Maxwell's electro-magnetic theory consists of two distinct parts. The essential elements of the first part are the law of induced electromotive force and the law of induced magnetomotive force. These two fundamental laws are an extension of the two experimental laws of magneto-electric and of electromagnetic induction, which extension consists in adding to what is already contained in these experimental laws the hypotheses. that the magnetic and the electric flux in dielectrics are not a mere mathematical fiction, as the older electro-magnetic theories supposed, but an actually existing state in the dielectric whose rate of variation through any imaginary surface in the dielectric is proportional to the reacting electro-motive, respectively magneto-motive force, induced around the circuit of the boundary line of this surface. The value of the reacting force is independent of the nature of the substance through which the circuit passes. Hence the physical constants of the electromagnetic field (that is specific inductive capacity, etc.) do not appear in the first part of Maxwell's electro-magnetic theory. The second part of this theory takes account of these constants by adding to the two fundamental laws just mentioned the ordinary form of law of flux. The first two laws are a cross-connection between the varying flux of one type and the reacting force of the other type; the law of flux, on the other hand, is a direct connection between the force and the flux of its own type. Combining the two fundamental laws with the law of flux, we obtain the fundamental differential equations of the second part of Maxwell's electro-magnetic theory. These equations are a mathematical statement of the laws of propagation of an electro-magnetic disturbance through various media, exhibiting the remarkable fact that, on the one hand, the velocity of propagation of an electro-magnetic disturbance is approximately and the wave-form of propagation is exactly the same as in the case of light; on the other hand, however, the velocity of propagation of an electro magnetic wave through a non-absorptive dielectric is independent of its periodicity, a fact which does not hold true in the case. of light.* Hence although both parts of the Maxwellian theory agree well with experimental facts which were brought to light by the labors of Hertz and of other physicists who extended the Hertzian methods of investigation, yet this theory falls short of being a satisfactory theory of light, because it represents the propagation of electro-magnetic waves as inde * An apparent exception to this is the propagation of light through the most perfect of all dielectrics, namely, the ether in a perfect vacuum The velocity of propagation of light through this dielectric is, within the narrow limits of the visible spectrum, independent of the wave length. The question, however, whether all wave-lengths travel through pure ether with the same velocity has not yet been answered definitely. The statement. therefore, that the velocity of Hertzian waves in ether is the same as that of light is somewhat indefinite. pendent of the periodicity of these waves, and therefore fails to account for some of the most important optical phenomena like dispersion, absorption, etc. These phenomena we know to be due to the dependence of the optical constants of a substance on the periodicity of waves; hence, if Maxwell's electromagnetic theory is to become a satisfactory theory of light, it must be extended so as to represent the electro-magnetic constants of a substance as functions of the periodicity of the impressed electro-magnetic forces. This extension of the Maxwellian theory will affect, therefore, the law of flux only, and not the first part of this theory, since this part does not and should not contain any reference whatever to the nature of the medium of which the electro-magnetic field is composed. This opinion is confirmed by the recent developments of the electro-magnetic theory of light, since in these the law of flux. and not the two fundamental laws of Maxwell's theory appear in a modified form. Besides, it is obvious that Maxwell's form of the law of flux holds true for constant and slowly varying forces only. In every other case its application is only tentative. The second part of the Maxwellian theory should, therefore, be considered as a tentative and therefore approximate description of the laws of propagation of electro-magnetic disturbances. A more accurate description of these laws must proceed from a more general form of the law of flux, a form which will hold true for forces of all periodicities. That such a form of the law of flux actually exists is rendered highly probable by considering the most general relation between the electric and magnetic forces and their fluxes which will satisfy Maxwell's fundamental hypotheses, and also the observed fact that the electric, respectively magnetic, energy of the medium through which an integral electric, respectively magnetic, current has passed is a quadratic function of that integral current. This general relation is expressed by the forms (C) and (D) deduced above. Maxwell's provisional form of the law of flux is a special case of this general relation. This general relation suggests also that many other forms of the law of flux are permissible in which those physical constants which determine the propagation of an electro-magnetic disturbance appear as functions of the periodicity of the disturbance. But these constants are not specific inductive capacity, magnetic permeabilities and resistivity of the medium. It still remains to be shown that according to Maxwell's electro-magnetic theory a law of flux containing explicitly the periodicity of the impressed forces is not only admissible. but also necessary. When that is shown, then this theory will have fulfilled one of the most essential conditions which every satisfactory theory of light must fulfill. Effingham Park, West Islip, L. I., Aug. 20, 1895. AM. JOUR. SCI-THIRD SERIES, VOL. L, No. 298-OCTOBER, 1895. SCIENTIFIC INTELLIGENCE. I. CHEMISTRY AND PHYSICS. 1. Conversion of the black to the red mercuric sulphide.-In a series of careful experiments W. SPRING has obtained the following values for the specific gravity and specific volume of the black mercuric sulphide precipitated, and for the red sulphide both precipitated and sublimed. 8.1587 Red, sublimed. 