city-conducting power of which almost nothing was known. Observations have been made on it in seven proportions of solution; and thereby a sufficient knowledge for all purposes has been gained. The method used for measuring the resistances was the one first described in the Nachrichten (1869, Nov. 14)-that of quickly alternating currents, which, with some subsequent improvements (namely, the production of a convenient inductioncoil for the alternating currents, the application of Wheatstone's bridge to the dynamometer, and the introduction of platinized electrodes*), in simplicity and accuracy leaves nothing to be desired. Indeed the determination of the temperature of the liquid under experiment now presents greater difficulties than the measurement of the resistance, if the same degree of accuracy is demanded for both. We have been most careful to make sure of the sensibly complete exclusion of polarization of the electrodes, which, when constant currents were used, hindered the exact measurement of the work of the current in decomposable conductors. To mention one of the tests applied in this direction, we experimented upon a solution of sulphate of zinc, first between electrodes of amalgamated zinc (which generally give no polarization), and then between the platinized platinum electrodes of 2000 square millims. surface which were employed for all the subsequent measurements. The greatest difference between the resistances found corresponded to a temperature-error of about of a degree. With the zinc electrodes, constant as well as alternating currents were used; and at the same time, by the accordance between the results, it was established that the work of the alternating currents follows the same laws as that of a constant current. From the commencement onward, the materials for observation were so arranged as to facilitate comparison when put together in Tables. The solutions contained approximately 5, 10, ... (or, in the case of nitric acid, 6-2, 124, ...) per cent.; and the temperatures were near 0°, 18°, and 40°; so that reduction to exactly these proportions was attended with no risk of error. Our thanks are due to Professor Büchner, Dr. Heumann, and Dr. Rössler for the preparation and analysis of the most concentrated solutions of each substance. The other solutions were prepared from these by weight. The specific gravities (at 18°; water at 4° equal 1) present a second definition, independent of the analysis, of each solution. The electric conducting-powers k, given below, are all referred to that of mercury at 0° as unity. Siemens's standards no. 1135 and no. 1143, which were made use of for the reduction of th * Pogg. Ann. Jubelband, p. 290, vol. cli. p. 378. mercury unit to absolute measuret, served for this reduction. The resistance of a column of liquid of 1 square millim. base and 1 millim. length, is found to be, in absolute measure, 9717000 millim. millim.2.milligr. is at the same k sec This, in sec. time the work of the unit current which passes this column in a second. Scarcely any property of bodies depends to so great a degree upon the temperature as the conductivity of electrolytes, which at middle temperatures is influenced as much as ten times as powerfully by heat as the pressure of a gas. On this account observations of the resistance without statement of the temperatures of liquids possess but little value. But even apart from this, the influence of temperature is here of singular importance, precisely on account of its unusual magnitude; for it follows that the electro-chemical work of the current stands in intimate relation with the thermal condition of the liquid, the tracing-out of which relation may supply an invaluable explanation on the nature of electrolysis. We have comprehended the observations of each solution in the formula k=ko(1+at+Bt2), in which k, signifies the conductivity at temperature t. Besides these constants ko, a, and B, the following Table contains the conducting-power at 18° multiplied by 108, and, finally, the increment for 1° in the vicinity of 18°, ex under ( 1 dk " k dt 18 pressed in fractions of the conducting-power at 18°. The percentages denote parts by weight of anhydrous salt, or of nitric-acid hydrate, in parts by weight of the solution. The specific gravities are for 18°. The solutions marked with an asterisk (*) have not been analyzed; but their content was taken, according to the specific gravity, from R. Hoffmann's Tabellen für Chemiker. The conducting-powers &c. set down for the bracketed percentages were interpolated from a graphic representation of the results, and are here and there uncertain to a few units in the last place. The most concentrated solution of NH4Cl precipitated some crystals at 0°, when a leap in the conducting-power was not observed. Two solutions of MgCl, were examined only at 18° and 30°; and two of SrCl, at 18° only. The