tremely valuable feature when one is obliged to experiment in order to determine the strength of the current necessary for accomplishing a certain work, and one equally valuable to the physicist who may desire to elucidate obscure points in the theory of the machine. The Gramme machine has been still further improved by combining with its peculiar features the construction due to Wild and Ladd, by which an immense magnetic power is developed from a very slight initial movement of magnetism; by this means an instrument has been produced by which the same electric tension is attained with a velocity one half as great as that originally necessary. In the course of the numerous improvements that Mr. Gramme has made in his original machine, his latest construction seems to leave nothing to be desired. number of electro-magnets and of coils is now reduced, from six and twelve respectively, to two and four. The ring is virtually doubled, giving far more facility in the application of the same machine to very different objects, such as galvanoplasty, lighting, heating, etc.; in the machines, as originally constructed with a simple ring, each one was only convenient for use for the immediate purpose for which it was designed and proportioned.-13 B, III., 139.

THE FRICTION AND THERMAL CONDUCTIVITY OF GASES.

The

In a memoir by Messrs. Kundt & Warburg on the friction and thermal conductivity of gases for heat, the authors endeavor to investigate the accuracy, at high temperatures and low densities, of the laws deduced by Maxwell, Meyer, Loschmidt, Stefan, and Boltzmann, which for ordinary temperatures and densities hold good in gases; they find, first, that the co-efficient of sliding friction between moving gas and a fixed plane has a determinate value dependent on the nature of the gas, so long as this is present in layers thicker than fourteen times "the mean length of path of the molecules" as defined by the kinetic theory of gases; the co-efficient is also inversely proportional to the pressure. Second, the absolute value of the co-efficient of sliding friction is found to be 0.7 x1, on the assumption that the molecule of gas. is reflected from the moving surface used in the apparatus with velocities of translation equal to those of the surface itself. For air at 760 millimeters, 7=0.000083 millimeter,

H

therefore the co-efficient of friction should be 0.000058 X pressure; but actual observations give a result very nearly twice as great. Hence it is concluded that in the striking of the molecules against the walls, their velocities are not completely equalized. The absolute co-efficient of friction for the air is given by these authors at 0.000189, being exactly midway between the four previous determinations made by Graham, Maxwell, Meyer, and Puling. The co-efficients of friction for hydrogen and for carbonic-acid gas were determined by them to be respectively 0.488 and 0.806 (that of the air being 1), agreeing closely with the values deduced from the observations of Graham. The co-efficient of friction for pure steam at a temperature of 15° Centigrade resulted about one half of that of air. The investigation into the dependence of the co-efficient of friction on the density or barometric pressure of the gas shows that the diminution of friction with pressure is greater the rarer the layer of gas. Further experiments bearing upon the kinetic theory of gases were made by Messrs. Kundt & Warburg in that they attempted to determine the co-efficient of conductivity for heat. Their approximate result for the atmosphere is one eleventh less than that deduced a few years ago by Stefan; and from these same observations there resulted also the value of the radiating power of glass, which agreed nearly with that of Lehnebach. The variation of the radiating power with the temperature does not seem to them to have been reliably determined in the classical work of Dulong and Petit.-Monatsbericht der K. Akademie von Preussen, Berlin, 1875, 160.

THE CONNECTION BETWEEN FLUORESCENCE AND ABSORPTION.

Dr. Sorby, President of the Royal Microscopic Society, states that he has been surprised to find that some of those who have paid considerable attention to such subjects have so far misunderstood the question as to suppose that the light of fluorescence consists of rays which are, as it were, reflected by this solution, and do not penetrate through it, so that the spectrum of the fluorescence would show a bright band in the same place as some dark bands seen in the spectrum of the transmitted light. This is certainly an error, and his own observations agree more nearly with Lubarsch,

who shows that of eight different substances the spectrum of the light of fluorescence extends some distance on the red end side of the principal absorption band in the spectrum of transmitted light; so that the spectrum of fluorescent substances can never contain rays which are more refrangible than those which are most readily absorbed by a very dilute solution. This, although a very general rule, yet has some decided exceptions. In some substances, under strong illumination, the light of fluorescence does contain rays of greater refrangibility than those most readily absorbed by a dilute solution, and extends from the red end a little beyond the centre of the main absorption band. A number of little known and interesting fluorescent solutions are quoted by Sorby in illustration of his remarks.-Monthly Micr. Journal, p. 161.

THE ISOCHRONISM OF THE BALANCE SPRING.

William D. Glasgow, in a short article in the Horological Journal, on balance springs, states that the isochronism of the balance spring of a watch is a subject bristling with controversy. There are some who say that every spring must be isochronized; others that every length of spring has its isochronous point of suspension; others that mere length has absolutely nothing to do with isochronism. Mr. Glasgow holds that length has every thing to do with it, as shown by his own experiments. Too short a spring, whatever may be its form, will make the short arcs of the balauce's vibrations to be performed in a less time than the long arcs. Thus a spring with ten turns may be too short, and will lose in the short arcs and gain in the long arcs. A spring of two turns will be too long, and will describe its longer arcs in too short a period. The best length for a flat spring is, he finds, fourteen turns; but a flat spring, although the most common, is also the worst form, as it does not expand and contract properly. It will assist the action in this spring if it is always a little small, as this gives more freedom to the portion of the coil next to the stud. The Breguet spring, although differing very little in form. from the flat spring, is essentially different in action and principle, having perfect freedom to expand in a circle all around. From twenty to twenty-five turns is, he finds, the

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 porous 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 aqueous vapor by the porous 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 temperature 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 dif fusion 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 unequal 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 improvements introduced by him, and new forms of apparatus, which enable the phenomena of repulsion by radiation to be