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but, first, of the co-efficient of increase peculiar to the particular metal under consideration; second, of the co-efficient of increase dependent upon the co-efficient of the expansion of the metal; and, third, of a function of a co-efficient expressing the resistance of the material at the absolute zero. He finds his formula correctly applicable to the metals platinum, iron, copper, aluminum, and silver, at temperatures between zero and 350° C. The results of his experiments are given in detail, and afford a valuable basis for still further investigations.

In the second part of his memoir he states that, in 1860, when engaged in examining the electrical condition of the Malta and Alexandria telegraph cable, his attention was directed toward the fact that the increase in the temperature of the cable could be measured by the increasing resistance to the electrical current; and accordingly constructed coils of cable wire, of known clectrical resistance, inclosed here metically in iron tubes, out of which passed thick insulated wires; and placing these coils at vnrious points within the mass of the cable, he was able, by examining the varying electrical resistances, to ascertain that the interior of the large mass of coiled cable was steadily rising in temperature, and by pouring cold water thereon saved it from ultimate destruction. Following up this idea, he shortly afterward constructed thermometer coils, consisting of a spiral or insulated wire, inclosed in a cylindrical silver casing, which he used for measuring ordinary temperatures on land, and which could be used, he suggested, by physiologists and others. The instrument is extremely sensitive, being correct within one tenth of a degree Fahr., or even less; and a modified arrangement of this kind for measuring deep-sea temperatures was presented to the Berlin Academy in 1863. After describing the method adopted by him for determining the temperature of a distant spot, and also a similar apparatus furnished by him to the steamship Challenger in her exploring expedition, he gives in detail the method of conistruction of the pyrometer for measuring high temperatures. He states that Professor Bolzain, of Kasan, is at present employing his resistance thermometer for registering the temperatures below the surface of the earth, and measuring the temperature of the air above; and, furthermore, that Mr. Bell,

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the eminent metallurgist, habitually employs his pyrometer for the determination of the temperatures employed in various operations of the blast furnace.

The third part of Mr. Siemens's paper is a highly suggestive and valuable memoir on a simple method of measuring electrical resistances. He states that although a Wheatstone balance furnishes electricians with the means of measuring the resistances of electrical circuits with great accuracy, yet its application is, in many cases, rendered difficult on account of the delicacy of the apparatus and of extraneous disturbing causes. As a portable instrument, and one especially applicable to observations on shipboard and in exploring expeditions, he proposes what he calls a differential voltameter, which consists of two similar narrow closed tubes fixed vertically to a wooden frame, with a divided scale behind them, and whose lower ends, being enlarged somewhat, are fitted with wooden stoppers saturated with paraffin, and penetrated by platinum wires. Diluted sulphuric acid is admitted into these tubes, and kept at a proper height in each by a very simple device, and the evolution of gas that occurs, when a current passes from the electrodes, affords the measure of the strength of the current. By means of a commutator the current from the battery is easily reversed every few seconds, preventing polarization of the electrodes. By introducing the resistance of the voltameter, and the unknown resistance x, first on one, and then on the other side of the arrangement, the observations, by a simple arithmetical

process, give the exact value of the unknown resistance. The measurement of the quantity of decomposed gases serves merely to determine the relative intensity of the currents which flow in the respective positions of the commutator. He states that, having measured numerous resistances by this instrument, and compared the results with measurements obtained by a very perfect Wheatstone bridge arrangement, he finds that it may be relied upon within a half per cent. of error of observation. The instrument especially recommends itself for use on board of vessels, as not being in the slightest degree influenced either by the motion of the vessel or by the magnetic influence of moving masses of iron. One of its intrinsical advantages is that it gives the resistance to be measured in units of work done,

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independent of such momentary changes in the strength of the current as affect the readings of a magnetic needle. It is also portable and inexpensive.Jour. Soc. Teleg. Engineers, 1875, 296.

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THE ACTION OF AN ELECTRO-MAGNET UPON THE SPECTRA OF

RAREFIED GASES TRAVERSED BY ELECTRIC DISCHARGES.

M. Cheantard states that he has examined the spectra of rarefied gases illuminated by an electric spark, and subjected to the influence of powerful magnets, and finds that these spectra are characterized as to the position, the number, the separation, and the fineness of their lines by very curious traits peculiar to each gas. As regards the metalloids with which only his experiments have thus far been conducted, he states that the light given out by sulphur and by selenium experienced a notable diminution under the influence of the magnet, so that sometimes the spectrum, which was very apparent at first, disappeared for some moments. On the contrary, chlorine and bromine are characterized by an increase in brilliancy, and by the development of fine brilliant rays especially numerous in the green, whose appearance or disappearance at the moment when we turn on or interrupt the current has a truly magical effect. This phenomenon seems to have some importance in consideration of the obscurity which at present characterizes our knowledge of the relation of magnetism and light.—1 B, 1875, 283.

