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C. GENERAL PHYSICS.

ATTRACTION AND REPULSION RESULTING FROM RADIATION.

Mr. Crooks has lately published his investigations into the phenomena known as attraction and repulsion resulting from radiation. The apparatus constructed by him appears to be more sensitive than the ordinary thermo-multiplier. He considers that the experiments show that the repulsion is not entirely due to the rays usually called heat, viz., to the extreme and ultra red rays of the spectrum. The theory advanced by Professor Reynolds to explain his observations is not acceptable to him, although, on the other hand, he has not yet prepared one of his own. According to Reynolds and Balfour Stewart, these experiments constitute a direct proof, and the only known direct proof of the truth of the kinetic theory of gases as developed by Clausius and Maxwell.

Mr. Crooks has adapted his results to the construction of a very important instrument, which he calls a radiometer. It consists of four arms suspended on a steel-pointed axle resting in a cup, so that the arms are capable of revolving horizontally. To the extremity of each arm is fastened a thin disk of pith, lampblacked on one side, and the whole is inclosed in a very perfect vacuum within a glass globe. Under the influence of light, or heat, the little arms revolve with considerable rapidity. Mr. Crooks states that the repulsion experienced by these disks when any radiation falls upon them is proportional to the length of the vibrations, and varies at every point of the spectrum. Professor Guthrie has remarked that Mr. Crooks's research had, in an almost unequaled degree, every element of greatness.

INCREASE OF RADIATION WITH TEMPERATURE.

It is well known that as the temperature of a solid is gradually increased, the refrangibility of the emitted light increases likewise, and as the result we find red light emitted first; afterward the other colored rays gradually appear as the heat increases, until we reach the ultra-violet rays. This correlation between refrangibility and temperature was first

experimentally proved by J. W. Draper, and it would be a result of great importance to determine the law of growth of refrangibility with temperature. If this could be achieved, a very convenient and accurate pyrometer could be made of the ordinary spectroscope. Au accurate investigation of this subject has been undertaken by a committee of the British Association, which has in a preliminary report presented some observations on the simple increase of radiation with temperature. On this subject Becquerel has a great number of observations, whence he infers that the differences between the logarithms of the luminous intensities are proportional to the differences of temperature, a law which he thinks would hold up to 1200 degrees Centigrade; but the law as thus expressed mathematically by no means represents the true rate of increase of the total luminous intensity, which is, indeed, very much slower than that required by Becquerel's law. Again, if the law of Dulong and Petit for the velocity of cooling be true, then the amount of heat radiated, as also the temperature, could be calculated; but on comparison with actual observations at high temperatures it is found that their law gives too rapid an increase for the total radiation. Assuming, however, these laws to be even approximately correct, we may calculate the hypothetical temperature corresponding to the brightness and total radiation from the sun, and deduce in one case for the solar temperature 13,000, and in the other case 11,000 degrees Centigrade. -Report of the British Association, 1873, 461.

THE DIFFUSION BETWEEN DRY AND MOIST AIR.

An investigation that may have some interest in the future of meteorology has been conducted by Dufour, who has examined the question of the diffusion between dry and moist air traversing a porous disk. He finds that the activity of the diffusion does not depend directly, except possibly in a very slight degree, upon temperature. If we compare the observations made at different temperatures, we find that the activity of the diffusion also does not depend, with slight exception, upon the relative humidity; it depends principally upon the difference between the quantities of vapor, or the tensions of the vapor, on the opposite sides of the disk, and is in fact very nearly proportional to the difference of the ten

sions. If, then, we compare observations made at the same temperature, the activity depends upon the relative humidity of the two portions of air. The study of the influence of the diaphragm, its extent, etc., has enabled him to explain how a small porous vase can, by connecting its interior with a manometer, be utilized in studying the diffusion of vapor in the free atmosphere. Similarly, we may determine the tension of the vapor of water existing in the air, at any moment, by determining the difference of the pressures shown by the monometer in the interior, and by the barometer on the exterior. This suggests a new hygrometer, which, simple as it is, and easily applicable, may rapidly find favor among meteorologists so soon as Dufour has exactly developed the laws connecting the relations of pressure and the hygrometric state of the atmosphere.—Bibl. Univers. et Revue Suisse, 1874, 336.

ON THE LAWS OF APPARENT ADHESION.

