THE VOLATILE PAKT OF PLANTS. 73 



easier to understand since it takes place by the loss or 

 gain of several molecules of water. 



The formulae given are the simplest that accord with 

 the results of analysis. In case of many of the amyloses 

 it is probable that the above formulae should be multi- 

 plied by 2, 4, or 6, or even more, in order to reach the 

 true molecular weight. 



Isomerism.— Bodies which— like cellulose and dextrin, or like levii- 

 lose and dextrose— are identical In oomposltion, and yet are character- 

 ized by diflferent properties and modes of occurrence, are termed isom- 

 eric ; they are examples of isomerism. These words are of Greek deri- 

 vation, 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 

 number of bricks and a certain amount of mortar, a simple wall, an 

 aqueduct, a bridge or a castle. The composition of these unlike struc- 

 tures may be the same, both in kind and quantity ; but the structures 

 themselves differ immensely, from the fact of the diverse arrangement 

 of their materials. In the same manner we may suppose starch to dif- 

 fer from dextrin by a difference in the relative positions of the atoms 

 of carbon, hydrogen, and oxygen in the molecules which compose 

 them. 



By use of " structural formulae " it is sought to represent the different 

 arrangement of atoms in the molecules of isomeric bodies. In case 

 of substances so complex as the sugars, attempts of this kind have but 

 recently met with success. The following formulae exhibit to the 

 chemist the probable differences of constitution between dextrose and 

 levulose. 



Dextrose. Levulose. 



H H 



H-C-0 H H-C-0 H 



H— C-0 H C-0 



C— H H— C— O H 



H-C-0 H H C-O H 



H— C— O H H C— O H 



C— O H H C— O H 



i 



i 



To those familiar with advanced Organic Chemistry the foregoing 

 f ormulEe, to some extent, " account for " the chemical characters of 

 these sugars, and explain the different products which they yield 

 under decomposing influences. 



APPENDIX TO THE CABEHYDEATES. 

 Nearly related to the Carbhydrates are the following substances :— 



