Alum.
Gla.s.s.
_Litharge._ Common salt.
Nitre.
Phosphorus.
Sulphur. [Page 250]
Resin.
Spermaceti.
Iceland spar.
Tartaric acid.
Citric acid.
Water.
Alcohol.
Ether.
Sugar.
Starch.
Gum-arabic.
Wood.
Ivory.
Dried mutton.
Fresh beef.
Dried beef.
Apple.
Bread.
Leather.
Fresh blood.
Dried blood.
Caoutchouc.
Jet.
Turpentine.
Olive oil.
Hydrogen.
Carbonic acid.
Carbonic oxide.
Nitrous oxide (moderately).
Nitric oxide (very slightly).
Olefiant gas.
Coal gas.
Nitrogen is neither paramagnetic nor diamagnetic, and is equivalent to a vacuum. Magnetically considered, it is like s.p.a.ce itself, which may be considered as zero.
The term _magnetic_ Faraday proposes should be a general one, like that of _electricity_, and include _all_ the phenomena and effects produced by the power, and he proposes that bodies magnetic in the sense of iron should be called _paramagnetic_, so that the division would stand thus:
Magnetic ... { Paramagnetic, { Diamagnetic;
and it is this division which has been observed in the preceding tables.
All s.p.a.ce above and within the limits of our atmosphere may be regarded as traversed by lines of force, and amongst others are the lines of magnetic force which affect bodies, as shown in the table of paramagnetic and diamagnetic bodies, which have the same relation to each other as positive and negative, or north and south, in electricity and magnetism.
The lines of magnetic force are a.s.sumed to traverse void s.p.a.ce without change; but when they come in contact with matter of any kind they are either concentrated upon it or scattered according to the nature of the matter.
The power which urges bodies to the axial or equatorial lines is not a central force, but a force differing in character in the axial or radial directions. If a liquid paramagnetic body were introduced into the field of force, it would dilate axially, and form a prolate spheroid like a lemon, while a liquid diamagnetic body would dilate equatorially, and form an oblate spheroid like an orange. Plucker has demonstrated that if magnetic solutions are placed in watch gla.s.ses across the poles of the [Page 251] electro-magnet, they are heaped up in a very curious manner.
The poles of the electro-magnet are pieces of soft iron, which may be drawn away or approached at pleasure, and according as the poles are nearer or further asunder, the magnetic liquids, such as solution of iron, are heaped up in one or two directions, as shown at B and C in Fig. 243.
[Ill.u.s.tration: Fig. 243. Gla.s.s dish holding magnetic solution of iron, and placed in the magnetic field.]
"The diamagnetic power, doubtless," says Faraday, "has its appointed office, and one which relates to the whole ma.s.s of the globe. For though the amount of the power appears to be feeble, yet, when it is considered that the crust of the earth is composed of substances of which by far the greater portion belongs to the diamagnetic cla.s.s, it must not be too hastily a.s.sumed that their effect is entirely overruled by the action of the magnetic matters, whilst the great ma.s.s of waters and the atmosphere must exert their diamagnetic action uncontrolled."
Plucker has also announced--what at the time he believed to be true--the highly interesting and important fact that the optic axis of Iceland or calcareous spar is repelled by the magnet and placed equatorially--a fact which Plucker thought true of many other crystals when the magnetic axis is parallel to the longer crystallographic axis. A piece of kyanite, which is a mineral composed of sand, clay, often lime, iron, water, and is used in India, being cut and polished as a gem, and sold frequently as an inferior kind of sapphire, will, it is said, even under the influence of the earth"s magnetism, arrange itself like a magnetic needle.
Plucker believed that he had discovered an existing relation between the forms of the ultimate particles of matter and the magnetic forces, and he imagined that the results he obtained would lead gradually to the determination of crystalline form by the magnet. The experiments of Tyndal and k.n.o.blauch lead, however, to a very opposite series of conclusions, and by ingeniously powdering the crystals with water, and making them into a paste, which was afterwards dried and suspended [Page 252] as a model in "the magnetic field;" also by taking a slice of apple about as thick as a penny-piece, with some bits of iron wire through it, in a direction perpendicular to its flat surface, they were found to set equatorially not by repulsion but by the attraction of the iron wires; or instead of the iron by placing bis.m.u.th wires, the apple now settled axially, not by attraction but by the repulsion of the bis.m.u.th. Ipecacuanha lozenges, Carlisle biscuits also, suspended in the magnetic field, exhibited a most striking directive action. The materials in these two cases were _diamagnetic_; but owing to the pressure exerted in their formation their largest horizontal dimensions set from pole to pole, the line of compression being equatorial; and it is a universal law "_that in diamagnetic bodies the line along which the density of the ma.s.s has been induced by compression sets equatorial, and in magnetic bodies axial_." Hence they a.s.sume, from these and many other conclusive experiments, that crystallized bodies, such as Iceland spar, take their position in the magnetic field without reference to the existence of an "optic axis."
