between the edges of two chimneys and the photographic apparatus, and found it to be 2° 46′ 55′′. By means of a cathetometer the distance of corresponding parts of the flash and of the lateral spectra, and the distance of the edges of the chimneys, were measured. The first magnitude was 1.6 mm. and the second 5.2. From these data the angle of diffraction is calculated at 0°856, and the wave-length at 382.10-6 mm. The determination of the angle of diffraction is independent of a knowledge of the focal distance of the object-glass, and of possible misdrawing. This measurement is not accurate enough to determine to what body this line belongs; it can only be concluded from this that that portion of the light which is chiefly effective in photographing lightning-flashes has approximately the wave-length 382.10-6. This communication has been made with a view of inciting other observers, who have at their disposal better apparatus and can more frequently observe storms, to make experiments by this method. It is possible that additional ultra-violet lines may be established if we succeed in photographing flashes at several hundred metres distance, for in the present case the distance of the storm was more than 10 km. Measurements of the wave-lengths of lines of the spectrum of lightning have been made by Vogel and by Schuster, by means of a spectrum apparatus provided with a slit. The following table gives the wave-lengths arrived at in their experiments : To this must be added one in the ultra-violet part of the spectrum of the approximate value of 382.10-6-Wiedemann's Annalen, Feb. 1894. ON THE MAGNETIZATION OF SOFT IRON.. BY M. P. JOUBIN. In a note in the number of this Journal for last month I have shown that we may expect to find a characteristic equation of the intensity of magnetization of strongly magnetic bodies as a function of the susceptibility, and I proposed to verify this conclusion experimentally. But the experiments of Rowland * solve this question, at any rate for the bodies investigated by him, and which comprise different varieties of iron, steel, and nickel. * Phil. Mag. 4th series, vol. xlvi. p. 140. Phil. Mag. S. 5. Vol. 37. No. 227. April 1894. 2 F There is a characteristic equation with coefficients independent of the nature of the body; consequently, the theorem of corresponding states applies to magnetism. Let us imagine an ideal magnetic body for which the susceptibility corresponding to an infinitely small field vanishes, and for which the maximum intensity of magnetization I, is exactly equal to three times the critical intensity I.. The curve representing the intensity I as a function of K is a parabola, and by referring the variables to their critical values I, and Ke, the reduced equation of the parabola may be written, as is readily seen, This is an ideal case, for : : I. The susceptibility has a finite value K, when the field vanishes: K-K。. K.-K. II. The value I, of I corresponding to K, is not equal to 31. But to obtain the same conditions, let us measure the susceptibility, not from 0, but from K, and take the ratio If we construct curves for Rowland's experiments, it will be seen that is very appreciably constant for all substances and is equal to 2.66. It follows from this by a simple calculation that the characteristic equation is X= I (= + y = KK) x=1+0·33 (1−y) ± 1·3 √1−y. The following are some examples showing the agreement of this formula with experiments: 1000 2000 3000 0 1240 1060 816 IV. Steel: 4K, 50; I, 570; 4K, 320. V. Soft Iron: 4πK ̧ = 300; I, = 520; 4πK ̧ = = 960 245 245 = 3550. 50 140 290 -Communicated by the Author, from the Comptes Rendus, Jan. 15, 1894. ERRATUM in last paper. P. 337, line 7 from bottom, p. 338, line 2 from top, for Hm read Ix Im. ON THE THERMAL BEHAVIOUR OF LIQUIDS. GENTLEMEN, In the February number of the Philosophical Magazine, Professors Ramsay and Young have published a short paper on the thermal behaviour of liquids, in which the results of some of my experiments are attacked. In the interest of true knowledge I cannot leave these remarks unnoticed, as I consider they rest on an insufficient foundation. The principal remark concerning the defectiveness of the arrangements employed to obtain constant high temperatures does not apply to me, as the authors of the above-named paper may easily see. In my article (Wied. Ann. 1. p. 529, 1893) I expressly and clearly state that in the glycerine bath which contained the experimental tubes the temperature was perfectly uniform and could be maintained constant for hours. This I proved by repeated experiments. The conclusions arrived at by the authors and stated on p. 207 (Phil. Mag. vol. xxxvii. 