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best length for this spring. According to his experience, the length of the spring, and the length alone, is sufficient to secure perfect isochronism.--Horolog. Journal, June, 1875.
THE VARIATIONS OF TEMPERATURE ACCOMPANYING THE DIF
FUSION OF GASES. Professor Dufour, of Lausanne, Switzerland, as the results of an investigation into the variations of temperature which accompany the diffusion of gases traversing partitions of porous earthenware, states his conclusions as follows: When currents of dry air, of hydrogen or of illuminating gas, circulate along the walls of a porous vase, or of a vase which incloses fragments of porous material, they produce a lowering of temperature. The depression diminishes little by little, and finally ceases altogether. When the currents of the same gas, charged with moisture, circulate under the same conditions, there is produced a heating, which also diminishes gradually, and finally ceases. The warming and the heating are more or less considerable, according to the initial condition of the porouis vase. The greatest variations are produced when the dry current succeeds to a saturated current, or inversely. These variations of temperature are probably due to the absorption of aqueons vapor by the porons substance, or to the disengagement of this vapor. If the experiments are conducted under a constant barometric pressure, then, when the air on the one side, and the hydrogen or illuminating gas on the other side, are in contact with the two faces of the porous partition, the diffusion which takes place produces a change of temperature, but a change having a different sign on the opposite sides of the diffusing partition. There is a lowering of temperature on the side where the denser gas is found, or, in other words, on the side where the current arrives most abundantly. There is, on the other hand, a rise of temiperature on the opposite side. These variations of temperature have been observed when the gases taking part in the diffusion are dry, as well as when they are charged with aqueous vapor. When the gases are employed without drying, and without saturation, the diffusion also evidently occasions the variations of temperature just indicated; but it is probable that this variation is influenced by the pres
ence of the vapor of water. The extent of the variation of temperature which accompanies diffusion is different in different cases, according to the special arrangements of the experiments. It is always greatest when the diffusion is most abundant and most active. We can conveniently explain the facts established by supposing that in the diffusion the gaseous current produces a heating on the side where it comes into the porous partition, and a cooling on the side where it emerges. These currents having an inequal importance, depending on their density, we can comprehend that there is, as a result, a warming on one of the faces, and a cooling on the other face of the partition. When the experiments are made under different barometric pressures, we find that, when the endosmose of a lighter gas is accompanied by an increase of pressure in the porous vase, the temperature varies only very little, and generally augments during the endosmose, while the manometer falls after having attained its maximum, and the pressures tend to equalize themselves, the temperature diminishes more or less rapidly, and by a relatively considerable quantity. When the exosmose of a lighter gas gives rise to a diminution of pressure in the porous vase, the temperature varies only a very little, and more generally diminishes during the exosmose. When the manometer rises after having attained its maximum, and the pressures tend to equalize themselves, the temperature augments more or less rapidly, and by a quantity relatively quite considerable. This change of temperature, when the diffusion is accompanied with a change of pressure, is conveniently explained by admitting that the thermic variation due to the diffusion is conformable to the laws above indicated, and is due (but with a certain retardation) to the variation caused by the compression or the rarification of the gas which surrounds the thermometer. -Bibl. Univ., XLIX., 103.
ATTRACTION, REPULSION, AND RADIATION. Professor Crookes, whose first interesting paper on radiation was read in 1873, has recently made a second communication on the subject, in which are described certain inprovements introduced by him, and new forms of apparatus, which enable the phenomena of repulsion by radiation to be
observed and illustrated. A bulb, three inches in diameter, is blown at the end of a glass tube eighteen inches long. In this a fine glass stem, with a sphere or disk of pith at each end, is suspended by means of a fibre of silk. The bulb is then perfectly exhausted and hermetically sealed. Instead of pith, disks may be made of iron, metal, cork, or other substances. The apparatus, when constructed with proper precautions, is so sensitive to heat that a touch of the finger on a part of the globe near one of the disks of pith will drive the index around over a quarter of a revolution, while it follows a piece of ice, as the needle follows the magnet. With a large bulb very well exhausted, a somewhat striking effect is produced. When a lighted candle is placed about two inches from the globe, the glass stem with its pith disks oscillates to and fro through gradually increasing arcs, until several complete revolutions are made, when the torsion of the suspended fibre offers a resistance to the revolutions, and the bar commences to turn in an opposite direction. This movement is kept up with great energy and regularity, like the movements of the balance wheel of a watch, as long as the candle burns. A modification of this apparatus, in which a glass thread is substituted for the silk fibre, allows quantitative as well as qualitative observations. The sensitiveness of the apparatus to beat rays appears to be greater than that of the ordinary thermo-electric multiplier. Thus the obscure heat rays from copper, at a temperature of 100°, after passing through glass, produce a deflection on the scale of 3} divisions, while under the same circumstances no current at all is detected in the thermo-pile.-Nature, XI., 494.
