the voltaic action of the iron and platinum is in full vigour. If the solution be heated to the boiling point, the precipitation of the coloured matter takes place immediately. Zinc immersed in the red liquor, also accomplishes precipitation, and, in a short time, the liquor becomes colourless.

At first, I was inclined to attribute this phenomenon to the presence of manganese in the iron; but from a close examination of the circumstances under which it was exhibited, and more especially from a chemical analysis of the brown precipitate, in which nothing but oxide of iron could be detected, I am led to infer that the colouring of the liquid was due to the formation of ferric acid; or rather to that of ferrate of potassa. The production of ferrate of potassa, under these circumstances, may be easily explained. It may be attributed to a predisposition of the oxygen to combine with the iron, by which, and the influence of an electric current, ferric acid would result which, at the moment of its formation, would combine with a portion of the potassa of the solution; or the electric current may, in the first place, enhance the affinity between oxygen and potassium, and thus give rise to the formation of peroxide of potassium, and afterwards to the ferrate of potassa. This latter explanation appears to be correct, from the fact that, if ammonia be used, ferric acid is not obtained. The phenomenon is interesting on two accounts: first, because we are now, by the aid of voltaic electricity, enabled to form an acid with facility, which M. Fremy, its first discoverer, and other chemists, have hitherto obtained by purely chemical means, only with greater difficulty: and secondly, because it is obtainable only by the employment of cast iron, and not by wrought iron.

This latter circumstance is certainly very enigmatical. I have entered into extensive experimental inquiries for the purpose of ascertaining the cause; but, hitherto, without success. I have never been able to produce ferric acid by employing either wrought iron or steel; neither have I been successful with every kind of cast iron. I have employed four kinds, but with two of them only have I succeeded. This difference in the results, by the employment of different kinds of iron, seemed for some time, to be attributable to a difference in the force of the electric currents; but from a comparison of the electric forces given by a pile in which wrought iron was one of the metals, with those by a pile into which cast iron entered as an element, I found that only a very trifling superiority was due to the latter: that by which ferric acid was producible.

By lengthening the conducting wire of the cast iron battery, I reduced the power of the current to below that of the wrought iron battery; but the ferric acid was no less produced by the former though not by the latter.

In a theoretical point of view, the great similarity in the force of the currents produced by the two kinds of iron battery, appears to me, to be an interesting circumstance: for whether the liberated body (in this case oxygen), be insulated and in a free state, or it be

combined with the metals, the force of the current, and the electro-` motive power, seem not to be affected. This fact is very unfavourable to the opinions of Grove and others, who think that water is more easily decomposed by the electric current when oxygen combines with the positive metal of the pile. At least, it is favourable to nothing further than the generally received opinion that, in the chemical combination of two bodies, there is a great quantity of electricity brought into action. It is to be understood, however, the current must have a certain degree of intensity for the production of ferric acid. I immersed two plates of cast iron in a solution of potassa, and afterwards united them to the metals of one of Grove's simple piles, but in consequence of the small dimensions of the pile the current was too feeble to produce any more than a very small trace of ferric acid. On the positive plate, however, there was attached a small portion of oxide, an incident I have never had to occur on wrought iron; and proves that, under these circumstances, cast iron has a greater tendency to oxidize than wrought iron.

In general, cast iron, and especially that kind by which ferric acid is most abundantly produced, is more soluble in acids than wrought iron probably in consequence of the particles of carbon which are intermixed with it producing voltaic action, and thus facilitating its solution; as in the case with commercial zinc, the impurities of which cause it to dissolve more readily than pure distilled zinc; and it is possible that the production of ferric acid, by cast iron, may be traced to the same cause.

