this plate is held between the observer and the sun, or a candle, with the eye very close to the smooth side of the glass plate, there will be seen three beautiful haloes of light, at different distances from the luminous body. The innermost halo, which is the whitest, is formed by the images refracted by a pair of faces of the octohedral crystals, not much inclined to each other; the second halo, which is more coloured, with the blue rays outwards, is formed by a pair of faces more inclined; and the third halo, which is very large and highly coloured, is formed by a still more inclined pair of faces.

Each separate crystal forms three images of the luminous body, placed at points 120° distant from each other, in all the three haloes; and as the numerous small crystals have their refracting faces turned in every possible direction, the whole circumference of the haloes will be completely filled up.

The same effects may be obtained with other crystals; and when they have the property of double refraction, each halo will be either doubled, when the double refraction is considerable, or rendered broader, and otherwise modified in point of colour, when the double refraction is small. The effects may be varied, by crystallizing upon the same plate of glass, crystals of a decided colour, by which means we should have white and coloured haloes succeeding each other.-Edin. Phil. Jour.

Variation in the Bulbs of Ther

mometers.-In mercurial thermometers, with a perfect vacuum above the mercury, M. Flauguergues has observed, that the freezing point has gradually risen ninetenths of a degree, and has gone on increasing for years. He attri

butes this to a permanent change of form, produced by the constant pressure of the atmosphere on the bulb. He therefore recommends, that thermometers should be made with open terminations. The same fact had been long before observed by M. Angelo Bellani, of Milan, who mentions a very pretty experi ment, for shewing that it arises from the glass. Take a mercurial thermometer, which has not been exposed for some months to temperatures near that of boiling water, whose scale rises to the boiling point or higher, and whose degrees are at least a line long, so that tenths of a degree can be easily seen. Having carefully marked the freezing point, plunge it in boiling water, and, upon replacing it in melting ice, it will be found, that the freezing point has sunk of a degree, in consequence of the expanded glass not having resumed accurately its original form. Hence it follows, that the new barometer of the rev. Mr. Wollaston must be liable to error from this cause.— Bibl. Univers.

New Form of the Voltaic Appa ratus.-Mr. Pepys has constructed, at the London Institution, a single coil of copper and zinc plate, consisting of two sheets of the metals, each fifty feet long, by two feet broad, having therefore a surface of 200 square feet; they are wound round a wooden centre, and kept apart by pieces of hair-line, interposed at intervals between the plates. This voltaic coil is suspended by a rope and counterpoise over a tub of dilute acid, into which it is plunged when used. It gives not the slightest electrical indications to the electrometer; indeed, its electricity is of such low intensity, that well-burnt charcoal acts as an insulator to it;

nor does the quantity of electricity appear considerable, for it with difficulty ignites one inch of platinum wire of inch diameter. When, however, the poles are connected by a copper wire inch diameter and 8 inches long, it becomes hot, and is rendered most powerfully magnetic, and the instrument is admirably adapted for all electromagnetic experiments.

Expansive force of Coal Gas.The whole mechanical power of an explosion of 15,000 cubic feet of a mixture of coal gas, and common air, is equal to that of the explosion of 6 cubic feet, or 4 barrels of gunpowder; and if we suppose the heated gases in both cases to escape, and mix with the common air in a building containing 30,000 cubic feet, so as to produce an effect commensurate to the temperature of the whole mixture, the explosion of about 15 cubic feet, or 10 barrels of gunpowder, would be required, in order to produce, like the gas, a force of about 10 atmospheres for the whole space. It must, however, be recollected, that gunpowder, thus disposed, is very unfavourably situated for producing violent effects; and that a much smaller quantity, in ordinary cases, would be more formidable than the explosion of the coal gas.-A more precise idea of the effects of such an explosion may be obtained from the calculation of its projectile effects, which would carry some parts of the wall of the surrounding building to a height of nearly 150 yards, and others to a distance of nearly 300. If the walls were in immediate contact with the gasometer, the height and distance would be about twice as great. But a roof of carpentry and tiles being lighter, would be carried higher, while the

lateral force of the explosion would be diminished. Supposing the explosion of the gas to be unconfined, the shock would throw down a brick wall, 9 feet high, and 18 inches thick, at the distance of about 50 feet from the centre; it would probably break glass windows at 150 yards, and at 300, would produce an effect similar to the instantaneous impulse of a very high wind.

