ties, the strongest analogies to that substance. For instance, every thing being equal in both cases, olive oil required forty minutes to produce a certain deviation, while poppy oil, or the oil of the beech-mast, required only twenty-seven seconds to produce the same deviation. One-hundredth part of any other oil added to oil of olives reduces the time to ten minutes. It would, therefore, be easy to discover by means of this instrument the smallest traces of any oil fraudulently mixed with oil of olives.

If the proportion of the foreign substance be considerable, the difference of time necessary to produce the maximum of effect would no longer be sufficiently great, and could not be measured with sufficient precision to indicate the proportion of the elements; but the apparatus might easily be modified so as to adapt it to this kind of determination.

The solid fats are worse conductors than the animal oils, arising no doubt from the large proportion of stearine contained in the former; for M. Rousseau is satisfied, by comparative trials with stearine and elaine, prepared by M. Chevreul, that the conducting power of the latter much exceeds that of the former. The fat of an animal becomes a worse conductor in proportion to the age of the individual which afforded it.

By means of the same apparatus, we may also observe a notable difference between resin, gum lac, and sulphur, the most insulating of all known substances, and silk, crystal, and common glass.

M. Rousseau has not found any difference in the conducting power of liquids, whether spirituous or aqueous, acid, alkaline or neuter, the time required by the needle to arrive at the maximum of deviation being too short, in every case, to ascertain the inequality of its duration. But a modification of the apparatus, similar to that for determining the proportions in an oleaginous mixture, would easily appreciate that difference.

It would be equally possible, and very curious, to try the effect of the two kinds of electricity on different substances; all that would be necessary would be to place the two poles alternately in connexion with the ground. According to Ermann's results, it is probable that a difference would be found between some substances.

ARTICLE VIII.

Abstract of M. Bequerel's Paper on the Electro-motive Actions produced by the Contact of Metals with Liquids, and on a Process for ascertaining, by Means of the Electro-magnetic Effects, the Change which certain Solutions undergo by Contact with the Air. (Read before the Royal Academy of Sciences, April, 1824.)*

IN former papers presented to the Society, M. Bequerel had attributed the electrical effects observed during chemical action, solely to the play of affinities exerted between the combining bodies; concluding that during such action the alkali takes positive, and the acid negative electricity, and neglecting to take into the account the effect resulting from the contact of the acid with the platina cup which contained it, and that of the alkali on the jaws of the forceps (which were also of platina) in which it was held, an action, however, which must by no means be overlooked.

The apparatus which M. Bequerel employed in his present experiments is similar to the electroscope invented and described by M. Bohnenberger, in the Bibliotheque Universelle, Nov. 1820 (see also Annales de Chimie, 1821, vol. xvi.), but instead of two dry piles placed vertically, he uses only one placed in a horizontal position, on a wooden support; to each pole a metallic plate, about three inches long, is fixed vertically, between which the gold leaf is suspended, in contact with the lower plate of the condenser; the condensing plates being nine inches in diameter. The delicacy of this instrument is such that it is sensible to the action of an excited glass tube at the distance of eight or ten feet.

A brass capsule, containing an alkaline solution, was placed on the upper plate of the condenser, and a communication established between it and the ground by touching it with the finger, or a moistened slip of gold-beater's skin, the lower disc being also in connexion with the ground; in a few seconds after, the upper plate was removed, and the gold leaf flew to the positive pole; consequently the alkaline solution had become positive from contact with the copper, and the metal negative.

When sulphuric acid was substituted for the alkaline solution, the electricities were reversed.

To ascertain the electro-motive action of different metals by contact with acid and alkaline solutions, a capsule of the metal containing the solution was placed on the upper plate of the condenser; the lower plate was then touched with a slip of the

*From the Annales de Chimie.

same metal, and the liquid with the finger; thus the electromotive action arising from the contact of the metal under examination with the copper was destroyed, and only the electricity which it had acquired by its contact with the solution remained on the upper plate of the condenser. It is sometimes necessary to place a slip of paper between the metal experimented on and the copper, for the apparatus is so sensible, that a very slight difference in the state of their surfaces would modify the electromotive action.

Operating in this way, it was found that by contact with an alkaline solution, the metal, whenever its electrical state can be determined, becomes negative; and positive with an acid; but with silver, and in many other instances, the electro-motive action is too feeble to be rendered sensible.

These results confirm and extend the observations formerly made by Sir Humphry Davy on the electrical effects produced by the contact of metals with acids and alkalies, in a perfectly dry and solid state, between which there is consequently no chemical action; for they prove that similar effects ensue when the latter substances are in solution, and even when in some cases the contact is accompanied by incipient chemical action.

In order to understand what is the influence of the fluids interposed between the plates of the voltaic instrument, and whether it has any other action than that of a mere conductor to transmit the electro-motive action of the metals from one to the other, it is necessary to ascertain what happens when an acid or alkaline solution is placed between two dissimilar metals. For this purpose, the copper capsule, filled with either an acid or alkaline solution, was placed on the upper plate of the condenser, as before; the solution was then touched with a plate of zine (taking care not to touch the copper with it), and the lower plate of the condenser with the finger, and after a lapse of twenty seconds, the upper plate was removed; the gold leaf flew to the positive pole; consequently the copper capsule had become positive.

