This value is quite of the order of the value above given for hydrogen A≤1·1 × 1013, and that for iodine-vapour A1=1·2×1012 ergs; it is indeed somewhat greater than the latter. H From this it results that the whole work required for the dissociation of hydrogen and iodine molecules is used in overcoming the purely electrical attraction forces of the valency-charges. That this heat of dissociation is so completely. spent in electrical work shows, in agreement with many other facts, that the forces of chemical affinity are essentially of an electrical character; that the electrostatical forces which the charges exert on each other at the positions of valency are "by far the most powerful among the forces exerted among the atoms;" and that in H. v. Hemholtz's 'Faraday Lectures,' particular possible chemical forces of the charged atoms can be but infinitely small in comparison with electrical forces. This seems of great importance for the theory of chemical forces in general. Wiedemann's Annalen, No. 10, 1893. ALTERATIONS OF THE ELECTRICAL RESISTANCE OF AQUEOUS SOLUTIONS AND OF GALVANIC POLARIZATION WITH PRESSure. BY BRUNO PIESCH. A large series of liquids were investigated, both acids and solutions of salts. The method of measurement was such that the resistance and the polarization could be determined simultaneously. The great pressure was produced by a Cailletet's apparatus, which held up to 600 atmospheres. The resistance-vessels consisted of a combination of glass tubes, which could be connected externally by means of ebonite cemented in the metal cap, which was firmly screwed in the iron cylinder. The measurements gave the following results : Change of pressure produces a change of resistance, which decreased in all cases with increase in the pressure. No regular influence of concentration could be observed, but with most substances the variations of a less concentrated solution were greater than of a concentrated solution. As the pressures increased the decreases of resistance were less. Polarization also showed a change of value with higher pressure. These, however, are for the most part small. An increase of polarization with the pressure was in general observed. irregularities here shown were greater than with the measurements of resistance. 2 The In conclusion, a solution of NH,NO, in alcohol was investigated, and alterations in the same direction were here met with.— Wiener Berichte, May 25, 1894. THE LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE AND JOURNAL OF SCIENCE. [FIFTH SERIES.] IN OCTOBER 1894. XXXVIII. Fixed-Arm Spectroscopes. [Plates VIII.-X.] prismatic spectroscopes and spectrometers of the usual construction it is necessary, in order to observe different portions of the spectrum under the same conditions, to vary the angle between the axes of the observing and collimating telescopes by the rotation of the arm which carries one of these telescopes about an axis parallel to the refracting edge of the prism. Usually the arm which carries the slit and the collimating-lens is fixed, and that carrying the observing telescope is movable; but sometimes the apparatus which it is necessary to carry on the observing arm is so massive, or else requires such a degree of stability, that it becomes necessary to fix it in position and make the slit-arm the movable one. Then difficulties are at once encountered in the illumination of the slit, if a fixed source of light such as the sun or a star is under examination. Even if terrestrial sources be employed, it is ofttimes extremely undesirable to have a complicated system of collimator, slit, condenser, and source of light, swinging about on a long arm, and in particular cases such an arrangement is absolutely inadmissible. In such cases where both arms of the spectroscope are necessarily fixed, we * Communicated by the Author. Phil. Mag. S. 5. Vol. 38. No. 233. Oct. 1894. 2 A may easily bring different parts of the spectral field to the cross wires of the observing telescope by a rotation of the prism alone; but in none of the usual forms without violating the usual condition imposed in spectroscopic work, viz., that the prism shall always be in minimum deviation for the central ray in the field. Heretofore the only forms of "fixed-arm" spectroscopes which fulfil the latter condition have been of the Littrow type, in which the ray is, by a system of reflecting surfaces, made to traverse the prism twice in opposite directions, and to emerge finally in a direction parallel but reversed in direction to that in which it entered. The original form and the various modifications of this instrument have been already described *. While they fulfil the condition of bringing any part of the spectral field into the field of the observing telescope without changing the angle between the latter and the collimator, there are certain objections to all of them. The most serious objection to the original form, that of a general illumination of the field, has been simply and successfully overcome in the manner already pointed out by the author †, but the second objection, that of a close proximity of the slit to the observing eyepiece, remains. Young's and Browning's modifications overcome this latter as well as the former difficulty; but they are both complicated and expensive, necessitating as they do the use of at least three reflecting-surfaces and two independent telescopes, in addition to the