expected to exhibit great apparent differences of conduct, arising simply from the very various intensity of the forces brought into play. Many persons have thought it requisite to imagine occult forces producing the suspension of the clouds, and there have even been absurd theories representing cloud particles as minute water-balloons buoyed up by the warm air within them. But we have only to take proper account of the enormous comparative resistance which the air opposes to the fall of minute particles, to see that all cloud particles are probably constantly falling through the air, but so slowly that there is no apparent effect. Mineral matter again is always regarded as inert and incapable of spontaneous movement. We are struck by astonishment on observing in a powerful microscope, that every kind of solid matter suspended in extremely minute particles in pure water, acquires an oscillatory movement, often so marked as to resemble dancing or skipping. I conceive that this movement is entirely due to the vast comparative intensity of chemical actions when exerted upon minute particles, the effect being 5000 or 10,000 greater in proportion to the mass than in fragments of an inch diameter (vol. ii. p. 9). Much that was formerly obscure in the science of electricity, arose from the extreme differences of intensity and quantity in which this form of energy manifests itself. Between the instantaneous and brilliant discharge of a thunder-cloud and the gentle continuous current produced by two pieces of metal and some dilute acid, there was no apparent analogy whatever. It was therefore a work of great importance when Faraday demonstrated the identity of the forces in action, showing that common frictional electricity would decompose water like that from the voltaic battery. The relation of the phenomena became plain when he succeeded in showing that it would require 800,000 discharges of his large Leyden battery to decompose one single grain of water. Lightning was now seen to be electricity of excessively high tension, but extremely small quantity, the difference being somewhat analogous to that between the force of one million gallons of water falling through one foot, and one gallon of water falling through one million feet. Faraday estimated that one grain of water acting on four grains of zinc, would yield electricity enough for a great thunderstorm. The It was long believed that electrical conductors and insulators belonged to two opposed classes of substances. Between the inconceivable rapidity with which the current passes through pure copper wire, and the apparently complete manner in which it is stopped by a thin partition of gutta-percha or gum-lac, there seemed to be no resemblance. Faraday, again, laboured successfully to show that these were but the extreme cases of a chain of substances varying in all degrees in their powers of conduction. Even the best conductors, such as pure copper or silver offer some resistance to the electric current. other metals have considerably higher powers of resistance, and we pass gradually down through oxides and sulphides. The best insulators, on the other hand, allow of an atomic induction which is the necessary antecedent of conduction. Hence Faraday inferred that whether we can measure the effect or not, all substances discharge electricity more or less". One consequence of this doctrine must be, that every discharge of electricity produces an induced current. In the case of the common galvanic current we can readily detect the induced current in any parallel wire or other neighbouring conductor, and can separate the opposite currents which arise at the moments when the original currents begin and end. But a discharge of high tension electricity like lightning, though it certainly occupies time and has a beginning and an end, r Experimental Researches in Electricity,' Series xii. vol. i. p. 420. yet lasts so minute a fraction of a second, that it would be hopeless to attempt to detect and separate the two opposite induced currents, which are nearly simultaneous and exactly neutralise each other. Thus an apparent failure of analogy is explained away, and we are furnished with another instance of a phenomenon incapable of observation and yet theoretically known to exist. Perhaps the most extraordinary and fundamental case of the detection of unsuspected continuity is found in the discovery of Cagniard de la Tour and Professor Andrews, that the liquid and gaseous conditions of matter are only remote points in a continuous course of change. Nothing is at first sight more apparently distinct than the physical condition of water and aqueous vapour. At the boilingpoint there is an entire breach of continuity, and the gas produced is subject to laws incomparably more simple than the liquid from which it arose. But Cagniard de la Tour showed that if we maintain a liquid under sufficient pressure its boiling point may be indefinitely raised, and yet the liquid will ultimately assume the gaseous condition with but a small increase of volume. Professor Andrews, recently following out a similar course of inquiry, has shown that liquid carbonic acid may, at a particular temperature (30°92 C.), and under the pressure of 74° atmosphere, be at once in a state indistinguishable from that of liquid and gas. At higher pressures carbonic acid may be made to pass from a palpably liquid state to a truly gaseous state without any abrupt change whatever. The subject is one of some complexity, because as the pressure is greater the abruptness of the change from liquid to gas gradually decreases, and finally vanishes. As similar phenomena or an approximation to them have been observed in various other liquids, there is little doubt that we may make a wide generalization, sLife of Faraday,' vol ii. p. 7. and assert that, under adequate pressure, every liquid might be made to pass into a gas without any breach of continuityt. The liquid state, again, is considered by Professor Andrews to be but an intermediate step between the solid and gaseous conditions. There are various indications that the process of melting is not perfectly abrupt; and could the experiments be made under adequate pressures, it is believed that every solid could be made to pass by insensible degrees into the state of liquid, and subsequently into that of gas. These discoveries appear to open the way to most important and fundamental generalizations, but it is probable that in many other cases phenomena now regarded as discrete may be shown to be different degrees of the same process. The late Professor Graham was of opinion that chemical affinity differed but in degree from the ordinary attraction which holds different particles of a body together. He found that sulphuric acid continued to evolve heat when mixed even with the fiftieth equivalent of water that is added to it, so that there seemed to be no distinct limit to chemical affinity. He concludes- There is reason to believe that chemical affinity passes in its lowest degree into the attraction of aggregation'". The atomic theory is well established, but its limits are not marked out. As Mr. Justice Grove suggests, we may by selecting sufficiently high multipliers express any combination or mixture whatever of elements in terms of their equivalent weights*. Sir W. Thompson has suggested that the power which vegetable fibre, oatmeal, and many other substances possess of attracting and condensing aqueous vapour is probably continuous, or, in fact, ident'Nature,' vol. ii. p. 278. u 'Journal of the Chemical Society,' vol. viii. p. 51. tical with capillary attraction, which is capable of interfering with the pressure of aqueous vapour and aiding its condensation. There are many cases of so-called catalytic or surface action, such as the extraordinary power of animal charcoal for attracting organic matter, or of spongy platinum for condensing hydrogen, which can only be considered as exalted cases of a much more general power of attraction. The number of substances which are decomposed by light in a striking manner is very limited; but many other substances, such as vegetable colours, are affected by long exposure; on the principle of continuity we might well expect to find that all kinds of matter are more or less susceptible of change by the incidence of light rays. It is the opinion of Mr. Justice Grove that wherever an electric current passes there is a tendency to decomposition, a strain on the molecules, which when sufficiently intense leads to disruption. Even a metallic conducting wire may be regarded as tending to decomposition. Davy was probably correct in describing electricity as chemical atlinity acting on masses, or rather, as Grove suggests, creating a disturbance through a chain of particles". Laplace went so far as to suggest that all chemical phenomena may be regarded as the results of the Newtonian aw of attraction, applied to atoms of various mass and position; but the time is probably long distant when the progress of molecular philosophy and of mathematical methods will enable such a generalization to be verified or refated. The Law of Continuity. to the title Law of Continuity we may place many of the general principle of reasoning, that Poracical Magazine,' 4th Series, vol. xlii. p. 451. |