metre, in accordance with the plan of Dr. Wollaston, while their upper extremities communicate respectively with two ends of the wire of a galvanometer, whose coils are well isolated by means of resin. The supports which bear the platina wires are movable along a division in such way that the two extremities of the wires immersed in the water may be made to approach one another as closely as possible, or be separated very nearly the whole length of the stratum of water. By means of a micrometric screw, the relative distance of the two points of platina may be so varied as to be appreciable to nearly the tenth of a millimetre. These two points draw off an almost insensible proportion of the electric current which traverses the trough filled with water—a proportion, however, which suffices to act in a distinct manner on the needle of the galvanometer. The proportion drawn off depends for a current of constant intensity on the distance of the two points, so that, if the intensity be variable, it is the variable distance to which it is necessary that the two points shall be brought, in reference to one another, in order for the indication of the galvanometer to remain constant, which measures the proportion drawn off in each case, and thus, by a ratio easily determined, the absolute intensity of the current.

Finally, a good pneumatic pump, to which a second complementary one may be joined, enables us to bring the gas to an advanced degree of rarefaction. As to the elastic force of the gas, that is measured by a manometer of mercury very carefully constructed, with which, by means of a cathetometer, a difference of pressure of even the fiftieth of a millimetre may be appreciated.

§ I.-GENERAL PHENOMENA PRESENTED BY THE TRANSMISSION OF ELECTRICITY IN RAREFIED GASES.

The Ruhmkorff apparatus, of which I have availed myself, gives in the inducted wire two successive and alternately contrary discharges. Hence, if these discharges encounter in the circuit which they traverse only good conductors, such as metallic wires, and even distilled water, no deviation is re marked in the galvanometer, because the discharges being alternately in a contrary direction, and in rapid succession, their opposed double action is neutralized. But if the circuit comprises an elastic fluid very much rarefied, the resistance which it opposes to the passage of the two successive discharges causes one of them to predominate, so that the phenomena take place as though there were but a series of discharges all in the same direction. The explanation of this difference is, that the two discharges, or inducted currents, though equal in quantity, have not the same tension, the direct, which have a less duration, having a stronger tension. It thence results that when the circuit is interrupted by a body which is a bad conductor, such as an elastic fluid more or less rarefied, the direct currents can alone be transmitted, so that the direction of the inducted current which traverses the elastic fluid is the same with that of the inductive current, and the latter changing, the other changes at the same time.

The pressure at which a discharge of a given intensity begins to pass through a gas varies with the nature of that gas, with its degree of rarefaction, and with the dimensions and form of the vessel which contains it. Moreover, the discharge does not pass immediately upon the electrodes being put in communication with the poles of the Ruhmkorff apparatus. For that a certain time is necessary-a time so much longer in proportion as the resistance is greater, whether arising from the nature or density of the elastic fluid, or from the effect of the form and dimensions of the vessel. Thus, in a long tube, from 2 to 5 centimetres in diameter and from 30 to 50 centimetres in length, it requires several minutes before the discharge can be transmitted, however rarefied the gas. But the first discharge having once passed, the succeeding ones pass with facility, and follow one another so rapidly as to produce on the galva

nometer the effect of a continuous current. The passage of the discharges may even be interrupted for many minutes without the loss of the capacity which the gas had acquired of immediately transmitting them. To lose it completely we must wait a long time, or renew the gas, and consequently again rarefy it. An equally important fact to be noticed is, that the discharges once transmitted, there may be gradually introduced, while they are passing, a new quantity of the same gas, which amounts to an augmentation of the density, without their ceasing to pass; the pressure may thus be carried to almost double what it was at the beginning. The direction of the discharges has no influence on this train of phenomena; for, the discharge having once effected a passage, its direction may be changed at will, without a cessation of immediate transmission. This result, which I have had occasion to verify in many and very different cases, would seem to show that the gaseous matter opposes a certain inertia to the establishment of that particular disposition which the transmission of elec tricity requires, and which determines the tension which precedes that transmission; but that this disposition once established, it subsists long after the passage of electricity has ceased, provided no disturbance intervene in the state of the gas. It had long been supposed, particularly in the theory of Grotthus on electrolytic decompositions, that something analogous occurred in the transmission of voltaic currents through liquids; it had thence been inferred that the tension of the poles of the piles induced in the liquid in which these poles were plunged, a polarization, which preceded the passages of the current. Only these two effects succeeded one another in a time so short as to be inappreciable, while with gases they would be found to be separated by an interval of more or less duration, but always appreciable.

