THE VOLATILE PAKT OF PLANTS. 89 



lime, in tobacco leaves, in the tubers of the Jerusalem, 

 artichoke, in the bulbs of onions, in beet roots, in coffee- 

 berries, and 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 liv- 

 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. Tire chemical 

 change consists merely in the removal of one atom of oxygen,. 

 Tartaric acid. Malic acid. 

 C 4 H 8 6 O = C 4 H 5 



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, ijectin, yield tar- 

 taric acid by the action of nitric acid. 



5t FATS AND OILS (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, hickory, 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 



