If the potential-gradient at the junction is dV/da, we have dV/dx = yr/A, where represents the total current, r the specific resistance of the solution, and A the area of cross-section of the tube. If v is the observed velocity, the specific velocity for unit potential gradient is given by A is determined by filling a known length of the tube with water or mercury, is read off on a galvanometer previously graduated by means of a Daniell cell and a box of resistance coils, and r is determined by Kohlrausch's method of a Wheatstone's bridge with alternating currents. The solutions must be of nearly equal conductivities, so that a mean value of r may be taken. This is the more important because, unlike the colour-boundary method, the formation of a precipitate is an irreversible process. Measurements cannot, therefore, be made with the current flowing in both directions, which, in the former paper, was shown to get rid of the disturbing effect of any remaining difference in conductivity. All that can be done is to choose solutions whose conductivities are very nearly equal, so that the uncertainty which must appear in the result shall be, at all events, as small as possible. The apparatus was immersed in a water bath, and the results all corrected to a temperature of 18° C., in order that they might be comparable with Kohlrausch's calculated values. The following results were obtained :— Barium.-Solutions used: decinormal barium chloride and sodium chloride, a little sodium sulphate being added to the latter. Temperature, 15.8°. Mean conductivity at 15.8° in reciprocals of legal ohms, 9.60 x 10-3. Mean current, 108/131 ampère. Area of cross-section of tube, 0430 sq. cm. Mean velocity of precipitate, 0.446 cm. in 10 minutes. vA/m =0'000372 cm. per sec. The temperature coefficient was found to be 2.5 per cent. per degree, so that we get for the specific ionic velocity of the barium ion, travelling through a decinormal solution of barium chloride in solid. agar jelly at a temperature of 18°, VBa=0'000393 cm. per sec. For an aqueous solution of this strength Kohlrausch gives ('Wied. Ann.,' vol. 50, p. 385) Ba0'000366 cm. per sec. With new solutions, containing only just enough sulphate to give a visible precipitate, the result was "Ba0'000386 cm. per sec. Calcium.-Solutions used: decinormal calcium chloride and sodium chloride, the latter containing a considerable amount of sodium carbonate, in order to get a visible precipitate; this increases the disturbing effect of the precipitation. Mean Temperature, 18.1°. Mean conductivity at 18.1°, 8·91 × 10−3. Mean current, 1.08/153 ampère. velocity, 0.376 cm. in 10 minutes. Kohlrausch gives Area, 0442 sq. cm. Ca=0'000349 cm. per sec. VCa=0'000290 cm. per sec. Silver.-Solutions used: decinormal silver nitrate and sodium nitrate, the latter containing a little sodium chloride. Mean velocity, Temperature, 17·4°. Mean conductivity at 17:4°, 8.96 × 10-3. Mean current, 1.08/164 ampère. Area, 0'442 sq. cm. 0-480 cm. in 10 minutes. VAg =0'000488 cm. per sec. Kohlrausch gives VAg=0'000462 cm. per sec. The Sulphate Group (SO).-Solutions used: decinormal sodium sulphate and sodium chloride, the latter containing a little barium chloride. Temperature, 15·2°. Mean conductivity at 15-20, 9.69 × 10−3. Area, 0430 sq. cm. Mean velocity, Mean current, 1.08/246. 0.257 cm. in 10 minutes. Another determination in a tube whose area of cross-section was The general result goes to show that the ionic velocities thus measured agree, within the limits of experimental error, with Kohlrausch's numbers.* It has already been shown (Phil. Mag.,' October, 1894) that when travelling through acetates whose concentration is 007 normal, the velocity of the hydrogen ion is about 0.000065 cm. per second, It is worthy of note that all the results for kations are slightly larger than indicated by theory, while the only measurement made for an anion gives a value which is slightly less. This may possibly be a result of the use of jelly. whereas in other solutions, such as chlorides, it is about 0.0030, that is, about 46 times as great. Now acetic acid at the concentration mentioned above has an abnormally low conductivity, only the 1/62 part of that of an equivalent solution of hydrochloric acid, so that, in such cases, the immediate cause of the low conductivity appears to be a reduction in the ionic velocities. An attempt was made to complete the investigation of acetic acid by measuring the velocity of the acetate group C2H3O2. I thought the red colour, which acetates give with ferric salts, might be used as indicator, and for this purpose set up solutions of ferric chloride and ferric chloride coloured red by ferric acetate. These ferric salts are said to be decomposed in solution into ferric hydroxide and the acid. Besides the chemical reasons in favour of this hypothesis, it is supported by the conductivities. Ferric