B.—CHEMISTRY. 43 
of transport numbers. Hittorf, like Faraday, thought that the trans- 
ference of the two ions through a solution was by means of a Grotthus 
chain, a view which now seems to us difficult to reconcile with the fact that 
the ions move at different speeds. 
Simultaneously, with the work on transference, a number of observers 
like Wheatstone, Wiedemann, and Beetz, were studying the conductivity 
of solutions in the light of Ohm’s Law. Ohm’s papers were actually 
published before Faraday’s, but Faraday knew no German and we are 
left to speculate as to what would have been the effect on him of realising 
the mathematical relationship between the factors that were so often in 
his mind. The problem was complicated by the polarisation of the 
electrodes, and progress was slow until Kohlrausch solved this difficulty 
by the use of alternating current. 
Kohlrausch must always remain the outstanding figure in the experi- 
mental study of the conductivity of solutions. For forty years his genius 
for exact measurement, his fine critical brain and his untiring industry 
were devoted mainly to this work. He devised the experimental methods 
we use to-day, and he surveyed for us with unerring accuracy the field of 
aqueous solutions. But his work went far beyond the mere collection 
of data. In 1876 he recognised the Law of the Independent Mobility of 
Ions. In 1878 he introduced that most convenient term, the equivalent 
conductivity of a solution, and he established the modern conception of 
ionic motion by calculating the mean velocities of the ions and showing 
that they were of the magnitude that would be expected if the ions were 
particles of molecular dimensions moving in accordance with the laws of 
hydrodynamics. In 1886, Lodge confirmed his calculation by measuring 
the actual velocities of the ions in a known field. 
| Kohlrausch quickly realised the theoretical importance of work on 
dilute solutions, devising special methods for their study, and perhaps it 
is only those who have tried to repeat his measurements both in this field 
and on the conductivity of pure water, who can really appreciate the 
experimental skill which attained such accuracy before the days of 
thermostats. To those of us who work in this field his papers are a 
constant source of inspiration, and may I acknowledge here my personal 
debt to him for the encouragement and the ready help which he gave so 
generously to an unknown beginner 4 
The evidence of Kohlrausch and Hittorf for the independent movement 
of the ions in dilute solutions was so clear that to us it seems surprising 
that the ionic theory as we know it to-day was not immediately forth- 
coming. As early as 1857 Clausius had pointed out that, since the 
resistance of a solution obeys Ohm’s Law, no work can be done by the 
current in separating the molecules into ions, which must therefore be 
already in existence, as a result, he supposed, of collisions between 
molecules. But just as the a priori conceptions of chemists had prevented 
the acceptance of Avogadro’s hypothesis for nearly half a century, until 
the chemical evidence in its favour was overwhelming, so again convergent 
evidence from different quarters, chemical as well as physical, was necessary 
before the idea that electrolytes might be largely dissociated into 
electrically charged ions was even entertained. The difficulty in chemists’ 
‘minds was twofold—firstly, how could mere solution separate a molecule 
