765 University of M General and Physical Chemistry. Influence of Pressure on the Index of Refraction of Water for Sodium Light. By L. ZEHNDER (Ann. Phys. Chem. [2], 34, 91-121). This investigation was carried out principally with the view of deciding whether the so-called specific refraction or some similar expression was truly a constant. Gladstone and Dale (Phil. Trans., 153, 317) have investigated the effect of change of pressure on the index of refraction of liquids, and they came to the conclusion that when D is the density of the liquid, the quantity (u 1)/D is a constant. Landolt (Ann. Phys. Chem., 123, 595) arrived at a similar conclusion. Rühlmann (ibid., 132, 1) and Wüllner (ibid., 133, 1) came to the conclusion that it was not perfectly constant. According to Jamin (Ann. Chim. Phys., 52, 163), the quantity (u 1)/D is a constant. Mascart (Ann. Phys. Chem., 153, 154) denied the constancy of this quantity, and Quincke (ibid. [2], 19, 401) confirmed Gladstone and Dale's conclusion. L. Lorentz (ibid. [2], 11, 70) and H. A. Lorentz (ibid. [2], 9, 641) independently arrived at the result that (u2 - 1)/(u2 + 2) . 1/D was constant. The author, as the results of a series of observations carried out with minute precautions, arrives at the conclusion that Gladstone and Dale's result, (u 1)/D = constant, is a very close approximation to the truth, and that the other relations are not. G. W. T. Spectra of Oxygen. By J. JANSSEN (Compt. rend., 106, 11181119). Observations on the Pic du Midi show that the indistinct absorption-bands in the oxygen spectrum (Abstr., 1886, 749) can be seen in the solar spectrum if the atmospheric layer through which the sunlight passes is of sufficient diameter. The existence of these bands, and the law that their intensity is proportional to the product of the diameter of the layer of oxygen into the square of the density of the gas, is confirmed by a series of observations extending from normal pressure to a pressure of 100 atmos. with tubes varying from 0:42 to 60 metres in length. The author calculates that these bands should be just visible with a layer of liquid oxygen 4 to 5 mm. in thickness; and Olzewski has actually observed them with a layer of 7 mm. in thickness. C. H. B. Spectrum of Gold. By E. DEMARÇAY (Compt. rend., 106, 1228— 1230). The lines at 5230, 4437, 4338, and 4064, which Boisbaudran observed in the spectrum of gold, but which Krüss attributes to the presence of platinum and palladium, are not really due to these metals. They are visible although all the other lines of platinum and palladium are absent, and they do not actually coincide with VOL. LIV. 3 e any lines of these metais. The lines 5230, 4437, and 4064 are undoubtedly due to gold, but the author was unable to observe the line 4338 in a condensed spark spectrum. Boisbaudran has since found that the line 4338 is only visible with concentrated solutions of gold chloride, and it may be due to a nonmetallic element. C. H. B. Application of the Method of High Interferences to Quantitative Spectrum Analysis. By H. EBERT (Ann. Phys. Chem. [2], 34, 39-90).—The author points out that the researches of Fizeau and Foucault (Ann. Chim. Phys. [3], 26, 138, and [3], 66, 429; Compt. rend., 21, 1155, 26, 680, and 54, 1237), Billet (Compt. rend., 67, 1000), Mascart (Ann. de l'école Normal [2], 1, 57), Ketteler (Beobachtungen über die Farbenzerstreuung der Gase, Bonn, 1865), Müller (Ann. Phys. Chem., 150, 86), the author himself (ibid. [2], 32, 337), and Michelson and Morley (Amer. J. Sci. [3], 34, 427), on interference with large difference in length of path, show that although the possibility of interference is limited by conditions depending on the change in the vibrations of the luminous particles, the limits are wide ones, and the last-named authors observed cases of interference with a difference in length of path of as much as 200,000 times the wave-length. Müller was the first who applied this to the study of the source of light itself. The method is very suitable for investigating small alterations in the refrangibility of lines in the spectrum. With light corresponding with the green mercury line, the author has obtained interference phenomena with a difference in length of path 80,000 times the wave-length, and as a shifting in one of the bands of one-tenth its thickness can be observed, an alteration of 1/800000 of the wavelength, or 1/800 of the distance between the two sodium lines can be determined. The most homogeneous lines in the spectrum are formed by a number of component vibrations which differ very little in wavelength, and the amplitudes of which are a definite function of the wave-length; alterations in the interference bands simply giving evidence of changes in the mean wave-length. The determination of the greatest difference in length of path for which interference bands are obtained gives a measure of the time during which the luminous molecules maintain their vibrations unchanged, because two rays from the same luminous particle will interfere only if the vibration is the same at the moment of emission of each of the rays. Wiedemann has considered this question (Ann. Phys. Chem. [2], 5, 503; Phil. Mag. [5], 7, 77), and has shown that the value of this maximum length of path is intimately connected with the mean length of the molecular free path, and in the case of sodium vapour the method gave a number of the same order of magnitude as that calculated from the kinetic theory of gases. An increase in the density of the gas will narrow the limits within which interference is possible, as after contact between two molecules the vibrations will be altered, so that two rays to interfere must both come from a molecule during the interval between two encounters. Lippich has also shown (Wien. Ber., 82) that an increase of temperature has the same effect, but the increase of the density is of greater importance, as is easily seen, and was pointed out by Wiedemann in the paper referred to. This physicist has applied the method to the investigation of the pressure at the surface of the sun and stars (Phil. Mag. [5], 10, 123). It is clear from these considerations that the method can only give a lower limit for the duration of the undisturbed vibration of a free molecule. An important question to be determined is the influence exerted by the breadth of the spectrum line on the greatest length of path for which interference is possible. Müller, in the paper referred to, shows that the finite breadth of a spectrum line gives rise to secondary systems of interference bands, causing an alteration in the clearness of the primary bands, and he shows by theoretical reasoning that the effect of gradually widening the line, maintaining the difference in length of path constant, has the same effect as gradually increasing the difference in length of path while maintaining the breadth constant. In confirmation of this he states that when a bead of salt is gradually plunged deeper into a bunsen flame, thus increasing the quantity of vapour, the lines become wider. The author has been unable to observe this alteration, and believes that Müller must have been mistaken, as the effect could not possibly be sufficient to be appreciated by the eye. The author does find an increase in clearness when the bead is plunged deeper into the flame, but only when it passes into the cooler interior portion, so that the quantity of vapour is diminished, and therefore the spectrum line becomes more homogeneous. With regard to the limits of the method, the author shows that it is impossible to follow the shifting of the bands due to broadening of the line for much more than the width of the band, but the extent to which the widening of a line for a given difference in length of path can be observed is only limited by the thinness of the plates used to obtain the fringes; and on the other hand, by increasing the thickness of the plates, the earlier stages of the widening process can be observed. The authors made experiments with beads consisting of salts of lithium, potassium, strontium, and sodium. When the bead first touched the outside of the flame the illumination was feeble, and the interference bands became sharply defined. As the bead was plunged more deeply into the flame the bands became less distinct, and the illumination of the field of view increased, the lines being widened and the light becoming less homogeneous. The depth to which the bead had to be plunged into the flame to cause complete disappearance of the bands was greater for the less fusible salts, and for those which gave the least homogeneous light. When the bead reached the cool interior portion of the flame, the bands reappeared. This alternate appearance and disappearance of the bands are in the author's opinion the explanation of the alternation described by Müller. As the illumination gradually increased, the bands appeared to remain constant in position until at a certain point, corresponding with |