ROOD'S APPLICATION OF ZÖLLNER'S HORIZONTAL PENDULUM.
The paper of Professor 0. N. Rood, on the application of the horizontal pendulum to the measurement of minute changes and the diminution of solid bodies, although read in 1874, bas only recently been published; and from it we learn the details of the instrument proposed by him as an improvement on Zöllner's horizontal pendulum. This proposed improvement consists essentially in an inflexible rod placed horizontally, and supported in that position in mid-air by vertical wires or springs, stretched in such a manner that the influence of gravity on the rod is no longer sensible,
while its motion is entirely under the control of the observer. Professor Zöllner's apparatus was designed expressly to measure attractive forces and slight changes of level. Professor Rood proposes to apply his own similar apparatus to the study of minute changes in the dimensions of solid bodies, for which purpose he gives it such dimensions as to impart to it an unprecedented delicacy. The main difficulty in the use of this apparatus is the fact that it is exceedingly sensitive to the most distant and unseen sources of disturbance. Thus Professor Rood remarks that children, playing at a distance of 360 feet, caused temporary deflections of one or two scale divisions; and similar deviations were caused by the lower notes of an organ in a neighboring church, the middle and higher notes producing no sensible results. These effects upon the apparatus can be eliminated, however, by making a sufficient number of observations, the evils caused by them being only temporary. As usual in all investigations, the effects of temperature are the most insidious.
As illustrating the fineness of the measurements that can be made with the horizontal pendulum, Professor Rood gives some figures showing that the one eighteen-millionth part of an inch becomes a sensible quantity; whereas hitherto, with the best optical and mechanical means, it has been hardly possible to measure the two one-hundred-thousandth part
of an inch.- Am. Jour. Sci., 1875, IX., 441.
THE ELASTICITY OF BARS OF ICELAND SPAR. Dr. G. Baumgarten mentions that a lecture of Professor Neumann on the theory of elasticity, in which he called attention to the interest that would attach to the determination of the co-efficients of elasticity in crystalline bodies, led him to undertake this labor, and that, so far as he knows, bis own are, as yet, the first direct observations on the elastic properties of crystals. Voigt, however, has since then investigated the elasticity of the crystals of rock salt. Iceland spar was chosen by Dr. Baumgarten, among other reasons because, in reference to its physical and optical properties, it is better known than almost any other mineral. His determinations of its elasticity were made by measuring the bending of bars of spar, when pressed in various directions, and which had been cut in different directions from the crystal. The bars operated upon by him were two inches long, and had a square section of about the seventieth part of a square inch. He finds that the amount of deflection in the centre of a bent bar is a function of many quantities, but his observations allow him to state, first, that the deflection of a bar whose section is a perfect square is the same, no matter against which side the bending force is applied. Second, it varies with the dimensions of the bar as regards its thickness, breadth, and length, precisely as though the body were homogeneous; and the same laws apply to it within the limits of accuracy of his observations as apply to ordinary iron bars, the deflections being proportional to the cnbe, to the thickness, and to the length. Third, the deflection is dependent in a peculiar manner on the direction of the axis of the bar, in relation to the optical axis of the crystal from which it is cut. There exists, however, in this respect no symmetry with reference to the optical axis of the crystal. Bars cut parallel to the longest diagonal of the crystal give a minimum of deflection; those cut parallel to the shortest diagonal giving a maximum deflection. Inaugural Diss., Berlin, 1875.
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A NEW MANOMETER.
M. Fol has submitted to the Physical Society of Geneva a description of a manometer specially designed for deep-sea soundings. This instrument consists essentially of two spherical reservoirs, superposed, and connected by a capillary tube. The upper reservoir should be closed and filled entirely with a compressible liquid-for example, alcohol. The other sphere has an opening in its upper part, and is filled with mercury, which also fills the capillary tube. The quantity of mercury which shall have passed from the second reservoir into the first, when the apparatus has been submitted to a given pressure, will give the measure of this pressure, and consequently of the height of the column of water or the depth in the sea.—Mem. de Soc. d. Phys. de Genève, 1874, 483.
THE PHYSICAL PROPERTIES OF MATTER IN THE LIQUID AND
GASEOUS STATES. Professor Andrews, in a preliminary notice of his researches on the physical properties of matter in the liquid and gas