I shall restrict myself here to certain numerical results,-results of but little importance indeed, since it is impossible to deduce from them a law, in view of the numerous causes which occasion them to vary. They serve only to show the accuracy of the general principle which I have just indicated. We may also infer from them the great superiority of hydrogen over nitrogen and atmospheric air, as regards its conducting power-a fact already noticed by several experimenters.

În a tube 5 centimetres in diameter and 16 in length, the discharge, when the tube was filled with atmospheric air, only began to pass when the pressure was reduced to 20 millimetres; with nitrogen it passed under a pressure of 24 millimetres, and with hydrogen under that of 36 millimetres. that subsequently, under the same conditions of intensity, and still with hydrogen, the discharge passed at pressures of 42, and even 48 millimetres; when rendered still stronger, it has been transmitted under a pressure of even 72 millimetres. With a tube having a like diameter of 5 centimetres, but only one metre in length, the same discharge only began to pass through nitrogen under a pressure of from 4 to 5 millimeters; with hydrogen it passed only under a pressure of from 12 to 13mm. Afterwards, when stronger, and again with hydrogen, it passed under a pressure of 18, and even 20mm. When the discharge begins to be transmitted, it exhibits itself in very minute jets or streams, more or less intermitted; afterwards these streams combine to form a larger and more continuous one. In a jar filled with hydrogen the discharge passed from an isolated central ball to a ring 12 centimetres in diameter, making the distance of the transmission but 6 centimetres in a space of hydrogen which might be called unlimited. In this case it was transmitted under a pressure of 128mm in the form of streams more or less intermittent and undulating, darting from the central ball to all points of the ring indifferently. At 90mm the discharge gave rise to a continuous stream, susceptible of being influenced by magnetism.

We see, by the instances just cited, that the pressure under which, for any given gas, the discharge can pass varies with so many circumstances, that its

determination is of little importance. Not so, however, when the discharge is once transmitted in regard to the influence which the pressure of the gas it traverses exerts on its intensity. The following are two comparative experiments made with nitrogen and hydrogen. These two gases were successively introduced into a tube 5 centimetres in diameter and 50 in length; the discharge passed between two balls of platina one centimetre in diameter, placed respectively near each extremity of the tube, so that the passage across the gaseous medium was quite 50 centimetres in extent.

Nitrogen. The intensity of the discharge was measured by means of that of the derived current received by the two points of platina plunged, at a fixed distance of 120mm, in the distilled water which was placed in the circuit:

Hydrogen.-Proceeding at first as in the case of the nitrogen, the two points of platina which received the derived current were left at a fixed distance from one another:

For pressure of less than 9 millimetres the points of platina were in each case brought nearer together, so as to have a constant current. At a distance of 55mm, the derived current, which under a pressure of 9mm had been 50°, was reduced to 40°. The following series was then obtained; and here, in order to restore the derived current to 40°, it was requisite, in proportion as the pressure diminished, to bring the points closer together, so as to render the interval of derivation smaller:

Thus, as far as 2mm of pressure, the intensity of the derived current, and consequently the conductibility of the gas, goes on increasing as well for the hydrogen as for the nitrogen; but we see how much more considerable is the conducting power of the hydrogen than that of the nitrogen, since, under a pressure of 9mm, all other circumstances remaining the same, the derived current is, with the nitrogen, scarcely sensible, while it is 50° with the hydrogen.

In two other comparative experiments, the pressure and the distance of the electrodes were made to vary alike for the nitrogen and hydrogen. The two gases had been successively introduced into the same ovoid globe. The following table gives the intensity of the derived current for three different distances of the electrodes under different pressures, when nitrogen is in the ball:

When for nitrogen we substitute hydrogen, the results differ somewhat, especially in the lower pressures, and when the electrodes are in close proximity with one another, which proceeds probably from the circumstance that the gaseous medium, in view of the form of the vessel, may be regarded as having an almost indefinite breadth. In a large receiver, in effect, where the current passes between a central ball and a concentric ring having a diameter of 12 centimetres, the intensity of the derived current is very little increased by diminishing the pressure beyond 10mm. That intensity, measured by the derived current, amounts, under a pressure of 15mm, to 35°; under a pressure of 10mm, it attains 45°; then augments gradually as far as 5mm, when it reaches its maximum of 50°, which it never exceeds, manifesting rather a slight tendency to become less under 2mm. By raising the central ball so as to give to the electric sheet a conical instead of circular form, the conductibility is not sensibly diminished. Under the same circumstances the atmospheric air does not present a resistance much greater than the hydrogen; thus the intensity of the derived current is 35° at 5mm instead of 50°, and at 2mm is 45°. However, with the tube of one metre in length, hydrogen must be subjected to a much weaker pressure in order to transmit the discharge, but its conductibility increases very rapidly with the diminution of that pressure. Thus, the apparatus of derivation being placed in the circuit, we have:

