if there be ten. A scale will be thus formed of magnetic intensities, corresponding to a fraction of any given discharge. If after> wards, instead of using different branches of the same metal, similar wires of different metals be used, and a second discharge be passed through this new system, equal to one of known effect, it will be divided unequally between the wires, and the steel needles placed transversely across them will indicate by their degree of magnetization, whether a particular metal has transmitted a third, another a fourth, another a tenth, or any other proportion of the whole quantity of electricity discharged.-Ann. de Chimie, xxxiv. p. 7.

2. On Lightning Rods and Compasses, by Dr. Fischer.-Dr. Fischer states, that when iron is magnetised it loses much of its conducting power for ordinary electricity; and concludes from his observations, that iron, which is known to become magnetic by even feeble electric explosions, is a very improper metal for the construction of lightning-rods. He relates an instance in which a rod of this metal, placed as a lightning-conductor upon a powder-magazine, had entirely failed on several occasions in preventing the explosion of lightning close by its side. Upon examining this rod, it was found to have attractive and repulsive magnetic properties.

M. Fischer consequently recommends copper as the metal to be used, and advises that the end be gilt or made to terminate in a gold point, rather than with platina, because the former is so much superior in conducting power.

The conclusions of M. Fischer with regard to the decrease of conducting power in iron, when magnetised, are in singular contrast to the assertions of Mr. Abraham, who states that the conducting property is wonderfully increased by making iron magnetic, and recommends that all lightning-rods be of magnetised iron. We doubt whether any effect is produced either way.

Mr. Fischer endeavours to prevent the disturbance of the magnetic needle from neighbouring masses of iron, by placing it in a bowl of that metal of equal thickness throughout. He states, that the needle points north and south, and is not effected by the approximation of iron. The means are the same, generally, as those practised some years ago in this country by Mr. Jennings, who surrounded his needles with a strong ring of iron. The effect is also stated to be the same.-Bull. Univ. E. vii. 93.

Mr. Abraham used two cast-steel rods, properly hardened and tempered, each three feet in length and half an inch in diameter; one end of each bar was hammered to a fine point, and one bar was rendered magnetic; then a brass ball was placed two inches off from the prime conductor of a machine, and the points of the unmagnetised and magnetised bars brought alternately towards it; the magnetic bar (it is said) prevented the passage of sparks between the conductor and ball when it was twelve inches off, whilst the other required to be brought to within nine inches.— Phil. Mag. N. S. i. p. 267.

3. On the transference of Ponderable Matter in Electric Discharges, by M. Fusinieri.-This philosopher has made numerous experiments on this subject, but we think it sufficient to quote the following only. Two large Leyden jars were charged, one of which had a ball of silver, connected with its knob by a silver wire; a polished dish of copper, nine centimetres in diameter and two-thirds of a millimetre in thickness, was then held against the ball of the discharging rod, and the discharge made from the silver ball, through the plate to the brass ball, with which it was in contact. The distance was six or eight lines, such was the length of spark, and the following were the results:-1. A red stain (oxide of copper), with a greenish portion (oxide of silver), and white spots, (silver), was formed on the face of the disc, opposite the silver ball. 2. A stain, more or less green, existed on the other side of the plate, where it was in contact with the brass ball. 3. The brass ball had a white stain, at the point of contact. 4. Finally, the ball of silver had traces of oxide of copper. Besides these effects, the copper and brass were roughened at the points of discharge.

In another experiment, a ball of gold and a disc and discharger of silver were used with the former two jars. The disc was placed in contact with the ball of silver; a gold stain, a centimetre in diameter, was obtained on the external face of the disc, although the electric spark had to pass a distance of from two to four centimetres from the gold to the silver. This beautiful stain was like a very thin film of gold; in the course of some minutes, it became more rare, and totally disappeared in the course of some days,—an effect which sufficiently indicates that the gold had been reduced to a volatile state. There was also a stain of gold on the ball of silver, and a stain of silver on the ball of gold.

