THE VOLATILE PART OF PLANTS. 81 



Isomerism. Bodies which like cellulose and dextrin, or like levulose 

 and glucose arc identical in composition, and. yet are characterized by 

 different properties and modes of occurrence, are termed isorneric ; 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 brinks and a certain amount of mortar, a simple wall, an aqueduct, 

 a bridge or a 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. 



3 THE PECTOSE GROUP. The pectoso 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 fruits, 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 oy 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, etc. 



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 



4* 