18.0 8.1464 34.6 8.1199 56.6 8.0906 77.5 8.0979 Sp. grav.... The above values, treated graphically, prove that the volume varies with the temperature with both the sulphides. The expansion increases rapidly with the temperature then grows less and less, finally diminishing so as to give a maximum at about 56°. It is hence concluded that the cause of the difference in color is to be found in the constitution of the molecule and not in their orientation, that is in the crystallization. On comparing the specific gravity with the molecular weight (232), calculation shows further that the molecule of cinnabar must contain the group HgS fifteen times if the black sulphide contains it fourteen times. The following are determinations of the specific heat at different temperatures of the two sulphides, both precipitated: The above values of the specific heat correspond at different temperatures to those of the volumes as given above. On comparing the volumes of the black and red sulphides at the same temperature the values 0.888 and 0.932 are obtained. From these the pressure can be calculated at which the one variety should be converted into the other. The value deduced is 34,000 atmospheres, on the supposition that the compressibility does not vary, and this not being strictly true, the required pressure in fact would be still greater or beyond the limit of experiment. The author, however, has found it possible to prepare a form of black sulphide differing less from the red in density. By subliming mercuric sulphide under ordinary conditions the red crystallized variety is formed, but if the vapors are diluted with a sufficient volume of inert gas (as CO,, or N), a black powder is obtained containing black opaque crystals quite distinct from cinnabar and also distinct from the black variety obtained by precipitation. This variety was found to have a specific gravity of 8.0395 and a specific volume of 124-385 at 17°. For this a pressure of about 2,500 atmospheres, which it is easy to obtain, suffices to convert it into the red sulphide; the black sulphide being changed throughout its mass to the red vermilion. Finally the author finds that while the red sulphide if heated at 250° begins to turn black but assumes its red color on cooling, and while this is true up to 320°, if heated above 320°, it remains black permanently; this is then a critical temperature above which the red variety ceases to exist.-Bull. Acad. Roy. Belgique, III, xxviii, 238-257. 2. The color, density and surface tension of hydrogen peroxide. Recalling the observations of Bunsen that pure water has a bluish color in a layer 2 meters in thickness and in one greater than 7.5 meters a bluish green, W. SPRING states that his observations show that the color does not change with the thickness of the layer, the shade only varies. He ascribes the green tints to a yellow fluorescence produced by the play of the white light upon minute solid particles in suspension. He also notes the observations of Olzewski that liquid oxygen shows a blue color in a thickness of 30mm. Further, he remarks that ozone as a gas has also a blue color in a layer of 1 meter in thickness (Hautefeuille and Chapuis), while in the liquid form it is blue and almost opaque in a layer of 2mm (Olzewski). The author has now shown that hydrogen peroxide, like oxygen, ozone and water, also has a blue color, though hitherto it has been stated to be a colorless liquid. With great care he has prepared a considerable amount of the pure material, viz: 212 grams representing some 140cc at 13°. Of this the specific gravity was found by means of a pycnometer to be 1.4996 at 1°.5, and the value of the surface tension, compared with that of water, was determined to be 0.456. The color was observed in a tube 1 meter in length; this placed vertically showed, after the bubbles of gas had disappeared, a distinct blue color somewhat tinged with green; the green however is regarded as non essential. A quantitative determination of the color showed that the tint, while similar to that of water, was deeper, the relation between the two being 1:183; this was obtained by comparisons of each with layers of copper chloride of such thickness as to give the same tint. From this fact it is argued that in hydrogen peroxide the oxygen has lost less of its properties than in water, and this, in the author's opinion, confirms the view that the constitution is expressed as a feebly united compound of the group O, with H, rather than as a union of two hydroxyl groups, HO-Oй. The blue color of the sky the author regards as explained primarily by the color belonging to the oxygen, ozone, water and hydrogen peroxide present in it; it is not due, however, to transmission simply but to reflection from the earth and refraction, as a mirage of diffused light, through extensive layers of the atmosphere. The author promises to discuss this latter subject more fully later.-Bull. Acad. Roy. Belgique, III, xxix, 363-384. 3. Indirect Electrolysis of a liquid.-E. ANDREOLI has recently described a modified method for the electrical decomposition of a liquid which he calls indirect or secondary electrolysis. He employs a bath divided by plates of porous porcelain into three compartments; the end ones he fills with some conducting liquid, while the middle one contains the liquid to be decomposed; the anode and cathode are immersed in the first and last compartments, respectively. If the arrangement is simply as thus far described the passage of the electrical current results only in the decomposition of the first mentioned liquid:-thus, if it is water containing salt, chlorine is liberated at the anode and sodium hydrate formed at the cathode while the other liquid, e. g. gold cyanide, is unaffected. If, however, a metallic plate, or better, several plates are immersed in the liquid of the central compartment, but not connected with either pole, this liquid is now decomposed and in the case given gold is deposited on the metallic surface. Thus a lead plate opposite the diaphragm near the negative pole becomes peroxydized, while the other side is coated with gold. Metallic plates placed vertically or horizontally in the layer of liquid near the positive compartment are coated with gold. If the liquid in the end compartments is gold cyanide, no change is found in it if tested after a considerable time, while the same liquid in the central compartment has been decomposed even to the point of exhaustion. This method has also been employed to transform bisulphite of sodium into the hyposulphite; this liquid is placed in the center with two metallic plates on the positive and negative sides respectively, while any convenient conducting liquid fills the ends. The latter liquid is unaffected by the passage of the current, while the other is decomposed with the liberation of gas and a piece of linen immersed in the bath is rapidly bleached.-Le Génie civil, xxvii, June 29, 1895. 4. On the Constituents of Cleveite gas.-A paper upon this subject by C. RUNGE and F. PASCHEN is printed in the September number of the Philosophical Magazine (p. 297) translated from the Proceedings of the Berlin Academy (July 11). |