usual prisms. Moreover, with all instruments of this general type, it is necessary for the beam of light to pass twice through the prism or train of prisms, and this, where brightness is of more importance than large dispersion, as with very faint sources of light for example, is a disadvantage. For these reasons I have thought it would be of interest to describe some simpler forms of "fixed-arm" spectroscopes, which I have recently devised and used, and in which the usual arrangement of parts may be preserved, viz., a separate collimating and observing telescope, on opposite sides of a prism-train, through which train the light is passed but once. The slit and observing eye-piece are thus far removed from each other, and the axes of the observing and collimating telescopes may be arranged to make any fixed angle with each other. The general solution of the problem is effected by the introduction into some part of the spectroscopic train, between "An Improved Form of Littrow Spectroscope," F. L. O. Wadsworth, Phil. Mag. July 1894. + Ibid. the slit and the focal plane of the observing lens, of a reflecting surface, which has an angular motion equal to onehalf the change in angular motion of the ray refracted at minimum deviation. In the instruments of the Littrow type a reflector is also used, but for an essentially different purpose (to return the ray through the prism) and in an essentially different way from that here indicated. It hardly seems possible that the use of a reflecting surface in this way can be new, as it is something which would naturally suggest itself to anyone who might consider the problem; but no reference has been found to its employment, perhaps because the necessity for a "fixed-arm" spectroscope does not often seemingly arise. I hope to show, however, that at least some of the forms which have been developed during the last three years are simpler, more convenient, and less expensive than the ordinary form, and for spectrometric work quite as accurate. I will briefly describe these forms in the order of design and use, for this, perhaps, naturally is also the order of increasing simplicity. Case I.-The case which first led me to a consideration of the problem was one in which it was desired to examine the radiations from a Geissler tube by means of the wavecomparer. Here the instrument which took the place of the usual observing eyepiece was a massive apparatus weighing about 500 pounds, which required great steadiness of mounting, while the Geissler tube, which itself served as the slit, was attached to a mercury-pump on one side and to a sodiumamalgam generating apparatus, with its attached dryingtubes, mercury-valves, &c., on the other, in such a way as to make its movement impossible. At first, indeed, the tube was mounted on a slit-arm of the spectroscope, sufficient flexibility being secured to allow of different lines being brought on the slit of the wave-comparer by the use of long lengths of glass tubing, but experiments soon showed that it was necessary to keep the passage from this Geissler tube to the pump as short and large as possible. If any considerable length of tubing intervened, unusual precautions had to be taken to keep all parts of it perfectly dry and clean, for only under these conditions, which, as the tubes were frequently changed, were almost impossible to maintain, could the McLeod gauge attached to the pump be relied upon to give, even approximately, the true pressure in the tubes, particularly when the electric discharge was passing. The arrangement adopted in this case was that shown in plan *See a paper by Professor A. A. Michelson, " Application of Interference Methods to Spectroscopic Measurements," Phil. Mag. Sept. 1892. in fig. 1, Pl. VIII.; where t is the Geissler tube which serves as the source; m a mirror, mounted on a vertical axis on the movable spectroscopic arm R; and S the slit of the wavecomparer, which separates out the radiation which is to be analysed by the latter. To the lower end of the shaft which carries the mirror m is fixed a drum connected with a second drum a, fixed on the axis of the spectroscope by two steel cords. The drum a is just one half the size of b, and hence, as the arm R revolves about the axis of the spectroscope, the mirror m revolves on its own axis with an angular velocity just one half that of the arm, but in the opposite direction. Hence, if the mirror is once adjusted so that the direction of the reflected ray is parallel to the axis of collimation, it will remain parallel to that axis for all positions of the arm, and we have therefore a virtual rotation of the slit. There will be a slight lateral displacement of the ray, which can be easily investigated. For a certain position of the arm, say a b, fig. 1, the reflected ray b a will lie in the principal axis of the collimating lens, which we will suppose passes through the centre of rotation of the mirror m. Let be the angle of deviation for this position, then for any other position of the system in which the deviation is 0+e, the reflected ray will be parallel to the axis of collimation, but will be displaced laterally from it a distance Ay. Finally, let a be the angle between the mirror-face and the axis of collimation in the position a b, then, from the construction of the apparatus, this angle for any position b'a will be a e/2. From the two small triangles bbd and bb' b we get at once € |