chloride, which gives hydrochloric acid, is known to have abnormally great conductivity, and measurements I have carried to great dilution show that the molecular conductivity reaches a maximum at a certain concentration, and, as the dilution is pushed further, sinks again. This behaviour is characteristic of the solutions of acids. In the case of ferric acetate, acetic acid is produced, and the molecular conductivity is abnormally low. It seemed likely, then, that the red colour produced by acetates, when added to solutions of ferric salts, might be used as a means of measuring the velocity of the acetate group in acid solutions. When the experiment was made, however, it was found that the colour boundary travelled in the wrong direction for an anion, viz., with the current, the specific velocity being 0-00028 cm. per second. Now it is unlikely that an ion should behave thus, and an experiment on the migration phenomena of a solution of acetic acid showed that there was no accumulation of acid round the kathode. The result of further investigation was to show that the red colour of such solutions is due to the presence of soluble ferric hydroxide, and that, under the influence of a current, this is transported through the solution without decomposition in the direction of the current. Among other experiments, a direct measurement of the velocity of the transport was made. If a solution of ferric chloride be dialysed through parchment paper, hydrochloric acid escapes, while a red solution of soluble ferric hydroxide, known as "dialysed iron," remains. This was used to colour a solution of ferric chloride, which was set up in contact with an ordinary aqueous solution of ferric chloride of the same concentration. The specific velocity of the hydroxide could thus be determined by observing the motion of the colour boundary, and came out 0.00033 cm. per second in the direction of the current. It is evident that this is what we were measuring in the case of the acetate described above. The conductivity of the dialysed iron solution is very low, and an investigation on its value for solutions of different concentration led to the conclusion that, in such solutions, the whole work of carrying the current is done by the residual ferric chloride, which is itself probably decomposed to some extent into hydroxide and acid, though perhaps the proportion decomposed is not so large as in solutions without an excess of ferric hydroxide, which is one of the products of the decomposition. An experiment on the migration of acetic acid showed that the velocity of the acetate group (C2H2O2) was, at all events, very small, so that, as in the case of mineral acids, the conductivity is mainly due to the motion of the hydrogen. The following table gives the velocities of all ions which have been experimentally determined : The values are calculated from Kohlrausch's theory for the same strength of solution as that used for the direct observation. In the case of copper, in decinormal copper chloride solution, there are no migration data for this. The velocity of the copper ion at infinite dilution is given by Kohlrausch as 0.00031. The sum of the ionic velocities of cobalt nitrate in alcohol, as calculated from the conductivity, comes out 0.000079, and that for cobalt chloride 0·000060. These numbers are to be compared with the sum of the observed velocities given above, namely, 0.000079 and 0.000048 respectively. The Society adjourned over the Whitsuntide Recess to Thursday, June 13. Transactions. Presents, May 30, 1895. Austin:-Texas Academy of Science. Transactions. Vol. I. No. 3. 8vo. Austin 1895. The Academy. Belgrade :--Royal Servian Academy. Glas. 46, 47. [Servian.] 8vo. Belgrade 1895; Spomenik, 28. [Servian.] 4to. Belgrade 1895. The Academy. Cambridge, Mass.:-Museum of Comparative Zoology. Bulletin. Vol. XVI. No. 15. Vol. XXVI. No. 2. 8vo. Cambridge, Mass. 1895. Coimbra :-Universidade. 1895. Annuario. 1894-95. The Museum. Kazan :-Imperial University. Uchenuiya Zapiski [Scientific The Trustees. The Society. No. 6. The Society. Royal Meteorological Society. Quarterly Journal. Vol. XXI. No. 94. 8vo. London 1895; The Meteorological Record. Vol. XIV. No. 55. 8vo. London. The Society. Royal United Service Institution. Journal. May, 1895. 8vo. London. The Institution. Manchester:-Manchester Geological Society. Transactions. Vol. XXIII. Parts 5-7. 8vo. Manchester 1895. The Society. Naples:-Accademia delle Scienze Fisiche e Matematiche. Rendiconto. Anno XXXIV. Fasc. 4. 8vo. Napoli 1895. The Academy. Paris-Comité International Permanent pour l'Exécation Photographique de la Carte du Ciel. Bulletin. Tome II. Fasc. 3. 4to. Paris 1895. Académie des Sciences, Paris. École Normale Supérieure. Annales Scientifiques. Tome XII. No. 5. 4to. Paris 1895. Muséum d'Histoire Naturelle. Bulletin. Année 1895. No. 3. 8vo. Paris 1895. The School. The Museum. |