March 25, 1898.] 



SCIENCE. 



407 



ties mean the relative distance through 

 which the ions move between the salt mole- 

 cules, or express their relative velocities in 

 reference to the solution, the change in 

 concentration being a function of the rela- 

 tive ionic velocities. 



Hittorf's analj'ses enabled him to give 

 numerical values to these relative veloci- 

 ties. The experiments of Nernst, Loeb and 

 others have extended Hittorf's results, and 

 have shown that in dilute solutions the rela- 

 tive velocities, of the ions are independent 

 of the difference of potential between the 

 electrodes and are only slightlj', if at all, 

 influenced by temperature. Hittorf pointed 

 out that a knowledge of the conductivity 

 of electrolytes should give valuable inform- 

 ation in reference to the nature of electro- 

 lytic action. A great deal of work has 

 been done in this direction by Horsford, 

 Wiedemann, Beetz, the Kohlrauschs and 

 others. The most notable, perhaps, was 

 the work of P. Kohlrausch, who devised a 

 method of measurement, using alternating 

 current by which results of high accuracy 

 were obtained. Kohlrausch's results give 

 the sum of the ionic velocities, and thus 

 combined with the results of Hittorf on 

 change of concentration, which gave the 

 ratios, the absolute velocity can be ob- 

 tained. It appears from these results 

 that the velocity of the ion in very dilute 

 solutions depends only on its own na- 

 ture and not upon the nature of the other 

 ions with which it may be associated. 

 For example, the velocity of the chlorine 

 ion is the same when determined from 

 solutions of KCl, NaCl or HCl. The im- 

 portant general law has also been found 

 that the conductivities of neutral salts are 

 additively made up of two values, one de- 

 pendent on the positive, the other upon the 

 negative ion. If, then, the velocities of the 

 ions themselves be known the conductivity 

 of a salt may be calculated. The results of 

 Kohlrausch received strong confirmation 



from some very ingenious experiments by 

 Lodge and Whefcham in which the migra- 

 tion of the ions was made to produce a 

 change of color in the solution, and could 

 thus be directly observed. 



In 1887 the theory was advanced by 

 Arrhenius and Ostwald that dissociation is 

 directly effected by solution or fusion and 

 that in very dilute solutions the dissocia- 

 tion is practically complete. Arrhenius 

 holds that the ions carry charges of electri- 

 city, positive or negative, dependent upon 

 their nature, but of equal quantity in every 

 ion. The remaining part of the theory is 

 similar to that of Clausius and others. Ac- 

 cording to this theory the ratio of electric 

 conductivities for different densities of solu- 

 tion gives a measure of the relative dissocia- 

 tion or ionization. If the act of solution 

 effects the dissociation necessary to admit 

 of electrolysis chemically pure substances 

 ought not to be decomposed by the electric 

 current, and this is found to be the case. 

 It is curious that two substances like hydro- 

 chloric acid and water, which separately are 

 insulators, should, when mixed, conduct 

 readily, and that practically only one of 

 them should be decomposed. This, how- 

 ever, is only one of the many problems still 

 to be solved. Another question is how do 

 the ions obtain their electric charge ? Still 

 another, what is the nature of the force 

 which causes ionization ? There are many 

 more. 



When we turn to the commercial appli- 

 cation of electro-chemistry we are met with 

 astonishing evidence of activity. Only 

 twenty years ago there was comparatively 

 little evidence of the importance of this 

 branch of applied electricity. At the elec- 

 trical exhibition in 1881 electro-chemistry 

 was apparently of comparatively little 

 prominence. A factory which could pro- 

 duce a few hundreds of tons of copper 

 electrolytically was considered a wonder. 

 The production of thousands of tons a 



