PROGRESS IN CHEMISTRY. 249 



tetriid. or tetravalent, and ,so on. This is evident from inspection of 

 the fornuihe of their conipoimds with hvdrogen. thns: 



/H H. /H 

 H-Cl . H-O-H ! H-N<; >C<f 



^H H^ ^H 



Instances of penta-, hexa-. and even hepta-valency are not wanting. 



KKY TO STRUCTUKK OF ( HEMICAL COMPOUNDS. 



This was the ke}' to unlock the structure of chemical compounds; 

 and Franklands' views, just stated, are still held })y chemists. The 

 determination of the constitution of compounds, chietl}^ those of car- 

 bon, occupied the attention of chemists almost exclusivel}^ until 1880, 

 The plan of action is much the same as that of a mechanician who 

 wishes to imitate a complicated mechanism. He must first dissect it 

 into o-rou]3s of mechanical contrivances; these are next constructed, 

 and thev are finally built together into the complete machine. In 

 certain cases the atoms of carbon are arrranged in "chains,''' as, for 

 example, in pentyl alcohol. 



H3C-C-C-C-C-O-H 

 H,H,H,H„ 



each atom ])eing tetrad, and its '"affinities," or powers of combination, 

 saturated either with hydrogen or with those of neighboring atoms of 

 carbon; in others they are in the form of a " ring," as in b<Mizene, the 

 fornuda of which was first suggested by Kekule, viz: 



H H 

 HCt >CHI, 



H H 



or in both, as in ethvl l)enzene. 



H H 

 H H 



One or more atoms of nitrogen, or of oxygen, may form part of the 

 circle, as in pyridine, 



and so on. By means of conceptions such as these many interesting 

 compounds have been built up out of the elements which they contain, 

 e. g., urea and uric acid, constituents of urine; theobromine and 



