February 16, 1900.] 



SCIENCE. 



245 



substances are called non-electrolytes. But 

 this relation does not hold for any sub- 

 stance which, when dissolved in water, con- 

 ducts the current ; and this class, called the 

 electrolytes, includes all the acids, all the 

 bases and all the salts. The exceptions to 

 Van't Hoff's generalization are quite as 

 numerous as the cases which conform to 

 it ; and the former are even more interest- 

 ing than the latter. 



To account for these exceptions it was 

 assumed that in aqueous solutions of acids, 

 bases and salts, the molecules are more or 

 less broken down or dissociated into ions ; 

 the amount of the dissociation increasing 

 with the dilution of the solution. An ion is 

 an atom, or group of atoms, charged either 

 positively or negatively. This suggestion, 

 which has been placed upon its present 

 quantitative bases by Arrhenius, is the di- 

 rect outcome of the analogy between the laws 

 of gas-pressure and of osmotic pressure, 

 pointed out by Van't Hoff. The theory of 

 electrolytic dissociation thus originated. 



The importance of the discovery of the 

 applicability of the gas laws to osmotic 

 pressure can now be seen, when we con- 

 sider that this relation, together with its di- 

 rect consequence, the theory of electro- 

 lytic dissociation, are the two corner-stones 

 of modern physical chemistry. Indeed, the 

 physical chemistry of to-day has grown up 

 almost entirely around these two concep- 

 tions. It is from these two generalizations, 

 directly, that ISTernst has calculated the 

 electromotive force of primai'y cells, and has 

 furnished us with the first satisfactory 

 theory of the action of such cells. Still 

 further, he has gone into the cell itself and 

 analyzed its action, calculating the amount 

 of potential which exists at the several 

 sources of potential, and has shown that 

 molecules as such have nothing to do with 

 the action of the primary element ; only the 

 ions into which the molecules dissociate 

 coming into play. 



These generalizations have, on the other 

 hand, entirely revolutionized our concep- 

 tion of chemical activity. We now know 

 that a large majority of the chemical reac- 

 tions with which we have to deal, take place 

 entirely between ions ; atoms and mole- 

 cules as such, playing no role whatsoever 

 in the reaction. Indeed, we have already 

 reached a stage where it is safe to say that 

 very few chemical reactions are other than 

 ionic ; and very recent work makes it more 

 than probable that atoms and molecules 

 are entirely incapable of entering into any 

 chemical reaction. The chemistry of atoms 

 and molecules is thus rapidly giving place 

 to the chemistry of ions. But space will 

 not allow further applications of these all- 

 important generalizations. 



In addition to these three monumental 

 pieces of work, any one of which would 

 secure permanent fame for its author, Van't 

 Hoflf has made a number of important con- 

 tributions to science. His paper on Solid 

 Solutions has called attention to an interest- 

 ing and important class of phenomena, 

 which had never been in any sense con- 

 nected with solutions. And the activity 

 which characterized the earlier years of his 

 life still manifests itself to an unusual de- 

 gree. This is seen in the number and 

 nature of the investigations which are com- 

 ing from his laboratory in Berlin, and in 

 the volumes which are appearing from his 

 pen. 



If we take into account the nature and 

 significance of the entire work of this re- 

 markable man, it seems quite safe to pre- 

 dict that he will be regarded in the future 

 as occupying a place in the same rank with 

 men like Pasteur and Virchow, Helmholtz 

 and Kelvin. 



The celebration in Eotterdam in honor 

 of the first twenty-five years during which 

 Van't Hoff has worked for pure science, is a 

 memorable event especially in the history 

 of modern physical chemistry. There were 