Stefan, who is well known as one of the most industrious investigators in all matters relating to molecular physics, has recently published in full a memoir relating to preliminary investigations on the subject of adhesion. The true phenomena of adhesion are easily confounded with what he calls apparent adhesion. That is to say, if two perfectly flat plates be brought quite near together, but not in actual contact, and an attempt be made to separate them, a slight force is required, which Stefan states, on further investigation, he is persuaded is simply the result of the resistance which the viscosity of the liquid or gas offers to its immediate inflow into the space between the two disks. The effect of this viscosity, of course, would disappear if the experiment could be performed in an absolute vacuum. On the other hand, the study of the experiments made by Stefan in the atmosphere at its ordinary pressure, and of such liquids as alcohol, water, solutions of salt, etc., has enabled him to determine the true co-efficient of viscosity or internal friction for these substances. The values he arrives at are, for water, 0.0108, for air, 0.00183; figures which agree exactly with those deduced by Maxwell and Meyer by entirely different methods of experiments. The actual separation by means of a slight force of the disks experimented with by Stefan is, he states, a dynamic, and not a static phenomenon. The time

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required in order to separate these plates through a given distance was inversely proportional to the force that was applied. It increased nearly as the inverse square of the original distance between the plates, and as the fourth power of the radius of the plates; and in whatever liquid he immersed his plates, the times were proportional to those required by equal volumes of these liquids to flow through capillary tubes.19 C, VIII., 60.

THE DISSIPATION OF ENERGY.

In some remarks on the dissipation of energy, Lord Rayleigh states that the chemical bearings of this subject are very important. A chemical transformation is impossible if its occurrence involves the opposite of dissipation; but it is not true that a transformation which involves dissipation must necessarily take place; otherwise the existence of explosives, like gunpowder, would be impossible. The possi bility of chemical action must often depend upon the density of the reacting substances. Thus, in the case of a mixture of oxygen and hydrogen in proper proportions at a certain density, the mixture may be exploded by an electric spark, and energy be dissipated; but beyond a certain point of rarity the explosion can not be made, as it could not then involve any dissipation. It may probably be found that many mixtures which show no tendency to explode under ordinary conditions, will become explosive when sufficiently condensed.

THE TRANSMISSION OF MECHANICAL POWER BY MEANS OF ELECTRICITY.

Magneto-electric machines have not yet attained that point of perfection which permits them to be placed among the industrial apparatus; but they at least offer a method of producing electricity very economically. The principal machines are those of Siemens and Halske, of the French Society of L'Alliance, and of Gramme. The currents produced in the last-named machines are analogous to those furnished by the galvanic battery, thus permitting its ap plication to all works where voltaic electricity is useful. The Gramme, as adapted for electro-plating in the silverworks of Christoffle, of Paris, has a height of four feet and a

length of two and a half feet, and the resulting electric current deposits eight hundred grammes of silver per hour, requiring a motive force of one horse-power. The experiments made at London by this machine give it the first rank among apparatus for producing electric light. A curious experiment was performed with one of these machines at the Exposition of Vienna. The principal magneto-electric machine being driven by an air-engine on the Lenoir system, the electricity thus produced was carried to the electro magnets of two Gramme machines of more feeble dimensions, which, acting as an electric motor, kept a small centrifugal pump in action. Thus the mechanical effect of the electricity was by the connecting wires carried to a great distance from the motor. The principal advantage of the transmission of force by electricity is found in the possibility of overcoming vertical spaces which are inaccessible to the cables or belts in use in manufacturing works. - Bulletin Hebdomadaire, XVI., 8.

CRYSTALLIZATION ILLUSTRATED BY THE MICROSCOPIC

PHOTOGRAPH.

It is often very important in chemistry or in crystallography to be able to seize exactly the delicate forms of crystals, as examined by the microscope; but the most conscientious draughtsman can not always reproduce the fineness of these crystallizations. According to Girard, many crystals can be photographed. As a preliminary, it is necessary generally to dilute the solution to different degrees, and to allow one specimen of each kind of crystal to be formed by itself. We thus acquire the faculty of choosing that degree of concentration where the crystallographic characters are most appropriate to the methods of photography, either by reason of the delicate grouping of the forms, or by their transparency to the penetration of light. The most simple. method of photographing the crystals consists in the use of an ordinary microscope, the body of which is placed in a horizontal position. In a dark room upon a table near a window the microscope is placed, and the object is illuminated by a beam of light coming through the window from a heliostat without. The luminous rays pass through the thin film of water containing the crystallized salt, traverse

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