that a magnetic needle might have been substituted in place of the unmagnetised one, during a few seconds when he left the apartment, yet such a supposition was completely excluded by the high integrity of Morichini, and of the young Italian Nobleman who assisted him in the experiment.

It is not, however, on the testimony of Morichini alone that we are disposed to give credit to this new property of light. The very same results were obtained by Dr Carpi at Rome, and M. Cosimo Ridolfi at Florence; and though, under a more northern and less serene sky, the experiment has been unsuccessful, yet, when we consider this difference of circumstances, we cannot regard the evidence of those who have failed as in the least degree invalidating the evidence of those who have succeeded. Dr Carpi maintains, that the temperature is a matter of indifference, but that the clearness and dryness of the air are of essential importance. M. Ridolfi exposed his needles to the influence of the extreme border of the violet ray. He magnetised some in thirty, and others in forty-five minutes; and he considered the chemical rays as contributing to the success of the experiment. His experiments were performed under almost every variety of circumstances. He magnetised the needles when the apartment was rendered very humid; but when the violet ray passed through a column of water in vapour, or when the needle itself was immersed in water, no effect was produced. When the violet light was transmitted through the thick smoke of burning sugar, the needle received only a very slight degree of magnetism *.

M. Berard, an eminent chemist of Montpellier, repeated the experiment of Morichini, when he was engaged in examining the invisible and the deoxidating rays. As we have not seen his own account of the results which he obtained, we must content ourselves with quoting the abstract of them, given by M. Cuvier in his account of the labours of the Institute for 1813. Equally decisive results have not been obtained respecting the property of magnetising steel, ascribed to the violet ray by M. Morichini, a well-informed Roman chemist. Although needles exposed to this ray appeared to be magnetised in cer

• See Brugnatelli's Journal 1816, 5th bimestre, or the Bibliotheque Universelle 1817, tom. v. p. 1.

tain experiments, they underwent no such change in many other trials; and at present no reason can be assigned for this difference, for in both cases every other known source of magnetism had been carefully removed. The summer of 1813, indeed, was not favourable to these kind of experiments, it was so bad."

About the end of April 1817, when Professor Playfair was at Rome, he availed himself of the opportunity which was presented to him of witnessing this singular experiment, which was performed by Dr Carpi, in the absence of Morichini, before a party of English and Italian gentlemen. The violet light was obtained in the usual manner by means of a common prism, and was collected into a focus by a lens of a sufficient size. The needle was made of soft iron, and was found, upon trial, to possess neither polarity nor any power of attracting iron filings. was fixed horizontally upon a support by means of wax, and in such a direction as to cut the magnetic meridian at right angles. The focus of violet rays was carried slowly along the needle, proceeding from the centre towards one of the extremities, care being taken never to go back in the same direction, and never to touch the other half of the needle. At the end of half an hour

It

after the needle was exposed to the action of the violet rays, it was carefully examined, and it had acquired neither polarity nor any force of attraction; but after continuing the operation twenty-five minutes longer, when it was taken off and placed on its pivot, it traversed with great alacrity, and settled in the direction of the magnetical meridian, with the end over which the rays had passed turned toward the north. It also attracted and suspended a fringe of iron filings. The extremity of the needle that was exposed to the action of the violet rays, repelled the north pole of a compass needle. This effect was so distinctly marked, as to leave no doubt in the minds of any who were present, that the needle had received its magnetism from the action of the violet rays *.

M. Dhombres Firmast, who resides, we believe, at Alais in

An account of this experiment was drawn up at Professor Pictet's request by Mr James Playfair, and published in the Bibliotheque Universelle 1817, vol. vi. p. 81.

† Ann. de Chim. et Phys. Mars 1819, p. 286.

the South of France, having repeated at an early period the experiments of Morichini without success, was induced, by Mr Playfair's account of them, to re-examine the subject with great attention. On the 18th October 1817, about 2 o'clock in the afternoon, when the external thermometer stood at 14°, the one in his chamber at 15° and 15° 2, and Saussure's hygrometer at 41°, he admitted the sun's direct rays through an aperture three-fifths of an inch in diameter. A prism placed behind this aperture formed the solar spectrum upon a piece of card perforated so as to allow only the violet rays to pass. A lens was fixed behind this perforation; but instead of making the focus of the violet rays pass over the needle, he found it easier to make the needle pass through this focus always in the same direction. After continuing this process for an hour, the needle being nearly in the direction of the meridian, he could not discover that the slightest degree of magnetism was communicated to it.

