MAGNETIS M 263 the magnecrystallic effect to be described presently, it is often advisable to reduce certain substances to powder before testing them ; the powder is filled into a thin glass tube and then tested like a liquid. By means of powdered bismuth the tendency of a diamagnetic to pass from places of stronger to places of weaker force can be very prettily shown. If the powder be strewn upon the circular end of the core of an electromagnet, it will leave the edges and collect in the centre, whereas iron filings will leave the centre and arrange themselves round the edges, the fact being that at the edges the force is much more intense than in the centre. 1 f Faraday arranges the metals in the following order of descending magnetic susceptibility : Iron. Nickel. Cobalt. Manganese. Bismuth. Antimony. Zinc. Tin. Cadmium. Sodium. Paramagnetic. Chromium. Cerium. Titanium. Diamagnetic. Mercury. Lead. Silver. Copper. Gold. Palladium. Platinum. Osmium. Arsenic. Uranium. Rhodium. Iridium. Tungsten. Silicium is given as strongly paramagnetic, and beryllium, 2 aluminium, potassium, and sodium 3 as weakly magnetic; the last three were given as diamagnetic by Faraday ; the magnetic character appears to depend on the method by which the material is prepared, being doubtless deter mined by the presence or absence of slight impurities. The copper of commerce is magnetic, owing to traces of iron ; but when it is reduced by means of zinc from the chloride or sulphate it is diamagnetic. It would appear that the paramagnetism of titanium, palladium, platinum, and osmium is due to iron impurity. 4 Platinum 6 reduced from very pure chloroplatinate of ammonium by heating in a current of air is diamagnetic. According to Graham the magnetism of palladium when charged with hydrogen is due to the presence of hydrogenium ; Blondlot, however, has recently found that palladium is less magnetic when charged with hydrogen than when uncharged, from which he concludes that condensed hydrogen is pretty powerfully diamagnetic. Tellurium, sulphur, selenium, and thallium are strongly, and niobium and tantalum weaklydiamagnetic. Magnetic Properties of Gases. The earliest results of Faraday were of a negative description, but the discovery by Bancalari 7 of the powerful diamagnetic action of flame again drew the attention of Faraday, 8 Pliicker, 9 and s Becquerel 10 to the subject. Faraday caused the gas under examination to stream vertically upwards or downwards (according as it was lighter or heavier than the surround ing gas) between the poles of an electromagnet, and observed how the stream was deflected. In the case of colourless gases the deflexion was observed by allowing small traces of hydrochloric acid to mix with the gas, and then placing in different parts of the field small tubes con taining pieces of filter paper moistened with ammonia ; by noticing in which of these the white fumes of ammonium chloride were formed the course of the gaseous current could be determined. Another method employed was to fix two thin glass tubes containing gases to be tested to the ends of a cross piece on one end of the arm of a torsion 1 This fact explains the astonishing behaviour of a flat disk of thin iron when placed on the centre of the pole. 2 See Wiedemann, Galvanismus, Bd. ii. 552. 3 Laing, Ann. d. Chim. et d. Phys., 1857. 4 Wiedemann, I.e. 5 Wiedemann. 6 Comptes Rendus, 1877. 7 Zantedeschi, Fogy. Ann., 1848.
- Phil. May., 1847 ; or Exp. Res., vol. iii. p. 467.
9 Pogg. Ann., 1848, &c. l Ann. d. Chim. et d. Phys., 1850. balance ; the tube containing the most magnetic then moved towards the axial line. Another method, employed both by Pliicker and by Faraday, is to blow soap bubbles with the gas to be tested, and observe their behaviour in the magnetic field, allowing of course for the feeble diamagnet- ism of the water film. Faraday s results are as follows : In Air. In Carbonic Acid. In Hydrogen. In Coal Gas. Air
j_ + weak , Nitrogen - stroii <* Ox yen + + -f stroll* + stroii" Hydrogen . strong o Carbonic acid . . . Carbonic oxide . Nitrous oxide... Nitric oxide Nitrous acid . ... Olefiant gas Coal gas - ? weak + I weak - strong
- weak + - weak - weak - weak
Sulphuric acid.. Hydrochloric acid . - ? weak Hydriodic acid . Fluosilicic acid. Ammonia Chlorine - - - weak
Iodine .... Bromine Cyanogen - strong List of gases. + means magnetic relatively to the surrounding gas, diamagnetic ; tlio epithets strong and weak relate of course to the apparent behaviour under the circumstances of the experiment. It appears therefore that oxygen is the most paramag- Oxygen, netic of all the gases ; on this account Faraday conceived that it probably played an important part as a cause of terrestrial magnetism. 11 Becquerel 12 has concluded from recent experiments of his own that the specific magnetism of ozone is still greater than that of oxygen. Faraday was able by filling thin glass bulbs with oxygen at different densities to show that the magnetic susceptibility decreased with the density, apparently in simple proportion. Some numbers giving an idea of the magnetic susceptibility of the various weakly magnetic bodies are given below. In all experiments with gases or fluid media, and indeed Faraday s in every possible magnetic experiment more or less, it is ex P e - important to notice that the resulting magnetic action is ^ r s _ the difference between the action of the movable body and en ti a i the action on the surrounding medium. This was first magnetic pointed out by Faraday. 13 He prepared three solutions action, of ferrous sulphate. No. 1 contained 74 grains of the hydrated salt for every ounce of water ; No. 2 was formed by diluting one volume of No. 1 with two volumes of water, No. 3 by diluting one volume of No. 1 with fifteen volumes of water. Three glasses g v </ ? , f/ 3 and three tubes t v t.-,, t 3 were filled with the respective solutions. The glassies were placed in succession between the pointed poles of the electromagnet, and the tubes tested in them with the following result : In g,. In r/,. In y 3 .
_u
+ +
Jj
+ + means pointed axially ; + + the fame with greater decision ; - pointed cquatorially ; was indifferent. We have here the experimental confirmation of the important theoretical conclusion (see above, p. 248) that any body will behave paramagnetically or diamagnetically according as it is surrounded by a medium less or more magnetic than itself. In cases where the square of the See Exp. Res., 2847 sq. ; also art. METEOROLOGY.
Comptes Rendus, 1881. 13 Exp. Res., 2362, 1845.
