THE PRESIDENTS ADDRESS 21 



negative electricity, and so a positive oxide like that of potassium 

 may- combine with a negative oxide such as that of sulphur to 

 form a salt. This theory dominated chemistry for nearly half a 

 century. 



The next important step toward an insight into the nature of 

 chemical combination was the discovery of Faraday, in 1833, that 

 the same current passing through a succession of electrolytes lib- 

 erates equivalent quantities of elements or radicals at the elec- 

 trodes immersed in the different solutions. This discovery points 

 very clearly to a definite unit quantity of electricity which is 

 directly connected with those primary units of matter which we 

 call atoms. It is not a unit of energy, since the energ\- absorbed 

 in the decomposition of the different electrolytes is different. The 

 fundamental conception which we are forced to. was stated very 

 clearly by Helmhotz in his Faraday lecture, in 1881 : "Tf we 

 accept the hypothesis that elementary substances are composed of 

 atoms, we cannot avoid the conclusion that electricity, positive as 

 well as negative, is divided into definite elementary portions, 

 which behave like atoms of electricity." Maxwell, too. spoke of 

 a '"molecule" of electricity in 1873. titit evidently had little sym- 

 pathy with such an idea. This idea is quite different from that 

 ,in the theory of Berzelius. who considered that differences in 

 affinity were caused by differences in the quantity of electricity 

 in diff'erent atoms. The next advance, therefore, seems rather a 

 step backward than forward, as it consisted in the overthrow and 

 complete abandonment of the old electrochemical theory. In that 

 theory chlorine was always negative and hydrogen was always 

 positive in their compounds. But Dumas showed that three 

 derivatives of acetic acid could be prepared in which one, two or 

 three^ atoms of the positive hydrogen could be replaced by one, 

 two or three atoms of the negative chlorine, and yet the resulting 

 compounds were monobasic acids resembling acetic acid in their 

 salts and in their decompositions. In his attempts to explain 

 these and other compounds on the basis of the electrochemical 

 theory, Berzelius found himself more and more in conflict with 

 well established facts of organic chemistry, and in spite of his 

 tremendous authority as one of the greatest chemists of his time, 

 the chemical world gradually abandoned the dualistic point of 

 view and accepted a unitary conception of chemical compounds. 

 Then, beginning in the fifties, there came the period of the rapid 

 development of the theories of valence and of structural organic 



^ In the formulas used by Dumas, two, four or six atoms. 



