678 POPULAR SCIENCE MONTHLY. 



radicles began to occupy the attention of chemists, and a definite 

 line of attack upon organic matter was recognized. 



Two years later the second great step was taken. Dumas, 

 studying the action of chlorine upon acetic acid, showed that the 

 chlorine could replace hydrogen atom for atom, or volume for vol- 

 ume, and that his observations explained other reactions which had 

 been unintelligible hitherto. This research led him to the famous 

 theory of substitutions, which at first was received with ridicule, 

 but soon found general acceptance. Electro-chemical conceptions, 

 the Berzelian doctrines, were then in vogue, and it seemed strange, 

 even absurd, to suppose that electro-negative chlorine could be sub- 

 stituted for electro-positive hydrogen. But the facts were stronger 

 than the preconceived ideas, and the latter soon gave way. In 

 this discovery by Dumas the first germs of the modern theory of 

 valence are to be found. 



For the study of inorganic substances, however, the dualistic 

 theory was long retained, with the result that inorganic chemistry 

 degenerated to a great extent into analysis and compound making, 

 without any general conceptions which could stimulate scientific 

 advance. It became a science of details rather than of principles, 

 and was soon overshadowed by the organic branch. In the latter, 

 theory after theory sprang up,, flourished, and died aAvay, each one 

 having partial truth, but none being exhaustive and final. Still, 

 the intellectual activity led to discoveries, and the warfare between 

 doctrines, unlike the warfare between men, was productive of good 

 instead of destruction. From the conflict of ideas the truth gradu- 

 ally emerged, and a new system of chemical philosophy was devel- 

 oped. The theory of compound radicles, the nucleus theory, the 

 theory of types, the conception of conjugated compounds, followed 

 rapidly one after the other, until in the discovery of valence all 

 discrepancies were reconciled, structural chemistry came into exist- 

 ence, and a single doctrine, applicable alike to organic and inor- 

 ganic substances, had possession of the field. 



The theory of valence was a logical outgro\vth from its prede- 

 cessors, whose valuable features it included in a wider generaliza- 

 tion, but it was the work of no one master mind. Many chemists 

 contributed to its up-building, Frankland and Ivekule being among 

 the leaders; but its foundations are to be detected in the atomic 

 theory itself, from which it is legitimately derived. To under- 

 stand its full significance we must take a step backward in history, 

 and trace the change in atomic weights from their first form to the 

 modern system. 



In the early days of the atomic theory, in the determinations 

 by Wollaston, Berzelius, and others, attention Avas chiefly paid to 



