberry, and the tallow-tree of Nicaragua, the 
fat is solid-at ordinary temperatures, and must be extracted 
by aid of heat; while, in most cases, the fatty matter is 
liquid. The cereal grains, especially oats and maize, con- 
tain oil in appreciable quantity. The mode of occurrence 
of oil in plants is shown in fig. 17, which represents a 
highly magnified section of the flax-seed, The oil exists 