Magnetism by Percussion. Mr. Scoresby has instituted a series of experiments, to determine magnetism by percussion with more precision; and some of his results deserve attention. When a bar of soft-steel, six inches and a half long, and a quarter of an inch diameter, held vertically, and resting upon freestone, was struck 17 blows with a hammer, it acquired the power of lifting 6 grains? 22 blows did not augment the force. When the bar rested vertically upon a parlour poker (previously deprived of magnetism), 42 blows gave it the power of lifting 88 grains, and 90 blows, with a larger hammer, augmented the lifting power to 130 grains. The poker was also rendered magnetic. Farther hammering rather diminished than increased the power. inverting the bar, a single blow. nearly destroyed the magnetism; two blows changed the poles. Hammering the bar in the plane of the magnetic equator, also destroyed the polarity. The magnetism by percussion was augmented when the length of the bars was increased.

On

On the Effects of Magnetism on Chronometers, by P. Lecount, Esq. I find, says he, that the subject of the iron in ships affecting chronometers has employed Mr. Barlow's attention as well as my own,

and that he attributes it to the same cause that I do, viz. a portion of fixed magnetism in the steel of the balance or its spring. For my part, I think it will not be found possible to ascertain any ship-rate for chronometers which shall correct the errors arising from this cause, from the direction and strength of the attraction of the iron in a ship undergoing such considerable changes as it does in different dips. I always considered the remedy to lie alone in the hands of the maker, who should carefully ascertain that no steel whatever in a chronometer possesses any fixed magnetic quality; and I pointed this out to a chronometer-maker in London in November 1820, shewing him, amongst a number of balances, those which had any portion of fixed magnetism, and those which had not, &c.; but it is requisite, that, in this respect, not only the balance and its spring should be attended to, but that all the steel in the instrument should be deprived of this quality, particularly the steel-spindles of the fusee, barrel, &c. for it is to magnetic attraction, residing wholly in the machine, that I attribute the alteration which takes place in the rates of chronometers on shore in different parts of the world, and which is often very considerable. These attractions may act in several ways. If there is fixed magnetism in the balance, and variable magnetism in the spindles of the wheels, the rate may be altered by any considerable alteration in the dip, as the direction and strength of the variable magnetism will thereby become changed; the same effect may be produced if the fixed magnetism is in the spindles of the wheels, &c. and the variable magnetism in the steel of the balance.

The balance-spring will likewise be acted on under similar circumstances; I should therefore think it absolutely necessary, that all the steel in the machine should be divested of the fixed magnetic quality; the variable ones will have no effect on each other: this can always be done by the action of fire, and if the mechanic, in the process of hardening and tempering the steel, always carefully cools it in a direction at right angles with the dipping-needle, it will rarely be found to possess any portion of fixed magnetism, as, on the contrary, it will be found, that small steel bodies, if heated redhot, and cooled in the direction of the dipping-needle, will often require this quality.

I am of opinion, that this fixed magnetism, if carefully excluded from the machine at first, will not be found to return from the continued motion of its parts.

A very necessary precaution with respect to the use of these instruments, is always to hang them up on board ship at a considerable distance from the compasses. I have known an excellent chronometer rendered useless for the time, by being kept within two feet of the cabin compass, and which, when removed to a different part of the cabin, performed remarkably well.

New Chemical Combination.-M. Dobereiner, professor of Chemistry in the University of Jena, by a series of entirely new experiments, has ascertained that platina, the heaviest of all elementary substances, when reduced into very fine particles, produces by simple contact with hydrogen gas, (the lightest of elementary substances), an electrical or dynamic combination, which, if brought into contact with hydrogen gas or with atmos

pheric air, instantly dissolves itself yielding fire and water. To prove this important fact by a brilliant experiment, M. Dobereiner makes hydrogen pass from a reservoir, by a capillary tube, curved below, upon pure platina in powder, which is contained in a glass tunnel, hermetically sealed at the point, so that the gas mingles with the atmospheric air before it touches the platina. The moment that the current of gas reaches the surface of the platina, the power of that metal becomes red and burning, and this phenomenon continues as long as the stream of gas is directed upon it.