An increase of intensity in the current, to a certain extent, at least, does not interfere with the production of ferric acid. A current from two of Grove's batteries, transmitted through a solution of potassa, between cast iron terminals, produces a rapid formation of ferric acid. This mode of obtaining the acid, I consider preferable to that which I first resorted to; as it is not necessary to put the solution of potassa in contact with an argillaceous vessel, which, consequently, prevents it from becoming impure, and the vessel from destruction, which invariably happens when employed. I must here state, however, that this process is attended with some little inconvenience, arising from the loss of a portion of the ferric acid, by decomposition at the negative terminal metal. This loss, however, is amply compensated by the superabundance of acid which is formed at the positive cast iron terminal. I have called the loss sustained at the negative terminal a trifling one, which, to some persons may probable appear an inappropriate description, by supposing that as much ferric acid would be decomposed at the negative terminal as is formed at the positive one; but this is not the case.

A curious modification of this process is made by employing one cast iron plate, and one of wrought iron. When the latter is the positive terminal in the alkaline solution, oxygen gas will be liberated; but when the former is made the positive terminal, ferric acid will be found.

K

There is always a greater proportion of hydrogen than of iron liberated at the negative pole; hence, a greater proportion of water than of ferric acid is decomposed.

On the other hand, notwithstanding the formation of ferric acid, however abundant, oxygen gas is invariably liberated at the positive terminal plate; and this occurs under all circumstances, whether we employ a battery or only a single voltaic pair; but, in all cases the battery liberates the greater quantity of oxygen gas: it is therefore possible that, by a sufficient increase of intensity of the current, oxygen gas only, and no ferric acid would appear, even with cast iron terminals. This copious liberation of oxygen gas is very unfavourable for ascertaining the composition of ferric acid obtained by the employment of voltaic electricity.

We may rest assured, however, that the quantity of oxygen determined at the positive terminal metal, is the equivalent of the hydrogen liberated at the negative terminal; and if the former were wholly to combine with the iron (whether permanently or not, would be of little consequence), the quantity of liberated hydrogen would be a representative of the quantity of oxygen in the ferric acid; and by deducting the oxygen from the weight of the oxide formed, the quantity of iron in the compound would be determined. But the liberation of oxygen gas, and the uncertainty of its being wholly engaged in the formation of ferric acid, precludes the possibility of applying this process of analysis.

The ferric acid being susceptible of decomposition in so eminent a degree, by the feeblest influences, I have not attempted the insulation of either itself, or its salt. If there be a possibility of their insulation, it can be accomplished at low temperatures only.

In conclusion, I think there is a probability of ferric acid being found, ready formed in nature, and constituting the colouring matter of the amethyst, in which, besides silica and a trace of manganese, iron has also recently been discovered.

On the Preparation of Calomel. By M. SONBERIAN.

IN France and in England, medical practitioners, with very few exceptions, administer that calomel which is prepared by steam: for they find it more active and more certain in its effects than that prepared by other means. The method of preparing this calomel is that shown by Joseph Jewel, modified by M. Ossian Henry, and consists in conducting a mixture of steam and the vapour of calomel into a capacious receiver: a process carried on in France to a great extent. I have practised this process for many years at the Pharmacie Centrale, but by no means approve of it for it is difficult to conduct, requires much dexterity of manipulation, and is frequently attended with accidents, by which a considerable portion of the

Comptes Rendus.

product is lost. Moreover, the calomel prepared in France is neither so white nor in such a fine pulverulent state as that which we receive from England.

I have now to communicate to the Academy, a novel process for preparing calomel, which is very superior to any hitherto made known. It is of a peculiar character, and as it is applied to the production of an article in which we have never yet been enabled to compete with the English manufacturer, I hope the Academy will not think the description of it an intrusion on their attention.

Instead of the vapour of water being interposed between the particles of the vapour of calomel, which prevents them from uniting, as in the steam process, I substitute a current of air, which, by passing over the heated calomel, enters its vapour as it forms, and causes it to condense in an impalpable powder. I heat the calomel in an earthen tube, which passes through a furnace, and through the heated tube I propel a current of air, by means of a small blowing apparatus, which mixes with the vapour, and is driven with it into the receiver. Should the tube be perfectly straight, a portion of the calomel would be carried to the distance of twenty yards but to prevent this inconvenience I bend the further end of the tube, so that it may terminate in a vessel of water; the calomel thus received by the water, deposits in a fine powder, and the end in view is accomplished. To perfect the process, it now remains only to ascertain the most suitable form, and the best material for the vessels in which the calomel is heated. Not having yet met with these essential qualities in any ready made vessel, I am not prepared at present to carry the description further; and must content myself with the complete success in my first attempts with this process; which I have no doubt will be found applicable in the minute division of the volatile bodies.

