A piece of active platinum invariably failed to give light, if kept 24 hours on mercury, the effect being the same whether it was covered by an inverted jar or not. On keeping an active ball in contact with mercury during two hours, its energy was perceptibly impaired. The action of mercury is still more injurious, when a heated ball is plunged under it, and held there during a few seconds.

Spongy platinum absorbs water greedily when any part of it is dipped into that liquid, and then does not become luminous under the jet of hydrogen. A platinum ball absorbs water in like manner, and in consequence loses its activity. On putting a ball thus moistened into mixed oxygen and hydrogen gases, an exceedingly sluggish action ensued, during which a large quantity of water condensed on the glass. By removing the ball successively into fresh portions of the explosive mixture, it gradually lost much of its moisture, and regained a great part of its former energy, which was not, however, completely restored till it had been ignited. Similar to this was the effect of moistening a ball with alcohol and ether, only the activity returned more speedily under the same treatment. Spongy platinum, if moistened with very strong sulphuric ether, did become luminous under a continued jet of hydrogen, but when alcohol or water was used it did not. A platinum ball moistened with sulphuric, nitric, and muriatic acids, did not act at all on an explosive mixture, but ignition restored its activity.

When platinum is kept for hours in dry oxygen, hydrogen, or atmospheric air, it acts afterwards on an explosive mixture with its usual activity. An active platinum ball was kept in carbonic acid during an hour, and afterwards acted readily on an explosive mixture. The same ball was put successively into olefiant gas, carbonic oxide, and coal gas, for five minutes, and suffered a considerable abatement of its energy in consequence, but still acted. Put into muriatic acid gas, for the same space of time, it acted still more sluggishly. Kept for the same time in sulphurous acid, sulphuretted hydrogen and ammoniacal gas, it had completely lost its energy, for it occasioned no diminution of volume when brought in contact with the explosive mixture.

Spongy platinum was pressed between two pieces of clean metal, till it had become a compact leaf of platinum. Its effect on a jet of hydrogen was tried at different periods of the process. So long as any of the porous texture remained,

it still became luminous; but when this was wholly destroyed, it gave no light whatever. A very gentle heat still enabled it to become luminous, and to inflame the hydrogen. I have repeated with success the experiments mentioned by MM. Thenard and Dulong, of throwing a jet of hydrogen on a 'fold of platinum foil, and on a coil of the wire. The metal under this form requires to be sharply heated for this purpose, to a degree apparently beyond that at which mercury boils.

The lamp without flame, proposed by Sir H. Davy to be formed with a coil of platinum wire, may be conveniently made with spongy platinum. The porous mass need only be supported just above, but very close to the wick, by means of a small wire; and when the spirit-lamp is allowed to burn for a few seconds, and is then blown out, the metal becomes red, and continues so till the alcohol is consumed. By this means a considerable body of light may be obtained, by which one can read small print, or see even the seconds hand of a watch with perfect precision. Sir H. Davy proposes to suspend a coil of platinum within the safety-lamp, in this expectation, that, should the atmosphere of a mine become at any time so charged with fire-damp as to extinguish the lamp itself, the platinum, by maintaining a slow combustion, would afford a certain degree of light to the miner. Should this be really useful, still greater advantage would probably be derived from suspending spongy platinum, inclosed in a little wire cage of the same metal within the safety-lamp, as the mass of light would then be increased.

The product of the slow combustion of alcohol by means of the porous platinum, is only carbonic acid and water; I have not at least detected either the acetic or any other acid.

ART. XLVII.-Instructions respecting Paratonnerres, or Conductors of Lightning. Extracted from the Report of M. Gay-Lussac, in the name of a Commission appointed by the Royal Academy of Sciences of Paris.* [Ann. Phil.]

THE principal object of the report (which was drawn up at the request of the Minister of the Interior), is to direct workmen in the construction and mode of fixing conductors on From the Annales de Chimie. 55

VOL. II.-NO. 5.

buildings, &c. It is divided into two parts, one theoretical, the other practical.

THEORETICAL PART.

Principles respecting the Action of Lightning, or Electric Matter, and of Conductors.

LIGHTNING is the sudden passage of electric matter through the air, with the evolution of great light, from clouds highly charged with that fluid; its velocity is immense, far surpassing that of a ball at the moment it leaves the cannon, and is known to be at the rate of about 1950 feet per second of time.

The electric matter penetrates bodies, and traverses their substance, but with very unequal velocities; through some, which are therefore called conductors, it passes with great rapidity; such are well burnt charcoal, and water; vegetables, animals, and the earth, in consequence of the moisture they are impregnated with, and saline solutions; but, above all, metals afford the readiest passage to the electric fluid. A cylinder of iron, for instance, is a better conductor than an equal cylinder of water saturated with sea salt, in the ratio of at least 100000: 1, and the latter conducts a thousand times better than pure water.

