If V is equal to 1 volt and t to 10-, this is about 44. Thus, if we take the cases of distilled water and an electrically neutral solution of sodium chloride, which is an exceedingly weak solution; then, if there were a difference of potential of 1 volt between distilled water and air, the apparent surfacetension of the salt-solution would exceed that of pure water by about 44. The effect of the electrification on the surfacetension is proportional to the square of the potential-difference. Thus for a potential-difference of one tenth of a volt the difference between the surface-tension of distilled water and a weak salt-solution from this cause would only be about 44. It is doubtful whether the measurements of surface-tension could be trusted to detect differences as small as this; so that all we can infer from the observations on the surface-tension of solutions is that the difference of potential between distilled water and air cannot be much greater than one tenth of a volt.

There seems no reason for limiting the possession of this double coating to liquids. It is possessed by liquids of the most diverse characters, as is shown by the electrification developed by drops of water, mercury, molten metals, turpentine, &c. If, however, we suppose that solids possess such a coating, it is evident that the rubbing off of part of one of these coatings when two solids are rubbed together would show itself as electricity developed by friction. Indeed it seems quite possible that a large part, if not the whole, of the electricity developed by friction may be due to this cause.

Another phenomenon in which I am inclined to think this double layer of electrification over the surface of bodies plays an important part, is the electrification of metals and fluorescent liquids when exposed to the influence of ultra-violet light. These substances under such circumstances acquire a charge of positive electricity, the negative electricity going to the air. Now in mercury and molten liquids and solutions of the fluorescent substances eosine and fluorescene the positive layer of the double coating is next the metal or liquid, the negative layer next the air. It seems quite possible that when there is intense reflexion of light from the surface of these substances, the outer coating may get partially dissipated, leaving the metal or liquid with a positive charge. A solution of rosaniline also shows the same effect when exposed to ultra-violet light, though the electrification of its drops is at low temperatures of the opposite sign to that of

solutions of eosine or fluorescene. We have seen, however, that at high temperatures the electrification of a solution of rosaniline changes its sign; and the state of the outer layers when exposed to intense light may very well be more analogous to that of the liquid at a high temperature than at a low one.

Again, the formation of large drops of water by the impact of smaller ones is analogous to the splashing of the waterdrops against a wet surface, and is likely to give rise to analogous electrical effects on a smaller scale. If this is so, the large drops of rain which frequently accompany thunderstorms may be to some extent rather the cause than the effect of the storm. I believe that this has been suggested by Sir G. G. Stokes.

The difference in the behaviour of different gases with reference to the two electricities is very conspicuous in these experiments, oxygen and chlorine acquiring a charge of negative electricity when under the same circumstances hydrogen acquires a positive charge. This suggests that the energy possessed by an atom of hydrogen, for example, when charged with a unit of positive electricity is not the same as that possessed by the same atom when charged with a unit of negative electricity; or, as v. Helmholtz expresses it, the atoms of various substances attract the two electricities with different intensities. If the atoms possess this property they will tend to acquire definite atomic charges, and thus tend to have a definite valency. We can see this if we remember that the likelihood of the formation of a chemical compound is conditioned by the changes in the energy which accompany the formation of the compound. We shall for the sake of clearness confine our attention to the changes which go on in the potential energy. Any chemical change will tend to go on if it is accompanied by a decrease in the potential energy, and will tend to be reversed if it is accompanied by an increase in that energy. Let us consider the case of an element the potential energy of whose atom is diminished by Q when one unit of positive electricity is communicated to it. If C is the electric capacity of the atom, then, when the atom has a charge of n units of positive electricity, its electric potential energy is

1 no -Qn. 2 C

Thus the charging of the atom will result in a diminution of the potential energy until, supposing the unit of electricity to be indivisible, the charge on the atom is the integer just less than CQ: when the charge exceeds this an increase in Phil. Mag. S. 5. Vol. 37. No. 227. April 1894. 2 B

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On the Electricity of Drops.

the charge will be accompanied by an increase in the potential energy. Thus there will, on account of the property which may be expressed as a specific attraction of the atom for electricity, be a tendency for the atom to acquire the definite charge CQ. If, however, the atom has a definite charge it has a definite valency. We must remember, however, that when chemical changes are going on, the charges gained by one atom are lost by another; and it is the total change in the potential energy which determines whether the change shall go forwards or backwards. Thus in some cases we may have an atom charged to an extent which involved an increase in its potential energy, because the abnormal charging of this atom may have involved such a decrease in the potential energy of some other atom as to more than compensate for the increase in that of the atom under consideration. case the atom would not have its normal valency.