PROCEEDINGS OF THE ROYAL SOCIETY.

November 18th, 1841.

THE following papers were read:-1. "Variations de la déclinaison et intensité magnétique horizontale observées à Milan le 28 et 29 Mai, le 23 et 24 Juin, le 21 et 22 Juillet, le 27 et 28 Août, et le 22 et 23 Septembre, 1841." Par Sig. Carlini, For. Memb. R.S.

2. "Variations de la déclinaison magnetique et de l'intensité magnétique horizontale observées à Bruxelles le 23 et 24 Juin, et le 21 et 22 Juillet, 1841." Par M. A. Quetelet, For. Memb. R.S.

3. "Meteorological Register kept on board the Earl of Hardwicke, during a voyage from London to Calcutta and back to London, by Captain Alexander Henning." Communicated by Sir John F. W. Herschel, Bart., F. R. S., &c.

4. "Meteorological Register kept at Port Arthur, Van Diemen's Land, by Deputy-Assistant-Commissary-General Lempriere, from

Feb. 1, 1840, to Feb. 1, 1841." Communicated by Capt. Beaufort, R.N., F.R.S., Hydrographer to the Admiralty.

5. "Term Observations of the Variation, Magnetic Declination, Horizontal Intensity, and Inclination, at Prague, for June, July, August and September, 1841." By Professor Kreil. Communicated by S. Hunter Christie, Esq., Sec. R.S.

November 25.-The following papers were read, viz.—

1. "Explanation of the construction, positions, comparisons, and times of observation, of the Meteorological Instruments at the Royal Observatory, Greenwich, with which the Observations have been made that are contained in the sheets of Meteorological Observations, forms 1 and 2, for each month from 1840, November, to 1841, July, both inclusive, sent to the Royal Society in 1841, October 26." By George Biddell Airy, Esq., M.A., F.R.S., Astronomer Royal.

2"On the Laws of the Rise and Fall of the Tides in the River Thames." By George Biddell Airy, Esq., M.A., F.R.S., Astronomer Royal.

The conclusions arrived at by the author, and stated in this paper, were derived from an extensive series of observations of the tides, made, on this suggestion, at the royal Victualling Yard, at Deptford, under the superintendence of Captain Shireff, R.N. The object of the first series of observations was simply to ascertain the times of high and low water, for the purpose of ascertaining the duration of the rise and fall of the tide the height of the water was observed at every quarter of an hour, night and day, during half a lunation. The curves representing the law of rise and fall of the water were found to be different for high tides and for low tides; and both are sensibly different from the line of sines. The author then investigates mathematically the motion of a very long wave, such as a tide-wave, in a rectangular canal, whose section is everywhere the same, on the supposition that the extent of vertical oscillation bears a sensible proportion to the mean depth of the water; and deduces an expression for the vertical elevation of a particle at the surface. This expression supposes the canal unlimited at the end farthest from the If the canal be stopped by a barrier, the expression changes its form. The formulæ obtained by the author enable him to explain a circumstance, hitherto perplexing, namely, that the age of the tide is different as inferred from the height of the high water, or from the time of high water; being always greater in the former mode of estimation.

sea.

66

3. Register of Tides, observed at Coringa, from January 1st to June 30th, 1841."

4." Meteorological Journal, from the 20th April, 1840, to the 29th April, 1841. Kept at the Falkland Islands on board H.M. Ketch"Arrow."

5. Daily Thermometrical Observations at Cape Palmas, for May, 1841."

These last three papers were communicated to the Society by