Finally, with the same tube, one metre in length and 5 centimetres in diameter, a sensible and regular augmentation in the intensity of the derived current has been obtained, for the same pressure and in the two gases alike, by a corresponding diminution of distance between the electrodes. The comparison of the numbers indicates, that, when the gas is sufficiently rarefied to be a good conductor, that is, to permit the discharge to become, so to say, continuous, it follows, like liquids and solids, in its conductibility, the law of the inverse of the length. It has been already seen that the influence of the section and of the volume is very considerable; but I have not been able to determine its law in a precise manner.

Thus far I have considered the propagation of electricity in gaseous substances only in relation to the resistance they oppose to it-a resistance variable with their nature, their density, and their dimensions. I have but glanced at this part of my subject, to which I shall return at an early occasion, as I propose to extend my researches to a much larger number of gaseous substan

ces. I pass now to phenomena of quite another order, and which relate to the mode itself in which the propagation of electricity is effected in gases-a mode which manifests itself under the form of stratification of the electric light.

§ II.-INVESTIGATIONS REGARDING THE STRATIFICATION OF THE ELECTRIC LIGHT.

It is known that at a certain degree of diminution of the elastic force of a gas which transmits the electric current, that current becomes stratified-that is to say, is decomposed into strata alternately obscure and luminous. The stratification commences by the appearance of certain slight strix or furrows on the side of the positive electrode; then gradually, as the elastic force diminishes, the current, which was at first very narrow, dilates, and the striæ grow larger. Next appears an obscure space, separating the extremity of the luminous column from the negative electrode, which is itself surrounded with a bluish atmosphere. This atmosphere continues to dilate, and the obscure space to lengthen, in proportion as the rarefaction of the gas increases.

In order to obtain the stratification of the electric light, it is necessary to diminish the pressure of a gas in proportion as the gas offers more resistance to the transmission of electricity. Thus in hydrogen, under a pressure of 18mm, the electric stream, which consists as yet of but a small rose-colored filament from three to four millimetres in diameter, is seen to divide into very distinct circular sheets, alternately obscure and luminous, the breadth of which is one-fourth of a millimetre. These striæ, at first more distinctly marked at the positive electrode, become general throughout the whole electric current, whatever be its length; and, in proportion as the pressure diminishes, the stream becomes enlarged, so as even to occupy the whole interior of a tube five centimetres in diameter. At the same time the breadth of the alternately obscure and luminous divisions so increases that, under a pressure of 2mm, it is about 5mm. These divisions are themselves annular, as I have satisfied myself by closing the tube which contains the rarefied gas, at one of its extremities, with a glass disk, which permits the whole interior of the tube to be seen in the direction of its length.

When the striæ begin to appear, an obscure space, as has been said, is seen to form in front of the negative electrode, increasing in proportion as the pressure diminishes, so far as finally to occupy a length of ten centimetres—a length which is independent of that of the gaseous column. However, by observing with attention this obscure space, we discover, beyond an interval which is perfectly black, and of a well-defined length of from 2 to 3mm, a palish, rose-colored gleam, which is only visible in utter darkness. This gleam, which has the form of a cone, whose base is the last section of the luminous column, only appears when the pressure has become very slight and quite inferior to that under which the obscure space is manifested. It is accompanied by the appearance, in the same obscure space, and at unequal intervals, of several still more luminous rings, (I have counted as many as four,) which contrast, by their immobility and their well-defined outlines, with the agitated striæ or divisions of the rest of the current. Let us add, that the luminous and stratified part of the current, which is much the longest, is so much the more distinctly and sharply separated from the obscure or palish part, as the electric discharge is more intense.

The bluish atmosphere which surrounds the negative electrode is also enlarged in proportion as the pressure diminishes, and nearly in the same ratio as the striæ. At the same time, its brightness becomes less vivid, and its exterior outline less sharply defined. This bluish atmosphere, which at first enveloped only the negative ball, at last, and in proportion as the pressure diminishes, equally envelops, in all its length, the metallic rod which supports the ball; at least, if this be not covered with an isolating coat, which indicates,