ORGANIC CHEMISTRY. 299 



grand elements of which most of the organic world is constituted are rep- 

 resentatives or types of these four classes, hydrogen being a monad, H' ; 

 oxygen a dyad, 0" ; nitrogen a triad, N'" ; and carbon a tetrad, C iv . By 

 taking advantage of these points of attraction, a peculiar kind of pictorial 

 formulae may be constructed, called "graphic formulae." Thus the ordi- 

 nary formula for water is H 2 O; but if we represent the dyad oxygen by 

 O , and the monad hydrogen by H , we have by combining them 

 H O II, a graphic formula for water. Take a more complex example, 

 from organic chemistry. Alcohol has the formula C a H 5 .HO, graphically 



represented thus : 



H 



I 



H-C-H 



I 



H-C-O-H 

 I 

 H 



! being a tetrad, O a dyad, and the rest monad hydrogen, each 



bond of affinity is satisfied by arranging the atoms in this manner. Now 

 you will see how graphic formulae help to explain metamerism, the highest 

 kind of isomerism. Take the three bodies which have already served us 

 as examples. On the following page you have the ordinary formulae, the 

 constitutional formulae, and the graphic formulae side by side, showing how 

 differently the atoms are arranged in the molecules of each body. We can 

 not explain to you how chemists are able to arrive at any probable knowl- 

 edge of the arrangement of atoms in the interior of a molecule, for the sub- 

 ject belongs to the highest branch of chemical philosophy. Of late years 

 the most wonderful progress has been made in precisely this field, and 

 indeed the whole aim of modern investigators is directed to this study 

 of internal atomic structure. 