At the conclusion of a brilliant lecture at the Royal Inst.i.tution by Dr.
Tyndal "On the influence of material aggregation upon the manifestations of force," in which Plucker"s experiments respecting the repulsion of the optic axis were gracefully discussed and his theory refuted, the learned doctor said: "This evening"s discourse is in some measure connected with this locality; and thinking thus, I am led to inquire wherein the true value of a scientific discovery consists? Not in its immediate results alone, but in the prospect which it opens to intellectual activity--in the hopes which it excites--in the vigour which it awakens. The discovery which led to the results brought before us to-night was of this character. _That_ magnet[E] was the physical birthplace of these results; and if they possess any value they are to be regarded as the returning crumbs of that bread which in 1846 was cast so liberally upon the waters. I rejoice, ladies and gentlemen, in the opportunity here afforded me of offering my tribute to the _greatest workman_ of the age, and of laying some of the blossoms of that prolific tree which he planted at the feet of the great discoverer of diamagnetism."[F]
[Footnote E: Alluding to a splendid magnet made by Logeman, which was sent to the Exhibition in Hyde-park in 1851. It could sustain a weight of 430 pounds, and was purchased by the Royal Inst.i.tution for Dr.
Faraday.]
[Footnote F: Dr. Faraday.]
It was first observed by Father Bancalari, of Genoa, that when the flame of a candle is placed between the poles of a magnet it is strongly repelled. The flames of combustible gases from various sources are differently affected, both by the nature of the combustible and by the nearness of the poles. Faraday repeated Bancalari"s experiments, and by a certain arrangement of the poles of this magnet he obtained a powerful effect in the _magnetic field_, and having the axial line of the magnetic force horizontal, he found that when the flame of a wax taper was held near the axial line (but on one side or the other), and about one-third of the flame rising above the level of the upper surface of the [Page 253] poles, as soon as the magnetic force was exerted the flame receded from the axial line, moving equatorially until it took an inclined position, as if a gentle wind was causing its deflection from the upright position.
When the flame was placed so as to rise truly across the magnetic axis, the effect of the magnetism was very curious, and is shown at A, Fig.
244.
On raising the flame a little more the effect of the magnetic force was to intensify the results already mentioned, and the flame actually became of a _fish-tailed shape_, as at C, Fig. 244; and when the flame was raised until about two-thirds of it were above the level of the axial line, and the poles approached very close, the flame no longer rose between the poles, but spread out right and left on each side of the axial line, producing a double flame with two long tongues, as at B, Fig. 244.
[Ill.u.s.tration: Fig. 244. Effect of magnetism on candle-flame between the poles of the magnet.]
It was these experiments that led to the important discovery of the paramagnetic property of oxygen, and proved in a decided manner that gaseous bodies when heated became more highly diamagnetic. Oxygen, which (tried in the air) is powerfully magnetic, becomes diamagnetic when heated. A coil of platinum wire heated by a voltaic current, and placed beneath the poles of Faraday"s apparatus, occasioned a strong upward current of air; but directly the magnetic action commences the ascending current divides, and a descending current flows down _between_ the upward currents.
The discovery, says Silliman, of the highly paramagnetic character of oxygen gas, and of the neutral character of nitrogen, the two const.i.tuents of air, is justly esteemed a fact of great importance in studying the phenomena of terrestrial magnetism. We thus see that one-fifth of the air by volume consists of an element of eminent magnetic capacity, after the manner of iron, and liable to great physical changes of density, temperature, &c., and entirely independent of the solid earth. In this medium hang the magnetic needles used as tests, and as this magnetic medium is daily heated and cooled by the sun"s rays, its power of [Page 254] transmitting the lines of magnetic force is then affected, influencing undoubtedly the diurnal changes of the magnetic needle.
For a complete digest of Faraday"s discoveries in diamagnetism the reader is referred to the second edition of Dr. Noad"s comprehensive and learned work ent.i.tled "A Manual of Electricity."
Coming always from the highest walks of philosophy to lower and "_common things_" one cannot help being reminded of the old-fashioned method of _drawing up_ a sluggish fire, and the natural query is suggested whether the poker is to be considered as a weak magnet, and does influence and draw towards the fire a greater supply of magnetic oxygen gas? (Fig.
245.)
[Ill.u.s.tration: Fig. 245.]
[Ill.u.s.tration: The interior of the optical box at the Polytechnic--looking towards the screen. The a.s.sistants are supposed to be showing the dissolving views.]
[Page 255]
CHAPTER XXI.