THE VOLATILE PART OF PLANTS. 81 
Isomerism.— Bodies which—like cellulose and dextrin, or like levulose 
and glucose—are identical in composition, and yet are characterized by 
ditferent properties and modes of occurrence, are termed isomeric; they 
are examples of isomerism. These words are of Greek derivation, and 
signify of equal measure. 
‘We must suppose that the particles of isomeric bodies which are com- 
posed of the same kinds of matter and in the same quantities, exist in 
different states of arrangement. The mason can build from a given num- 
ber of bricks and a certain amount of mortar, a simple wall, an aqueduct, 
a bridge ora castle. The composition .of these unlike structures may 
be the same, both in kind and quantity; but the structures themselves 
differ immensely, from the fact of the diverse arrangement of their ma- 
terials. In the same manner we may suppose starch to be converted 
into dextrin by a change in the relative positions of the atoms of carbon, 
hydrogen, and oxygen, which compose them. 
8. Tae Pxcrosr Grovup.—The pectose group includes 
Pectose, Pectin, Pectosic, Pectic, and Metapectic acids, 
These bodies exist. in, or are derived from, fleshy fruits, 
including pumpkins and squashes, berries, the roots of 
the turnip, beet, onion, and carrot, and in cabbage and 
celery. They are an important part of the food of men 
and cattle. 
Pectose is the name given to a body which is supposed 
rather than demonstrated to occur with cellulose in the 
flesh of unripe fitits, and in the roots of turnips, carrots, 
and beets. Its characters in the pure state are as good as 
unknown, because we are as yet acquainted with no means 
of separating it from cellulose without changing its nature. 
Pectose is thought to constitute the chief bulk of the dry 
matter of the above-mentioned fruits and roots, and is con- 
cluded to be a distinct body by the products of its trans- 
formation, either such as are formed naturally, or those 
procured by artificial means. In what follows, we shall as- 
sume, with Fremy, (Ann. de Chim. et de Phys., XXIV, 
6,) that pectose exists, and is the source of pectin, ete. 
Pectin is produced from pectose in a manner similar to 
that by which dextrin is obtained from cellulose or starch, 
viz., by the action of heat, of acids, and of ferments. When 
the flesh of fruits, or the roots which consist chiefly of 
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