NOVEUBEB 5, 1909] 



SCIENCE 



619 



recall to them that neither do dry hydrogen 

 and oxygen apparently unite at ordinary 

 temperatures, even in the presence of plat- 

 inum. Yet Bodenstein has proved that 

 they do unite appreciably at temperatures 

 above 500° C. in the presence of platinum, 

 and he has calculated that if the observed 

 temperature coefficients hold it would re- 

 quire the volumes of hydrogen and oxygen 

 with which he worked several hundred 

 years to form a small fraction of a milli- 

 gram of water at ordiyiary temperatures. 

 We sincerely hope that our ardent admirers 

 of ions wiU start these experiments for 

 their posterity to finish as a monument to 

 them. On the other hand, the defender of 

 the divine right of molecules to enter into 

 instantaneous chemical reactions ignored the 

 small minority of the ions present with the 

 large number of molecules in such mixtures 

 as hydrochloric acid and copper oleate in 

 benzene solution. It does not foUow that 

 because such solutions hardly conduct the 

 electric current there may not be present, 

 in practically instantaneous equiKbrium 

 with the molecules, minimal amounts of 

 ions which react with each other with enor- 

 mously high velocities. 



This point is perhaps best illustrated by 

 the beautiful work of "^ATiitney, Melcher, 

 Kuester, Bodlaender and others, on solu- 

 tions of the complex cyanides and am- 

 monium compounds of copper, cadmium 

 and silver, in which only minimal traces 

 of the silver, copper and cadmium ions 

 exist. A normal solution of KAg(CN)2 

 contains so few silver ions that the disso- 

 ciation constant, 



Ag(CN). 

 AgX(CN)»' 



has the enormously high value 10-', which 

 expresses in liters the volume of water 

 which flows over the Niagara Falls in 

 500,000,000 years! Yet the silver can be 

 electrolyzed out of this same solution in 



a very .short time. The dissociation con- 

 stant of the complex cadmium cyanide ion, 



Cd(CN). 

 Cdx(CN)*' 

 is 10" and that of hydrogen sulphide, 



HXSH~ ' 



is 1.75 X 10^, and yet in a mixture of 

 solutions of these two substances the cad- 

 mium sulphide is precipitated almost in- 

 stantaneously. Indeed it seems possible 

 that ions with the mass which we are accus- 

 tomed to ascribe to them must be so far 

 apart in such solutions that they can not 

 possibly migrate rapidly enough to allow 

 all of them to come together in such a short 

 time, and it may be that further investiga- 

 tion will show that these complex cyanides 

 and ammonium compounds react in some 

 cases through their complex ions or mole- 

 cules. Kahlenberg has done great service 

 to physical chemistry by insisting that some 

 reactions may be those of molecules. It is 

 quite evident then that in such inorganic 

 reactions, which usually take place with 

 very high velocities, it is sometimes very 

 difficult to decide whether the constituents 

 directly concerned are molecules or ions. 



But organic chemistry offers us a rich 

 field in which all kinds of reactions can be 

 found. Most of these take place compara- 

 tively slowly and can be studied easily, and 

 the values of the various constants can be 

 determined accurately. In some reactions 

 we have already shown that both ions and 

 molecules undergo transformation. Of 

 course many organic reactions do not lend 

 themselves to quantitative work, on account 

 of complex or disturbing side reactions 

 which can not be followed accurately, or on 

 account of the difficulty of finding a good 

 analj'tical method for determining the con- 

 centration of each constituent at any mo- 

 ment. But it is certain that we could not 

 have had the best development of the law 



