October 9, 1896.] 



SCIENCE. 



513 



an amount of patience and experimental 

 skill rarely found ; Ms contributions will re- 

 main classical. In connection with Hittorf 's 

 work, Kohlrausch. recognized that, accord- 

 ing to Faraday's law,the conductivity should 

 be represented by sums of the velocities of 

 the ions, each carrying its electric charge. 

 Thus, having from experimental data on 

 conduction the sum of the velocities, and 

 from Hittorf 's migration constants, based 

 upon changes in concentration, the ratio of 

 the ionic velocities, the absolute velocities 

 of the ions would be calculable. Inasmuch 

 as the quantities he was to deal with were 

 groups of atoms or molecules he determined 

 at once to make the molecule his unit of 

 quantity, and not mass alone. This expedi- 

 ent simplified the comparison of results and 

 has been neglected by physicists. The com- 

 parison of results obtained by making the 

 molecule the unit, revealed at a glance re- 

 lations between the physical behavior of 

 different substances which would have been 

 obscure if the mass had been chosen as unit. 

 The selection of the most convenient proper 

 unit is of great importance in the interpre- 

 tation of results and the enunciation of 

 physical laws. 



Kohlrausch expressed the concentration 

 in gramme molecules per unit volume of solu- 

 tion, the unit solution containing a number 

 of grammes of the electrolyte equal to the 

 number expressing the chemical equivalent 

 on the hydrogen scale, in one litre of water. 

 The measurements were then made upon 

 solutions, the relative numbers of molecules 

 in which were known. The ratio between 

 the conductivity and the number of gramme 

 molecules contained in the solution will 

 then give molecular conductivities. 



The results of such measurements show, 

 that as dilution increases there is an increase 

 in molecular conductivity, that in very di- 

 lute solutions it approximates a limiting 

 value. This increase of conductivity is 

 considerable for bad conductors, less so for 



good conductors. The limiting value in di- 

 lute solutions of good conductors can be 

 reached. In bad conductors, even at the 

 extremest dilution accessible to measure- 

 ment, the molecular conductivity is still far 

 from the limiting value. 



In general there is an increase of conduc- 

 tivity with increase of temperature, usually 

 amounting to about 2 per cent, per degree 

 Centigrade. 



The conductivity of equivalent quantities 

 of neutral salts is of much the same order 

 of magnitude, usually reaching the limiting 

 value at a dilution of 2wo gramme equiva- 

 lent. 



From Kohlrausch's numerical values and 

 Hittorf's constants, the absolute velocity of 

 a large number of ions was calculated. It 

 appears from this, that the velocity of the 

 ion in very dilute solutions depends only 

 upon its own nature and not upon the na- 

 ture of the ions with which it may have 

 been associated ; thus the velocity of the 

 Chlorine ion was found to be the same 

 whether determined from solutions of KCl, 

 l^aCl, HCl, etc. 



This important general law was also 

 found, that the conductivities of neutral 

 salts are additively composed of two values, 

 one depending only upon the metal or 

 positive ion, the other upon the acid radical 

 or negative ion. According to this law the 

 conductivity of a neutral salt can be calcu- 

 lated from a knowledge of the velocities of 

 the ions independently, a test which has 

 been applied in many cases with very sat- 

 isfactory results when checked experimen- 

 tally. For quite a number of compounds, 

 however, the computed results were much 

 too high an abnormality to be discussed 

 later. This law confirms the idea of inde- 

 pendent migration of the ions. 



Kohlrausch's numbers expressing veloci- 

 ties were checked by some exceedingly in- 

 genious experiments by Oliver Lodge and 

 Wethan. By a change in the color of the 



