The fact being established that magnetism is essentially a molecular phenomenon, the next step is to inquire what is the constitution of a magnetic molecule, and why it is that some molecules are ferromagnetic, others paramagnetic, and others again diamagnetic. The best known of the explanations that have been proposed depend upon the magnetic action of an electric current. It can be shown that if a current i circulates in a small plane circuit of area S, the magnetic action of the circuit for distant points is equivalent to that of a short magnet whose axis is perpendicular to the plane of the circuit and whose moment is iS, the direction of the magnetization being related to that of the circulating current as the thrust of a right-handed screw to its rotation. Ferromagnetism was explained by Ampère on the hypothesis that the magnetization of the molecule is due to an electric current constantly circulating within it. The theory now most in favour is merely a development of Ampère's hypothesis, and applies not only to ferromagnetics, but to paramagnetics as well. To account for diamagnetism, Weber supposed that there exist within the molecules of diamagnetic substances certain channels around which an electric current can circulate without any resistance. The creation of an external magnetic field H will, in accordance with Lenz's law, induce in the molecule an electric current so directed that the magnetization of the equivalent magnet is opposed to the direction of the field. The strength of the induced current is -HScos[theta]/L, where [theta] is the inclination of the axis of the circuit to the direction of the field, and L the coefficient of self-induction; the resolved part of the magnetic moment in the direction of the field is equal to -HS²cos²[theta]/L, and if there are n molecules in a unit of volume, their axes being distributed indifferently in all directions, the magnetization of the substance will be -(1/3)nHS²/L, and its susceptibility -(1/3)S²/L (Maxwell, _Electricity and Magnetism_, § 838). The susceptibility is therefore constant and independent of the field, while its negative sign indicates that the substance is diamagnetic. There being no resistance, the induced current will continue to circulate round the molecule until the field is withdrawn, when it will be stopped by the action of an electromotive force tending to induce an exactly equal current in the opposite direction. The principle of Weber's theory, with the modification necessitated by lately acquired knowledge, is the basis of the best modern explanation of diamagnetic phenomena. Entry: J
The [alpha]-diketones may be prepared by boiling the product of the action of alkaline bisulphites on isonitrosoketones with 15% sulphuric acid (H. v. Pechmann, _Ber._, 1887, 20, p. 3112; 1889, 22, p. 2115), CH3·CO·C:(N·OH)·CH3 --> CH3·CO·C:(NHSO3)·CH3 --> CH3·CO·CO·CH3; or by the action of isoamyl nitrite on the isonitrosoketones (O. Manasse, _Ber._, 1888, 21, p. 2177), C2H5·CO·C = (NOH)·CH3 + 11C5HONO = C2H5·CO·CO·CH3 + C5H11OH + N2O. They condense with orthodiamines to form quinoxalines (O. Hinsberg, _Ann._, 1887, 237, p. 327), and with ammonia and aldehydes to form imidazoles. _Diacetyl_, CH3·CO·CO·CH3, is a yellowish green liquid, which boils at 87-88°C., and possesses a pungent smell. It combines with sodium bisulphite and with hydrocyanic acid. Dilute alkalis convert it into paraxyloquinone. Entry: DIKETONES