strongest nitric contained a little nitrous acid. + Nachrichten, 1870, p. 513. It is not unimportant to remark that the present comparison of the two standards gave, to within bo, the same ratio as that made four years previously. The dependence of the conducting-power of the chlorides on the temperature shows, according to the above, great simplicity in many respects. The universally small amount of the coefficient B proves that, with all solutions, the conducting-power increases in nearly equal proportion with the temperature; the positive sign of B, that each of the slight deviations consists of an acceleration. With so strongly pronounced a dependence as we have here (with which 30° rise of temperature about doubles the conducting-power), this nearly equal proportional augmentation could not à priori be expected. It has, however, been observed also in sulphate-of-zinc and sulphuric-acid solutions*, and appears to be a universal property of liquid conductors. Viscous substances only, such as concentrated solutions of chloride of calcium, chloride of magnesium, and sulphuric acid, exhibit greater inequality. A further, very remarkable fact is the near approximation to equality of the temperature-coefficients for the different chlorides in dilute solution. Those at 18°, for example, for all 5-per-cent. solutions, lie between (for LiCl) and (for NH, CI); the graphic representation permits the conjecture that with further dilution they would come still nearer together; nay, it is probable that they tend to the same limit (about). And certainly this limit cannot signify the temperature-coefficient of pure water, since the conducting-power of this is generally a vanishing quantity in comparison with the numbers in the above Table. The temperature-coefficient of sulphate-of-zinc solution, too, observed by Beetz, appears as the dilution is increased to approach towards about the same limit. With increasing amount of salt contained, all the temperaturecoefficients at first diminish. Afterwards the substances divide themselves into two groups: KCl, NH, Cl, and BaCl, show a diminution of the coefficients up to the greatest concentration, the coefficient sinking in the case of NH4Cl to the lowest value, 4. NaCl, CaCl2, and MgCl2, on the contrary, have a minimum between 10 and 20 per cent.; and thence onward the coefficient rises, that of MgCl, even to. This group-difference appears to be connected with a maximum of conducting-power with the salt-content, exhibited by the latter substances, but not by the former. (Compare what is stated below.) Nitric acid connects itself with the latter group. In the sign of B changing from to it agrees with sulphuric acid; yet the inequality of the augmentation between 0° and 40° is generally slight. The absolute amount of the influence of temperature is less than with the chlorides, and not very different from that observed with hydrochloric and sulphuric acids. * Beetz, Pogg. Ann. vol. cxvii. p. 21; Grotrian, ibid. vol. cli. p. 394. If we now consider, secondly, the dependence of the conductingpower on the amount of salt or acid contained, the only thing common to all the substances investigated appears to be the constancy of the variation. The annexed figure exhibits this better than the numbers in the Table. It has for abscissæ the percentage contents, and for ordinates the conducting-powers at 18°. LiCl, so far as it was investigated, very nearly coincides with NaCl, and is therefore not delineated. Both the absolute quantities of the conducting-powers and the laws according to which they depend on the content vary to a degree which is surprising in bodies which stand chemically so near one another. CaCl, has a maximum (at 24 per cent.), and so has MgCl, (at 20 per cent.). NaCl seems to go towards one; but it is questionable whether it reaches it before saturation (25.5 per cent.). The curve for SrCl, is moderately curved; those for BaCl, and NH, Cl are less so; with KCl the conductingpower at 18° is almost exactly proportional to the salt-content. Indeed, from the Table for 0° it is seen that at this temperature the conducting-power of the KCl solution increases somewhat faster than the percentage strength, which has not, till now, been observed in any liquid. As the above-mentioned minimum of the temperature-coefficient (see p. 421) and the maximum of conducting-power belong to the same liquids, the two properties appear to have an intimate connexion. In general BaCl, is the worst conductor; by far the best is NH, Cl, which in a 25-per-cent. solution conducts about half as well as the best-conducting acids known, and, at all events, is the best among all known salts. It is to be presumed, since the solubility of NH4Cl increases considerably with the temperature, that a solution saturated at 100° conducts at least as well as the best-conducting acid at the same temperature. Accordingly by |