Another experiment was made: after having covered one of the faces of the silver-plate with wax, and made a hole in the wax, 1 millimetres in diameter; the face not covered received a stain of volatile gold, as in a former experiment; but on the face covered with wax, there was a layer of gold of more thickness and solidity, as it did not disappear by spontaneous evaporation..

It is supposed, that these experiments prove the transportation of ponderable matter by the electric spark. The subject being reduced to such a state of division as to become volatile. After this it is supposed, that the light of the spark is due to the presence of molecules of ponderable matter detached by the electricity from the hardest bodies. The diversity of colour in the spark is in the same way supposed to be readily explicable. The light observed in vacuo between the poles of the voltaic light are due, without doubt, it is said, to solid particles carried by the current. The smell occasioned by the electric sparks of our machines, and by lightning, is considered as the odour of the transported matter.— Gior, di Fisica, 1825, p. 450.

4. New Phenomena produced by Vapour, observed by M. Clement

Désormes. When steam under high-pressure is allowed to escape in a roaring, violent jet, from rather a large orifice pierced in a plate, and a flat disc is opposed to it at a moderate distance from the aperture, the latter is powerfully forced away; but if it be pressed still more closely, as if to shut the aperture, although the vapour issue on all sides, like an artificial fire-work, and presses more on the disc than before, it may nevertheless be left free: not only will it not be raised, but if the issue for the vapour be turned towards the earth, and the disc consequently tend to fall, as well by its own weight as by the pressure of the vapour, still it will not descend. The same result is obtained if the experiment be made with the current of air from a strong blowing machine, belonging to an iron blast furnace.

Another fact, also very curious, though well known, is that a jet of high-pressure vapour, issuing from a very hot boiler, will appear as a refreshing wind, compared to another jet of vapour coming from a source much less elevated in temperature, and of less than one-twentieth the pressure of the former.

From the first experiment, M. Clement concludes that the common safety-valves, which are real discs placed upon apertures with flat edges, involve a danger inherent in their form. In fact, no sooner are they raised, to permit the escape of a thin film of vapour, than there immediately results the impossibility of rising any further; and if the production of vapour is too considerable for the small aperture which can exist, and for the strength of the boiler, an explosion happens, notwithstanding that the safety-valve is open. This, in fact, is what has actually occurred sometimes, although it has appeared incredible.

M. Clement attributes these effects to the vacuum which is formed in the jet of vapour, in consequence of the extreme velocity of its molecules, and to the conical form of the kind of tube produced by the two approximated plates between which the vapour is forced to pass. It dilates near the edges, in consequence of its momentum and elasticity far below the pressure of the atmosphere, and the latter acts with sufficient force on the moveable disc to resist the vapour.

The remedy to this danger may be found, without doubt, in a good proportion between the orifices and their edges; the former should be large, and the latter small. The addition of a conical tube to safety-valves will diminish the effect of the atmospheric pressure, and of the weight with which they are charged.-Bull. Univ. E. vii. p. 41.

In the next number of the Bulletin Universelle the following explanation of this phenomenon by M. Clement is given. When a jet of vapour moving with great velocity does not displace a moveable plate put over the orifice by which it escapes, it is because a partial vacuum exists between the fixed plate and the mobile disc, where a tension less than that of the atmosphere exists APRIL-JUNE, 1827.

2 I

throughout a space sufficiently large to permit the pressure of the atmosphere to be stronger than that of the vapour. M. Clement has demonstrated by several experiments that this partial vacuum really exists in the space which serves as an issue for the vapour, just as occurs in the conical tubes which are applied to orifices through which liquids are flowing. He has shown, that the interval which separates the two plates between which the vapour escapes is a true conoide. In fact, this interval is formed of concentric rings, of which the surface augments in proportion to the diameters; and, consequently, the elastic fluid as it passes from the smaller to the larger ring has successively filled the spaces in which it has assumed a decreasing density, until beneath that which corresponds to atmospheric pressure, and this in consequence of the active force with which each molecule is animated when it escapes from its first pressure.