New compound of Iodine. Iodide of Carbon-Messrs. Ferrari and Frisiani, whilst preparing the iodate and hydriodate of potassa, observed the production of a new compound of iodine. It may be obtained thus:-Heat an ounce of iodine, with a little water, on a sand bath, and add to it, by degrees, about two ounces of potash; when the two salts above mentioned will be formed. In order to saturate the excess of alkali, pour in, by degrees, a tincture composed of one ounce of iodine to six ounces of alcohol, specific gravity 837. When the re-action of the tincture on the potash is finished, pour the hot liquor on a filter, and the liquid which passes through will, as it cools, deposit yellow crystals, of the substance; they should be carefully washed in cold water, to remove all the iodate and hydriodate of potash. Another method is, to take the alcoholic solution of the two salts, prepared as above, and distil it and when the fluid which comes over ceases to be co loured, to change the receiver; the colourless liquor then obtained, upon cooling, deposits very pure

crystals, of the substance in question. If the distillation be suspended from time to time, and the retort allowed to cool, beautiful crystals of the substance form in it. If strong alcohol be used in the above operations, and but little water, then, upon adding water to the filtered liquor, the substance is precipitated in abundance. This substance is solid, of a lemon yellow colour, tastes like nitric ether, and has an odour like that of saffron. Its form is a compressed hexahedron (esaedro schiacciato). It is insoluble in water, alkalies, or acids, but soluble in alcohol and ether. It fuses and sublimes by a gentle heat, but at a higher tem perature becomes discoloured, is decomposed, and evolves vapours of iodine, leaving behind a mere trace of carbon.-Giornale de Fisica.

Vegetation in Atmospheres of different Densities.-The following experiments have been made by professor Dobereiner of Jena. Two glass vessels were procured, each of the capacity of 320 cubic inches, two portions of barley were sown in portions of the same earth, and moistened in the same degree, and then placed one in each vessel. The air was now exhausted in one, till reduced to the pressure of 14 inches of mercury, and condensed in the other, until the pressure equalled 56 inches. Germination took place in both nearly at the same time, and the leaflets appeared of the same green tint; but at the end of 15 days the following differences existed. The shoots in the rarefied air were six inches in length, and from nine to ten inches in the condensed air. The first were expanded and soft; the last rolled round the stem and solid. The first were wet on their surface, and especially towards the ex

tremities; the last were nearly dry. "I am disposed," says M. Dobereiner," to believe, that the diminution in the size of plants, as they rise into higher regions on mountains depends more on the diminution of pressure than of heat."

Sugar for preserving Fish.Dr. McCulloch, of Edinburgh, has ascertained that the antiseptic quality of sugar is sufficient to preserve fish in the most excellent condition. He states, that this substance is so active, that fish may be preserved in a dry state, and perfectly fresh, by means of sugar alone, and even with a very small quantity of it. He has thus kept salmon, whitings, and cod, for an indefinite length of time; and by this simple means fresh fish may be kept in that state some days, so as to be as good when boiled as when just caught. It is added, that "if dried and kept free from mouldiness, there seems no limit to their preservation; and they are much better in this way than when salted. The sugar gives no disagreeable

taste. This process is particularly valuable in making what is called kippered salmon: and the fish preserved in this manner are far superior in quality and flavour to those which are salted or smoked. If desired, as much salt may be used as to give the taste that may be required; but this substance does not conduce to their preservation. In the preparation, it is barely necessary to open the fish and to apply the sugar to the muscular part, placing it in an horizontal position for two or three days, that this substance may penetrate. After this it may be dried; and it is only further necessary to wipe and ventilate it occasionally, to prevent mouldiness. A table spoonful of brown sugar is sufficient in this manner for a salmon of five or six pounds weight; and if salt is desired, a tea-spoonful or more may be added; saltpetre may be used instead, in the same proportion, if it is desired, to make the kipper hard.”

On the destruction of Caterpillars on fruit trees. In May last I (Mr. Sweet, Bristol,) found the gooseberry caterpillar had begun its usual ravages on the leaves of a quarter of gooseberries in my nursery near Bristol. In the last and former season, among many other applications which had been tried, in vain, lime had been used, but in a dry state. I resolved this year to try the effect of it in a different way a bushel of stone lime was therefore slacked, and covered up

for a few hours, to make it more soft and fine; it was then sifted through a mason's sieve of the finest kind, which made it almost an impalpable powder. A man with a light garden-engine was then directed to play water in different directions among the leaves, so that every part of the plants was wet; another man followed closely with a coal-box full of the fine lime-powder, scattering it with his hand, so as to cause it to appear like smoke, and to spread itself on