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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°. LICI, 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, CI, 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 NH, Cl 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
no means so high a place belongs to the acids as is generally assumed for them. In galvanic piles, for example, a nearly saturated solution of sal ammoniac can with advantage be employed in preference to the strongest acids that can be used for
In another salt of ammonium, also, namely the nitrate, Wiedemann found a high conducting-power*.
The behaviour of MgCl, is remarkable. When the conducting-powers of its solutions are compared with those of the other chlorides of equal concentration, the former take the second place when very dilute, at 10 per cent. the fifth, and from 22 onward the last
Nitric acid shows a maximum of conducting-power, namely for 18° when it contains 29.7 per cent. HNO. It was already found previously that a maximum belongs also to sulphuric and hydrochloric acidst. It appears remarkable that these maximal conducting-powers of all three acids have nearly the same magnitude. Attention has already been called to this by Quincke.
If we try to express the conducting-power k as a function of the salt-content P, we find that for the chlorides the form k=ap+bp + cp8 renders the observations with tolerable completeness; but the conducting-power of nitric acid is not even approximately represented by this expression. As, moreover, empirical laws in which the number of terms is considerable present for calculation no advantage over a Table with an equidistant argument, nor exhibit in their coefficients a recognizable physical meaning, it would be superfluous to go further into this subject.
On the contrary, it is evidently important to compare quantitatively the different substances in those solutions in which they are at once comparable—that is, in but slight concentration. For the conducting-power of pure water is, in comparison with the above numbers, to be put sensibly equal to zero; and the course of the curves (p. 422) shows that the conducting-power constantly increases; consequently dilute solutions have a limit which the ratio of the conducting-power to the salt-content approaches : it may be named the specific conducting-power of the substance in
solution. If the observations for the contents 0·05 and 0-1 (i. e. 5 and 10 per cent.) be expressed in the form
k=ap+bp?, a will represent very nearly the specific conducting-power just
• Pogg. Ann. vol. xcix. p. 228.
424 Electric Conducting-power of the Chlorides of the Alkalies, &c. now defined. At the same time it is immaterial for a whether the solution be reckoned in parts by weight (as is done here), or (as is more rational according to the definition of the conducting-power) by volume, since for dilute solutions the volume is equivalent to the weight. Also the quantity b, which denotes the initial deviation from proportionality, has a definite signifi. cation for each salt.
Only the results for 18° shall here be given, as their form for the other temperatures is very similar. They are :
k=0.000177 p-0.0000011 p?
k=0.000077 p-0-0000008 p
HNO3. k=0.000534p-0.0000101 p. According to this, the total character of each curve already shows itself while the content is yet very small: those substances which have a maximum of k at a definite degree of concentration, are distinguished by a relatively high value of b. (It may therefore be conjectured that LiCl also will show a maximum.)
If now we seek to connect the specific conducting-power a with other physical properties of the substances dissolved, we readily perceive that for the chlorides the quantities a stand nearly in the inverse order in a series to that of the equivalentweights A of the anhydrous salts—indeed so that, with equal amounts of chlorine in solution, the conducting-power of dilute solutions is not very different. Still the deviations of the pro. ducts A.e from their mean amount to as much as 22 per cent. (Vide infrà.)
On the other hand, another accordance of an arrangement is self-evident-namely, according to the specific gravities s of the anhydrous salts. The products 8.a are, for the chlorides of the alkalies and alkaline earths, constant quantities, the greatest deviation from the mean being 12 per cent. Although this deviation is not inconsiderable, yet so simple a relation is deserving of notice. If it were rigorously exact, it would signify that equal volumes of anhydrous salts in solutions imply equal conductingpowers.
In the following Table the salts are placed in the order of their conducting-powers a, and together with their equivalentweights A and specific gravities s. For the latter I am indebted partly to the memoirs*, and partly to the information by letter, most kindly communicated, of H. Schröder.
