The action of magnetism and electricity on light is similarly illustrated by the rotation of the plane of polarization. Sir John Herschel was the first who tried to rotate the plane of polarization of a ray of light by surrounding it with a spiral wire electrized by the great battery of two enormous plates of copper and zinc at the London Institution, but he obtained no evidence of any such action. Long afterward Professor Faraday succeeded by sending a ray of light through a piece of silico-borate of lead, which formed the core of a magnetic helix. The silico-borate took on a quasi-crystallised state during the passage of the electric current round it, giving it for the moment the property of circular polarization, analogous to that of glass in a state of tension or compression.
Substances vary exceedingly in the facility with which they transmit electricity; even the same substance under another form differs remarkably in that property: charcoal, which next to the metals is the best conductor known, when under the form of diamond is quite impervious to electricity. In general, substances that are the best conductors of heat are also the best conductors of electricity, as for example the metals, which however, possess the transmissive property in very different degrees. Silver and copper are the best conductors, lead one of the worst; its resistance to the passage of electricity is twelve times greater than that of silver and copper, consequently it becomes twelve times as hot, for when a current of electricity is impeded it is changed into heat. So great is the resistance offered by a fine platinum wire, that the heat amounts to 3280° and the wire is melted, a striking instance of the correlation of electricity and heat, and of the power of the cohesive force.
When electricity is passing through conducting substances or when it is static, it induces an electric state in bodies at a distance by transmission through non-conducting substances or air, for it gives polarity and tension to the adjacent atoms, and these to the next, and the next in succession, throughout the whole intervening mass,—a strong proof of the individuality and polarity of the atoms of matter.
Motion, which is the result of all the physical powers, has itself a strong action upon the ultimate elements of matter; in cases of unstable equilibrium it accelerates and even determines their chemical union. Some substances will remain merely mixed as long as they are at rest, but no sooner is their inertia disturbed by a slight motion than they rush into permanent combination. In newly sublimed iodide of mercury the vibration impressed by the scratch of a pin is so rapidly transmitted through the mass that its colour is immediately changed from yellow to bright red. By a new arrangement of the molecules their action on light is altered.
Catalysis or the chemical decomposition and composition of substances by the contact of a foreign body, is well illustrated by the chloride of nitrogen, that explodes when touched by substances which at ordinary temperatures would neither combine with the chlorine nor with the nitrogen. The iodide of nitrogen explodes if touched by a feather, and M. Becquerel decomposed the iodide of nitrogen by the vibrations of sound. When substances only exist in consequence of the inertia of their atoms, the instability of their chemical attractions and repulsions is only increased by an external agent, so that a great effect is produced by a slight cause, as in an avalanche, the snowy mass is on the point of falling, and the smallest motion, a breath of wind, hurls it down. In such cases the potential energy of the unstable mass is in a moment changed into vis viva or impetus. Daguerreotype impression shows the power of the chemical rays on substances in unsteady equilibrium, and the length of time required to make the impression under the same circumstances is a measure of the instability.
Most of the fulminates are compounds of nitrogen; of that the fulminate of aniline is a recent instance, since it is formed by the slow action of nitrous acid on aniline. Explosion takes place on the sudden evolution of gas, or the sudden change of a solid into vapour. In these cases fire or percussion are the foreign causes of change. They are all particular instances of the general principle of catalysis, which is the chemical combination of heterogeneous atoms by the action of a substance that does not participate in the change. Thus it has long been known that when platinum is plunged into a mixture of oxygen and hydrogen it combines these gases into water. Acids in some cases seem to have the same effect; for when rags or starch are dissolved in an acid the starch is changed to dextrine and the liquid has acquired the power of turning the plane of polarised light to the right. The acid has undergone no alteration, but it has changed the properties of the starch though not its chemical composition. After a time, a second transformation takes place, the liquid ceases by degrees to turn the plane of polarisation to the right, and ends by turning it to the left. The acid is still unchanged, but the dextrine has now disappeared: it has combined with the water and is transformed into glucose or sugar of grapes.
The quantity of the physical powers, active and latent, is inappreciably great. The quantity of heat or potential energy generated by chemical combination alone is enormous.
SECTION III.
ATOMIC THEORY, ANALYSIS AND SYNTHESIS OF MATTER, UTILITY OF WASTE SUBSTANCES—COAL-TAR COLOURS, ETC.
The chemical combination which forms the infinite variety of substances in the organic and inorganic creation consists in an intimate union of their ultimate atoms which produces substances differing from their constituent parts in every respect except gravitation, the sum of the weights of their constituent parts being invariably equal to the weight of the resulting substance. Thus the chemical union of oxygen and hydrogen forms water, and the weight of the water so formed is exactly equal to the sum of the weights of the two gases.
All chemical changes whether of analysis or composition are subject to definite unalterable laws of weight, measure and number; nothing is by chance or casual, the relative weights of the invisible atoms of matter, and their combination in definite proportions reveal the laws which prevailed in the primeval structure of created things. By the wonderful discovery of these laws Dr. Dalton has placed chemistry on a strictly numerical basis.