In this

It would appear that if the atoms possess this specific attraction for the two electricities, a large rotating body ought to produce a magnetic field. For consider a substance the atoms in whose molecules attract the two electricities with different intensities, and suppose, for example, that the electro-negative component is the more energetic of the two then, in consequence of the uncompensated attraction of this atom for a negative charge, there will be in the neighbourhood of the atom an electric intensity in the same direction as that which would exist if the atom had a positive charge. This effect may be inappreciable at finite distance from the molecule and produce no external electrostatic effects. When, however, the molecule is set in motion, the specific attraction of the molecule would make the positive electric tubes move in a different way to the negative ones. This differential motion of the tubes would produce a magnetic field of the same general character as that due to a positive charge moving in the direction of the molecule. In the case of a rotating sphere the maximum magnetic force at the surface would be proportional to wa2, where is the angular velocity of rotation, and a the radius of the sphere. If we take the earth's magnetic force as an index of the superior limit of the magnetic force due to a rotating sphere of the size of the earth, we find that the magnetic force due to a sphere 1 foot in radius rotating 100 times a second would not exceed more than about one hundred millionth part of the earth's magnetic force.

I have much pleasure in thanking my assistant, Mr. E. Everett, for the assistance he has given me in these experiments.

XXXII. The Densities of Solutions of Soda and Potash. By SPENCER UMFREVILLE PICKERING, F.R.S.*

IT

T is somewhat remarkable that there exist no modern or exact determinations of the densities of solutions of soda or potash. Berthelot, it is true, made series of observations with both substances, but his results have no pretensions to accuracy, the values being given to the third decimal place only, and applying to temperatures varying from 10° to 15°. The tables which are generally reproduced in text-books, as well as in the works of Schiff and of Gerlach on densities, are those which Tünnermann constructed as far back as 1827. These tables are based on half a dozen determinations at most, and on a theory which can scarcely be accepted at the present day; they are fairly voluminous, and as Tünnermann calculated the values for percentage strengths not expressed by round numbers, they seem to have been generally accepted as experimental instead of calculated values. They show errors extending up to a unit in the second decimal place, and when plotted out give figures bearing very little resemblance to the true ones. Dalton also made some determinations ('Elements,' ii. p. 315 †), and possibly also Richter (Stöchiometrie, iii. p. 332†), but as I have failed in procuring his work (or Dalton's either) I am uncertain whether they were original determinations or not. In any case determinations of such a date could only be roughly approximate.

Apart from the desirability for practical purposes of having accurate tables of the densities of such familiar substances as soda and potash, there were questions of theoretical importance which induced me to investigate their solutions. My work on Sulphuric Acid (Chem. Soc. Trans. 1890, pp. 64, 331) had indicated the existence of numerous hydrates in solution, but beyond the indications afforded by the changes of curvature themselves there was very little independent evidence as to the particular hydrates indicated, one only had been known to exist in the solid condition, and one other was subsequently isolated. But in the case of soda, my recent work (Chem. Soc. Trans. 1893, p. 890) has shown that the hydrates which can be crystallized from moderately strong

*Communicated by the Author.

These references are taken from Watts's 'Dictionary of Chemistry,' v. p. 339.

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solutions are very numerous, and if it is established that the properties of the solutions show changes of curvature at points corresponding to these isolated hydrates, we shall have further evidence of a very strong character in favour of the reality of these changes, and of the view that they indicate the existence of hydrates in solution.

The Determinations.

The soda used was made from sodium, and the potash was purified by crystallization from alcohol. The lumps were first washed in well-boiled water to remove any adhering carbonate, and were then dissolved to form solutions nearly saturated at ordinary temperatures, and filtered through glass-wool. These strong solutions were subsequently diluted to the required strengths by the addition of accurately weighed quantities of water. The solutions were made up twelve to twenty-four hours before their densities were determined, specially stopped bottles of common white glass being used for holding them during this time. The same solution was never used twice. In spite of every precaution the solutions could not be obtained entirely free from carbonate: the presence of a little carbonate, however, would have no appreciable effect on the relative accuracy of consecutive observations, which is the chief consideration in the present work.

The strength of the stock solutions was determined by titration, and was probably correct to 0.05 per cent. of the total alkali present. The relative strength of the consecutive solutions is dependent only on the balance error in diluting them, and is therefore represented by a much smaller quantity.

The density-determinations were made in a precisely similar manner as in the case of sulphuric acid, the temperature, however, being 15°+01. The various determinations. of consecutive strengths were not always made consecutively. The results with soda, for instance, were made in five or six different series, each extending throughout the whole range of strengths investigated, and any changes which are noticed cannot, therefore, be explained away by the suggestion of there being a certain error affecting a certain day's work, and not affecting that of another day.