A.a may be called the specific conducting-power according to equivalents; s.a that according to volume.
Finally, the quantity b, on which the amount of flexure of the curve depends (p. 422), appears in the case of the chlorides in general to stand in relation to the internal friction of the solutions, so far as ocular inspection can warrant a judgment on the latter property; for the liquids which have b large exhibit in general decidedly more viscosity with greater concentration than the restt. On the other hand, nitric acid is still a very mobile liquid even when more concentrated, while it yet (as well as hydrochloric acid) has b large and exhibits again a diminution of conducting-power from a moderate percentage content onward.
It seems, then, that other molecular properties than the viscosity of solutions come into question here. At all events further data are requisite in order to accomplish a mechanical theory of electrolysis, perhaps upon the basis given by Quincke (l. c.).
Darmstadt, July 1874.
XLIX. The Electrolysis of certain Metallic Chlorides. By J. H.
GLADSTONE, Ph.D., F.R.S., Fullerian Professor of Chemistry in the Royal Institution, and ALFRED TRIBE, F.C.S., Lecturer
on Chemistry in Dulwich College I. WE
E have previously shown that nitrate of copper brought
into tension by silver and copper in conjunction is decomposed by free oxygen in solutiong. Thinking that chlorine might be substituted for oxygen, we commenced some experiments, employing chloride of copper, and observed some facts which seemed to have an interest from their bearing on the causation of galvanic action.
* Pogg. Ann. vol. cvi. p. 226, vol. cvii. p. 114, Suppl. vol. vi. p. 58; and a Monograph, 1873, Heidelberg.
T Compare Hankel, Pogg. Ann. vol. Ixix. p. 263; Wiedemann, ibid, vol. xcix. p. 229; and Beetz, ibid. vol. cxvii. p. 17.
| Read before the Physical Society, 1875. Communicated by the Society.
Š Proc. Roy. Soc. vol. xx. p. 290.
It is known that if metallic copper be placed in a solution of cupric chloride, it will slowly become covered with a crystalline deposit of the insoluble cuprous chloride :
Cu + CuCl,=2 CuCl. We found that when metallic copper and platinum are connected by a wire and immersed in cupric chloride, the insoluble salt forms not only upon the copper, but also on the platinum plate, as a white crystalline body. This deposit may generally be observed in about two minutes when the plates are three quarters of an inch apart. The formation of cuprous chloride upon the platinum plate takes place about equally rapidly in solutions containing 2.5 or 10 per cent. of salt. With a 20 per cent. solution the deposit was smaller, and with 40 per cent. practically nil, although there was abundant formation of cu. prous chloride upon the copper plate.
We satisfied ourselves that the action took place equally well in solutions from which oxygen had been rigidly excluded, and also that a current passed from the copper to the platinum through the liquid--that is, from the metal of higher to that of lower potential.
In order to test whether this electrolysis of cupric chloride into CuCl and Cl could be effected by weak currents ab extra, we tried the effect of a zinc-platinum cell excited by common water and with platinum electrodes, and found that cuprous chloride deposited upon the negative electrode and chlorine at the positive, a little of which entered into combination with the platinum, but the greater part passed into the liquid. A cell excited with dilute sulphuric acid acted in a similar manner. A single Grove's cell
gave for the first two or three minutes cuprous chloride on the negative platinum electrode, but afterwards metallic copper, while chlorine always formed at the positive plate.
As zinc immersed in a salt of copper is capable of throwing down that metal, an experiment was tried with plates of zinc and platinum in connexion immersed in the chloride; the result was a more energetic action than with a copper-platinum couple similarly arranged, and besides a thick coat of cuprous chloride the edges of the platinum were incrusted with metallic copper. A similar magnesium-platinum couple gave a similar result, but with a decidedly greater proportion of metallic copper.
As there are two chlorides of mercury, similar to the two chlorides of copper, analogous experiments were tried with solution of corrosive sublimate,