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THE FREEZING OF SALT WATER, Professor Guthrie, in continuation of former researches on the solutions of salt, has endeavored to ascertain the manner in which mixtures of salt act as cryogens, and to study their combination with water at various temperatures and in various proportions. He finds that when two salts, composed of different acids or bases, are mixed, and no precipitation occurs, it is generally considered that partial decomposition takes place, two new salts being formed; but be finds that if the salts a x and b y be mixed in atomic proportion, and dissolved in the smallest possible amount of water, a mixture is obtained identical with that produced on mixing a y with b æ; and the temperature and composition of the resulting cryohydrate are the same in both cases. Thus, a

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saturated solution of a mixture of nitrate of potassium and sulphate of sodium solidifies at -5°. A mixture of nitrate of sodium and sulphate of potassium also solidifies at this temperature; but the temperatures never fall as low as the point which could be reached by employing whichever of the salts a x, a y, b 2, or b y forms a cryohydrate with the lowest temperature. Thus, in the above case, the solidifying point of nitrate of sodium is -17°:- Nature, XI., 440.

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ON UNILATERAL ELECTRIC CONDUCTIVITY. Dr. Arthur Schuster states that in the course of many experiments he has had frequent occasion to remark that electric currents seem to traverse copper wires more easily in one direction than in the other; so that the galvanometer indicates different intensities when we reverse the direction of the current which traverses it. He gives to this phenomenon the name of unilateral conductivity. He first observed it in using the galvanic battery, but was able to make more accurate observations by means of magneto-electric machines. The phenomenon observed with this apparatus led him to the hypothesis that the current induced by one pole of the magnet traverses a circuit more easily than that induced by the opposite pole. In his second memoir Schuster records another phenomenon which is not without analogy with the preceding. He joined the electrodes of a galvanic battery to the apparatus which he had used in his first experiment, and found that, whatever the intensity of the continuous current might be, or the relative positions of the electro-magnet to the battery, it always happened that the initial deviation of the galvanometer needle augmented during the rotation of the magnet. On the other hand, as long as the magnet was immovable, it exerted no influence upon the deviation produced by the permanent currents. Noting then, at first, the initial deviation of the needle while the magnet was stationary, he interrupted the current before turning the magnet, and observed again the deviation produced at the first passage of the current during the rotation. The difference between these two deviations was sensibly proportional to the intensity of the permanent current, but decreased rapidly with the increase in the electric vibrations produced by the magnet. The cause of this singular

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influence of the rotation of the magnet upon the intensity of the permanent current is scarcely less obscure than that of the unilateral conductivity; and the explanation of the one phenomenon involves the apparent contradiction of the other.-Bull, Hebd., 1875, 297.

SINGULAR PROPERTY OF ALUMINUM ELECTRODES. A singular property of aluminum has been noticed by Ducretet. A voltameter, whose electrodes are respectively aluminum and platinum, allows an electric current to pass, or prevents it, according as the electrodes are respectively positive or negative. If the positive current passes from the platinum to the aluminum, no unusual resistance is experienced. If, on the other band, a positive current passes from the aluminum to the platinum, the current is nearly arrested, and the needle of the galvanometer marks in the first case, for instance, 22°, and in the second case 2o only. The explanation of this seems to be that in the former, or favorable case, the current disengages oxygen at the platinum and hydrogen at the aluminum pole. But in the opposite case the oxygen is produced at the aluminum pole, and forms a layer of alumina (or the oxide of aluminum), to the presence of which the arrest of the current must be attributed. If the poles are placed in hydrochloric acid, the phenomenon no longer takes place, and similarly does it not follow if we employ any alkaline liquid. In opposition to this explanation, however, it must be granted that the microscopic examination of the aluminum electrode does not reveal any apparent change in its appearance, no matter in which way the current flows. Whatever may be the explanation, the fact remains, and is certainly a very striking one. Of course, if both electrodes are formed of aluminum, the current will not flow in either direction. Ducretet proposes to apply this interesting property of aluminum in the construction of a telegraphic apparatus, which he calls a rheotome of constant direction.-13 B, III., 218.

ON THE ELECTRIC DISCHARGE.

The electrical sparks of a peculiar nature that have been called "feeble sparks” by Riess, by whom they were first discovered, are distinguishable from the ordinary bright

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