Fig. 108. 



Fig. 108. Here each figure is composed of eight white and 

 eight black squares; but though the number is the same 

 in all, the grouping is different. In a one and one; in b two 

 and two, in c and d four and four squares are so joined to- 

 gether as to make the figures look very different from 

 each other. If we imagine these squares to be atoms we 

 obtain an idea of isomeric substances, and can see how 

 there may be bodies of the same constitution and form, yet 

 presenting an entirely different appearance, and having 

 very different properties. 



Isomeric bodies are far more numerous in organic than 

 in mineral chemistry, because the molecules of organic sub- 

 stances are more complex than those of inorganic ; for the 

 difference in properties can not be owing to any thing else 

 than a difference in arrangement of the atoms, and the more 

 atoms there are in a molecule obviously the greater is the 

 range afforded for differences in arrangement. This may 

 be illustrated by reference to Fig. 108. Each of the squares 

 contains eight small black squares, and eight white ones, 

 sixteen in all. It is obvious that more and greater changes 

 in arrangement can be made here than there could be if the 

 number of small squares were less four, for example; and 

 so, also, more differences in arrangement can be had in a 

 molecule if it be composed of sixteen atoms than there can 

 be if there be only four atoms in it. 



417. Graphic Formulae. Another method of explaining isomerism 

 makes use of so-called graphic formula. You learned in 44 that the 

 elements differ in atom-fixing power, and that they are divided into groups, 

 monads, dyads, triad?, tetrads, etc., according to this power. The four 



