THE PRESIDENTS ADDRESS 23 



The development of the theory has demonstrated ahnost as cer- 

 tainly that atoms or groups of atoms carrying one or more units 

 of an electrical charge exist independently in a solution of an 

 electrolyte, as the study of gases has demonstrated that inde- 

 pendent molecules of nitrogen peroxide exist in the gas which 

 consists of a mixture of nitrogen peroxide. XO., and nitrogen 

 tetroxide, X.O4. What becomes of these charges when the ions 

 reunite, is a question which has scarcely been raised till recently. 



From the idea that the reactions between electrolytes in solution 

 take place between ions bearing electrical charges, it is a very 

 simple and natural step to the thought that other reactions may 

 occur in a simiilar manner. An application of this thought to the 

 reactions of the elements soon leads to the view that the molecules 

 of elementary substances may sometimes separate into positive 

 and negative atoms. This conclusion was. perhaps, first expressed 

 by Van't Hoflf in 1895, in an attempt to explain the formation of 

 ozone during the slow oxidation of phosphorus.^ Similar views 

 were expressed by myself- in 1901, and were quickly followed by 

 a statement from Professor Stieglitz that he had presented the 

 same idea some time before at the University of Chicago. Three 

 years later Professor Abegg, of Breslau, published a remarkable 

 paper in which he developed the idea of the electrical polarity of 

 thf" atoms in considerable detail, and proposed his theory of 

 normal valences and of contra valences, the sum of the two kinds 

 of valences for any given atom being eight. Thus a nitrogen 

 atom may develop toward hydrogen three negative, normal val- 

 ances or toward oxygen it may assume five, positive, contra val- 

 ences. In this paper Abegg points out for the first time a probable 

 connection between his theory and the theory of electrons. 



Lavoisier and many others of the earlier chemists seem to have 

 considered an atomic or molecular constitution of matter as prob- 

 able, but the first definite basis for an atomic theory was, of 

 course, laid by Dalton's discovery of the laws of combining 

 weights and of miltiple proportion, in the early days of the 

 nineteenth century. He did not discover the law of constant pro- 

 portion, and the evidence which he gave for the law of miltiple 

 proportion was exceedingly crude from the quantitative stand- 

 point: indeed, there is to-da>-, as far as I know, only a single 

 case for which the law has been demonstrated with an accuracy of 

 one part in a thousand, and the determinations for that single 



' Z. physik. Chemie, l6. 411 (1895). 

 - T. Am. Chem. Soc. 2S, 460 (1901). 

 Mbkl., -V. 797 (1901). 