The experiment was reversed by using a capsule of zinc filled with either solution; and the lower plate of the condenser was touched with a plate of zinc to destroy the electro-motive action between the capsule and the plate, and a plate of copper, held between the fingers, was immersed in the solution. On raising the upper condensing plate, the gold leaf flew to the negative pole, and consequently the zinc capsule had become negative. We see from these two experiments, that when zinc and copper are separated by an acid or alkaline solution, the copper becomes positive and the zinc negative; just the reverse of what takes place between these metals by simple contact.

"We have also," says M. Bequerel," examined what takes place on the contact of a metal with a saline solution; as cop

per with solution of sea salt; the copper becomes negative, and the solution positive. This result explains why a plate of copper in contact with zinc or tin, as lately ascertained by Sir H. Davy, is less acted on by the sea-water than when not in contact with an electro-positive metal. It cannot be denied, that two substances at the moment they combine are in different electrical states, and that there is a certain relation between those states and the chemical affinities. Now if we can modify those electrical states, it is almost certain that we shall also modify the play of affinities; but we have seen that a plate of copper, by contact with a solution of sea-salt, becomes negative; it follows that if we touch the same metal with an electro-positive metal, the copper will be placed between two bodies, each tending to impart the same kind of electricity, a condition which we know will tend to annul the electro-motive action of the copper on the solution of sea-salt. Thus, according to the electro-chemical theory, circumstances are so arranged as to weaken the chemical action of the solution of sea-salt on the copper.'

The memoir concludes with pointing out a method by means of electricity, of ascertaining the changes which some solutions experience by contact with the air.

Dissolve iron in nitric acid; filter the solution, and immerse into it two lamina of platina, each communicating with one of the extremities of the wire of the galvanometer; leave one of the wires in the solution, withdraw the other, and again immerse it; it will be positively electrified.

The nitrates of copper and lead give similar results for a short time; nitrate of zinc produces no such effect.

When the experiment is made in an atmosphere of hydrogen, no electrical current is established, though all circumstances, except the want of contact with the atmosphere, are precisely similar in both experiments.

"Hence the contact of atmospheric air is indispensable to the production of the electrical current by the immersion of platina laminæ in several fresh-prepared nitrates; but what is the modification that is instantaneously effected in the liquid adhering to the surface of the lamina withdrawn from the solution? We can, to a certain extent, explain this: The solution of a metal in nitric acid gives rise to several compounds: take iron, for instance; first deutoxide of azote is formed, and soon after nitrous acid, a protonitrate and a deutonitrate; by degrees the deutoxide of azote passes to the state of nitrous acid, the protonitrate to that of deutonitrate, and, after a certain time, only deutonitrate remains in the liquid. According to this statement, when we withdraw one of the platina laminæ from the solution, the liquid which adheres to it immediately, in consequence of the thinness of the stratum, experiences changes from

the action of the air, which do not take place till after several hours in the bulk of the solution. It follows, therefore, that when we re-immerse the lamina, we bring in contact two dissimilar liquids, and nothing in that case opposes the production of an electrical current.

"On the other hand, since the immersion of platina laminæ in a solution of nitrate of zinc does not produce any current, although it contains deutoxide of azote and nitrous gas, it is probable that this may be owing to the nitrate not suffering any change by contact with the air, in consequence of the metal being capable of forming only one oxide.'

ARTICLE IX.

On a deoxidating Property of the Vapour of Water.
By C. H. Pfaff.*

It was remarked by Hermbstadt, while making the experi ments from which he deduced the existence of a peculiar colouring principle in sea-water, and its superincumbent atmosphere,+ that if that liquid be boiled in a retort, and if, by means of a glass tube attached to the beak of the retort, the gases and aqueous vapour evolved be made to pass through a solution of nitrate of silver, the latter by degrees assumes the colour of red wine, and at the end of 24 hours, a brownish-yellow coloured sediment is. deposited. I observed the same appearance, on repeating this, experiment with sea-water from the bay at Kiel. As, howeven, I had reasons for suspecting that this change is not occasioned by any peculiar gaseous constituent, I prepared an artificial mixture of solutions of the muriates of magnesia and soda, in the proportions which constitute sea-water, and on making the experiment with this, I still obtained the same result. I observed also that the colour imparted to the solution of nitrate of silver at the beginning, and towards the conclusion of the experiment, is different: at first it is a weak violet, but after the experiment. has gone on for some time (provided there be a sufficient quantity of the metallic salt in the vessel through which the vapours pass), it has a considerable infusion of reddish-brown. Hence I considered it not unlikely, that in these experiments there are two distinct causes which produce discoloration. This induced me to repeat the experiment in a variety of ways; which at last conducted me to the following very interesting results.

The experiments themselves are extremely simple. The

• Schweigger's Journal für Chemie und Physik, xxxvi. 68.

+ This opinion was refuted by Pfaff in a small pamphlet, entitled. Das Kieler See-bad verglichen mit andern Seebädern an der Ostsee und Nordsee, Kiel, 1822.