The connection between light and magnetism, which appears to be indicated by the preceding results, has been supposed to exist to a still greater extent. In a paper published by Colonel Gibbs in Professor Silliman's Journal*, he goes so far as to consider light as the great source of magnetism. In 1817 he had visited the mine of magnetic iron at Succassanny, and was informed, that the ore in the upper part of the bed was magnetic, while that which was raised from the bottom acquired it only after exposure to the influence of the atmosphere. This ef fect he ascribed, without any sufficient reason, to the influence of light.

In support of this opinion, Colonel Gibbs attempted to ascertain by direct experiment, the influence of light upon a magnet. For this purpose he "determined the power of his magnet, as it had been shut up in the dark for a long time, and lying down. He then exposed it to the rays of the sun, also lying down, and remote from the iron support, and he found that it had gained 12 oz. power in forty minutes, and 14 oz. power only in five hours †.”

A very remarkable analogy between the phenomena of magnets and of glass, either transiently or permanently crystallised,

American Journal of Science, No. 1. p. 89.

+ Id. No. 2. p. 207.

when acting upon polarised light, has been pointed out in the Philosophical Transactions for 1816. This analogy, which we shall have occasion to explain at some length in a subsequent paper, is so complete, that there are few phenomena in magnetism which have not their counterpart in the action of crystallised glass and regular crystals upon polarised light.

ART. IV.-Examination of some Compounds which depend upon very weak Affinities. By JACOB BERZELIUS, M.D. F.R.S. and Corresponding Member of the Institute of France. Communicated by the Author.

(Continued from page 75.)

EXAMINATION of Magnesia Alba.—It is well known, that when this earth is precipitated from a warm solution, by carbonate of potash, an effervescence takes place. The magnesia alba, therefore, contains less carbonic acid than the potash abandons. Is it a subcarbonate? And, if so, what proportion subsists between the acid and the base? My attempts to resolve these questions led to a result very different from what I had at first anticipated.

The analyses which we possess of this substance give very discordant results, as the following table will testify:

The great ease with which the quantity of caustic magnesia, remaining after the calcination of this salt, may be accurately determined, forbids us to suppose that any of the analyses are erroneous in that particular. But the quantities of carbonic acid and of water, may have been somewhat modified by the method followed for obtaining them.

It is easy to see, that, according to none of these analyses, is the carbonic acid combined with twice as much oxygen, as in the ordinary carbonate. It is farther evident, that the first three analyses, in no respect agree with the laws of chemical proportion; whilst that of Klaproth makes the carbonic acid

and the water to contain each an equal quantity of oxygen, and the magnesia two-thirds of that quantity. Although this last result might have been expressed by a formula possessing great probability, and although it proceeded from our greatest master in the art of correct analysis, I thought it necessary to repeat the experiment before founding upon it, with confidence, a calculation respecting the real composition of magnesia alba. Simple as this analysis may appear, it cost me much more time and precaution than many other researches at first sight more difficult; and I did not succeed in finding what I conceive to be the true result, till after sixteen different trials.

I began with precipitating a solution of muriate of magnesia by carbonate of potash, the mixture being kept for some time in a state of ebullition. At first, I left the muriate of magnesia slightly in excess, because I had reason to think, that when the Jiquid contains an excess of alkali, the precipitate carries with it a small quantity of this alkali, which water is unable to extract. I collected the precipitate upon a filter, and washed it, till the water which passed no longer acted upon the nitrate of silver. When dried and examined in the manner indicated above, it gave, Magnesia, 41.60; carbonic acid, 36.58; water, 21.82. As a repetition of the analysis produced exactly the same result, there could be no inaccuracy in the experiment. But when I dissolved the caustic magnesia, procured by the analysis, in nitric acid, and mixed with the solution a little nitrate of silver, the liquid became clouded, and deposited a small quantity of muriate of silver. The precipitate which I had analysed, must therefore have contained a portion of the muriate of magnesia. Another precipitate, formed by employing the carbonate of potash in excess, gave for its composition, Magnesia, 42.37; carbonic acid, 37.17; water, 20.46. The magnesia in this case contained no muriatic acid.

I repeated these experiments by precipitating the magnesia from its sulphate. When the sulphate was in excess, the precipitate seemed to contain, Magnesia, 42.24; carbonic acid, 37.00; water, 20.76. This magnesia being dissolved in muriatic acid, formed a pretty abundant precipitate, by adding muriate of barytes. When, in the mixture of carbonate of potash and sulphate of magnesia, the former was in excess, the preci