890 



'wimuE. 



[N. S. Vol. XV. No. 388. 



of KCl, KNO3, KC2H3O,, etc. Kohlrauseh 

 gives these numbers for six cations and ten 

 anions. The results calculated from these 

 numbers agree well with the observed con- 

 ductivities. 



Methods have been devised for directly 

 observing and measuring the rate at which 

 ions travel. In this connection I may 

 specially mention the names of Oliver 

 Lodge, Whetham and Masson. These 

 measurements a^ee very well with the 

 rates calculated by Kohlrauseh. 



I now show an experiment indicating a 

 way in which such measurements can be 

 made. 



The apparatus* consists of a glass U- 

 tube, with a long stopcock-funnel connected 

 to the lower part of it. The tube is nearly 

 half filled with a dilute (about 0.03 per 

 cent.) solution of potassium nitrate, and 

 then about the same quantity of a solution 

 of potassium permanganate, of the same 

 conductivity as the other solution, is slowly 

 introduced by means of the funnel. The 

 permanganate solution is loaded with urea 

 (a non-electrolyte) so as to make it 

 denser than the nitrate solution; the per- 

 manganate solution now lies in the lower 

 part of the U-tube with a sharp interface 

 between it and the nitrate solution above 

 it in each limb of the tube. If now we 

 connect the electrodes, which were pre- 

 liminarily inserted into the upper parts of 

 the limbs of the tube, with a battery with 

 high difference of potential, a current will 

 pass, and a transference of ions will take 

 place, cations (K) towards the cathode and 

 anions (NO3 and MnO^) towards the 

 anode, and the column of pink color will 

 rise in the limb containing the anode and 

 fall by an equal amount in the other. By 

 this means an approximation can be made 

 to the rate of travel of the ions. 



We now come to a new chapter begin- 



* Experiment from a paper by Nernst, in the 

 Zeitschrift fur Elektrochemie, III., p. 308 (1897). 



ning with 1887; but before entering on it 

 we must turn aside for a little to a subject 

 which does not at first sight seem to have 

 a very close relation with the matter we 

 have in hand. The subject is that of what 

 may be called the osmotic phenomena. 

 These are all connected with the concentra- 

 tion or with the dilution of solutions. They 

 all involve the idea of the work done in 

 concentrating a solution. We need not 

 discuss the theory of these phenomena, we 

 are interested in them now only as they 

 give us methods of ascertaining the molec- 

 ular concentration of a solution. In 

 1883* Raoult showed that in the case of a 

 great many substances, equimolecular solu- 

 tions (with the same solvent) have the same 

 freezing-point In 1886 f he showed that 

 equimolecular solutions with the same 

 volatile solvent have the same boiling-point. 

 Molecular is here used in its ordinary chem- 

 ical sense. These discoveries were eagerly 

 taken up by chemists as promising an im- 

 portant addition to the means at their dis- 

 posal for determining the molecular weights 

 of substances. Convenient arrangements 

 for applying the methods were devised by 

 Beekmann,J and soon came into use in 

 nearly every laboratory. They were almost 

 exclusively used for the determination of 

 the molecular weight of organic substances, 

 and have been found trustworthy in such 

 cases. When, however, van't Hoff§ in his 

 study of the theory of solutions concluded 

 from theoretical considerations that the de- 

 pression of the freezing-point and the rise 

 of the boiling-point are proportional to 



* Raoult, Comjit. rend., XCIV., p. 1517; XCV., 

 pp. 187 and 1030 (1882); XCVI., p. 1653; 

 XCVII., p. 941 (1883). 



t Raoult, Compt. rend., CIII, {). 1125 (1886); 

 CIV., pp. 976 and 1430; CV., p. 857 (1887). 

 Zeitschrift f. physih. Ghemie, II., pp. 353 (1888). 



t Beekmann, Zeitschrift f. physik. Chemie, II., 

 pp. 638 and 715 (1888); IV., p. 632 (1889); 

 VIII., p. 223 (1891 ) . 



§ van't HofT, Zeitschrift f. physik. Ghemie, I., 

 pp. 500-508 (1887). 