1894) are in accord with my own observations, for I also found that when the experimental tubes were uniformly heated in the glycerine bath no striæ were visible, but only a rusty appearance. As regards the purifying of the liquid from air, the authors may see what particular pains were taken by referring to p. 530 of my article. St. Petersburg: Physical Laboratory of the Academy of Sciences, February 21, 1894. B. GALITZINE. ON THE THERMAL BEHAVIOUR OF LIQUIDS. To the Editors of the Philosophical Magazine. GENTLEMEN, In the February number of this Journal Messrs. Ramsay and Young have published a note on the critical temperature in which a statement appears of which, in the interests of science, I am compelled to ask them for an explanation; they have characterized as "very inaccurate a paper on the same subject by myself*. This would not be of much importance if my work had not attracted the attention of many physicists and especially of M. Galitzine, who would thus have been led into error by my mistake. To avoid this I venture respectfully to request Profs. Ramsay and Young to point out the errors of reasoning and of experimentation that I have made in the demonstration of my fundamental proposition:" The volume of a fluid is not always defined by the temperature and pressure only;" thus fluids of different density can correspond to the same temperature and pressure. On this proposition all my deductions rest. It would also be of service to show the error committed by M. Galitzine in his demonstration. I may also point out to Profs. Ramsay and Young that before 1880 I had arrived at the conclusion that the particles that constitute liquids have a different volume from the particles of vapour. It was by this means that I explained the maximum density of water t. Profs. Ramsay and Young think with M. Gouy that the only variation of density produced in liquids confined in a tube results from the variation of hydrostatical pressure; the great compressibility of fluids near the critical point would cause very considerable variations of density. The ingenious interpretation of M. Gouy, which would explain very simply an anomaly that at first had a paradoxical appearance, cannot now unhappily be admitted. If it were correct there should be produced immediately, at the critical temperature, a stable equipoise in a pipe vertically fixed. The experiments I published last year show that this is not the case; thus the mode of distribution of substance is different a little after the disappearance of the meniscus and twenty-four hours after. P. DE HEEN. "Sur un état de la matière caractérisé par l'indépendance de la pression et du volume spécifique," Bulletin de l'Acad. Roy. de Belgique, t. xxiv. for 1892. "De la dilatabilité de quelques liquides organiques et des solutions salines," présenté à l'Acad. Roy. de Belgique, May 6, 1879. "De l'influence du temps sur le mode de formation du ménisque à la température de transformation," Bull. de l'Acad. Roy. de Belgique [3] xxv. p. 14 (1893). THE PHILOSOPHICAL MAGAZINE AND JOURNAL OF SCIENCE. [FIFTH SERIES.] MAY 1894. IT XLI. The Internal Work of the Wind. [Plates V.-IX.] PART I.-Introductory. T has long been observed that certain species of birds maintain themselves indefinitely in the air by "soaring," without any flapping of the wing or any motion other than a slight rocking of the body; and this, although the body in question is many hundred times denser than the air in which it seems to float with an undulating movement, as on the waves of an invisible stream. No satisfactory mechanical explanation of this anomaly has been given, and none would be offered in this connexion by the writer, were he not satisfied that it involves much more than an ornithological problem, and that it points to novel conclusions of mechanical and utilitarian importance. They are paradoxical at first sight, since they imply that under certain specified conditions very heavy bodies entirely detached from the earth, immersed in, and free to move in, the air, can be sustained there indefinitely, without any expenditure of energy from within. These bodies may be entirely of mechanical construction, A paper read (by title only) to the National Academy of Sciences, in April 1893, and subsequently (in full) at the Aeronautical Congress, at Chicago, in August, 1893. Communicated by the Author, to whom we are likewise indebted for clichés of the plates. Phil. Mag. S. 5. Vol. 37. Nọ. 228. May 1894. 2 G |