Non-conductors, or insulating bodies, oppose great resistance to the passage of electricity through their substance; such are glass, sulphur, the resins, and oils; the earth, stones, and bricks, when dry; air and aeriform fluids.

No bodies, however, are such perfect conductors of electricity as not to oppose some resistance; which being repeated in every portion of the conductor, increases with its length, and may exceed that which would be offered by a worse, but shorter conductor. Conductors of small diameter also conduct worse than those of larger.

The electric particles are mutually repulsive, and consequently tend to separate and disperse themselves through space. They have no affinity for bodies, they determine only to their surfaces, where they are retained solely by the pressure of the atmosphere, against which they, in their turn, exert a pressure proportionate at every point to the square of their number. When the latter pressure exceeds the first, the electric matter escapes into the air in an invisible stream, or

in the form of a luminous line, commonly called the electric spark.

The stratum of electric matter on the surface of a conductor is not of equal density at every point of its surface, except it be a sphere. On an ellipsoid the density is greater at the extremity of the great axis than on the equator, in the ratio of the great axis to the smaller; at the point of a cone it is infinite. In general, on a body of any form, the density of the electric matter, and consequently its pressure on the air, is greater on the sharpest or most curved parts, than on those that are flat or round.

The electric matter tends always to spread itself over conductors, and to assume a state of equilibrium in them, and becomes divided amongst them in proportion to their form, and principally to their extent of surface. Hence, if a body that is charged with the fluid be in communication with the immense surface of the earth, it will retain no sensible portion of it. All that is necessary, therefore, to deprive a conductor of its electricity, is to connect it with the moist ground.

Of several conductors of very unequal powers, the electric fluid will always choose the most perfect; but if their differences be small, it will be divided amongst them in proportion to their capacity for receiving it.

A Paratonnerre* is a conductor which the electric matter of the lightning prefers to the surrounding bodies, in order to reach the ground, and expand itself through it: it commonly consists of a bar of iron elevated on the buildings it is intended to protect, and descends, without any divisions or breaks in its length, into water or a moist ground. An intimate connexion of the paratonnerre with the ground is necessary, in order that it may instantly transmit the lightning as it receives it, and thus defend the surrounding objects from its attacks. When lightning strikes the surface of the ground, for want of a good conductor it does not spread over it, but penetrates below it till it meets with a sufficient number of channels to carry it completely off. It sometimes leaves visible traces of its passage, even at a depth of more than 30 feet. When also a paratonnerre has any breaks in it, or is not in perfect communication with a moist soil, the lightninng

*I adopt the French term, as we have none in our language to express in one word, a conductor of lightning, meaning thereby, not merely the metallic rod, but the whole apparatus complete. At least we may as well use it as parasol, parachute, paraboue, &c.—Tr.

having struck it, flies from it to some neighbouring body, or divides itself between the two, in order to pass more rapidly into the earth. Frequent instances of serious accidents have occurred from both these causes.

Before the flash ensues, the influence of the thunder cloud disturbs the natural electrical state of all the bodies below it at the surface of the earth, and brings them into a state contrary to its own; and thus every object becomes a centre of attraction, towards which the lightning has a tendency to direct itself. In order that this effect may be suddenly produced, it is indispensable that the bodies influenced by the cloud be good conductors, and in perfect communication with a moist soil.

A paratonnerre perfectly connected with the ground, and terminating in a very sharp point instead of being rounded off, may become so intensely electrified by the influence of a thunder cloud, as to give off a continual stream of electric matter, which sometimes is visible in the dark, appearing as a luminous pencil at the extremity of the point, and must certainly tend, in part at least, to neutralize the electrical state of the thunder cloud. A rounded point may exert an equal, or even a greater, attraction on the thunder cloud than a sharp one; but if the flow of electric matter from the point become very rapid, the lightning will strike sooner, and from a greater distance between the cloud and the paratonnerre, than if its extremity were rounded; at least electrical experiments lead to this conclusion.

Thus the most advantageous form that can be given to a paratonnerre appears evidently to be that of a very sharp cone. The higher a paratonnerre is elevated in the air, other circumstances being equal, the more its efficacy will be increased, as is clearly proved by the experiments with electrical kites, made by MM. de Romas and Charles.

It has not been accurately ascertained how far the sphere of action of a paratonnerre extends, but, in several instances, the more remote parts of large buildings on which they have been erected, have been struck by lightning at the distance of three or four times the length of the conductor from the rod. It is calculated by Charles, that a paratonnerre will effectually protect from lightning a circular space, whose radius is twice that of the height of the conductor; and they are now attached to buildings after that rule.

A current of electric matter whether luminous or not, is