M. Clement finds that the theory which Daniel Bernouilli has given for the effect of conical or cylindrical jets applies perfectly to these curious phenomena with vapour, so that the effects were explained, as it were, before they were observed. The great degree of cold produced by compressed vapour at the moment of its expansion in the air, is also an effect of the active force which it possesses at the moment of its issuing forth, which enables it to acquire a very low tension. The jet is then formed of vapour, of the force of one-half or one-fourth of an atmosphere, and which consequently possesses only the corresponding temperature, i.e. 60 or 50 degrees C. This temperature is still further reduced by the quantity of air which the vacuum draws to it, and thus a wind rather cold than hot is occasioned.

M. Clement proposes some alterations in safety-valves, and considers the addition of a weak tube to steam-boilers as a perfect security. He recommends it to be of considerable diameter, and capable of supplying when burst a large vent for the vapour, so as to be sufficient to allow of the sudden productions of vapour which sometimes take place, and to which probably numerous accidents are due.-E. vii. p. 104.

5. Transference of Heat by change of capacity in Gas.-Many of the copper vessels in which gas is compressed at the Portable Gas-works are cylinders, from two to three feet in length, terminated by hemispherical ends. These are attached at one end to the system of pipes by which the gas is thrown in, and being so fixed, the communication is opened; it frequently happens, that gas previously at the pressure of thirty atmospheres in the pipes and attached recipients, is suddenly allowed to enter these long gas-vessels, at which time a curious effect is observed. That end of the cylinder at which the gas enters becomes very much cooled, whilst, on the contrary, the other end acquires a considerable rise of temperature. This effect is produced by change of capacity in

the gas; for, as it enters the vessel from the parts in which it was previously confined, at a pressure of thirty atmospheres, it suddenly expands, has its capacity for heat increased, falls in temperature, and consequently cools that part of the vessel with which it first comes in contact; but the part which has thus taken heat from the vessel being thrust forward to the further extremity of the cylinder by the successive portions which enter, is there compressed by them, has its capacity diminished, and now gives out that heat, or a part of it, which it had the moment before absorbed; this it communicates to the metal of that part of the gas-vessel in which it is so compressed, and raises its temperature. Thus the heat of temperature is actually taken up by the gas from one end of the cylinder, and conveyed to the other, occasioning the difference of temperature observed. The effect is best observed when, as before stated, the gas, at a pressure of thirty atmospheres, is suddenly let into the vessels: the capacity of the parts is such, that the pressure usually sinks to about ten atmospheres.

6. On the Change in the Zero of Thermometers.*-M. Arago remarks, relative to the gradual change which takes place in the indications of the thermometer, in consequence of the elevation of the mercury, that it throws much doubt over numerous observations made with that instrument, and imperatively requires some process by which it can be prevented. The process he recommends is one already known to artists, and practised by them with alcoholic thermometers, and consists in leaving air in the upper part of the tube sufficiently compressed to prevent the disengagement of that contained in the liquid, and the precaution he considers as not less necessary, whether the metal really contains the air in solution, or whether it be considered as interposed only between it and the glass.

Deluc has established, by numerous experiments, "that when from any cause the particles of air disseminated in a liquid gather together, they acquire a greater power to separate the particles of the liquid; and that this effect may be produced long before the small collections of air become visible, and even before they are capable of making their way through the fluid." The excess of force spoken of by Deluc, is manifested by an augmentation of volume, and when the fluid is contained in the tube of a thermometer, by the ascent of the zero; the effect is quick and considerable in thermometers, containing alcohol, water or oil, and should be smaller and less rapid in one containing mercury, though ultimately an analogous result will be obtained.

An experiment made by M. Arago is then described, in which the effect of inclosing air in the thermometer is accurately ascertained. Two similar thermometers were constructed at the same

Quarterly Journal of Science, xiv. 441; xv; 160, 369, 371.