its proper distance. A tendency then is a cause which may or may not be counteracted. In the second case of explanation an effect is shown to be due, not to the supposed cause directly, but to an intermediate effect of that cause. Instead of A being the cause of C, it is found that A is the cause of B, and B the cause of C, so that B constitutes an intermediate link. This explanation may seem to increase the complexity of the matter, but it really simplifies it; for the connection of A with B may be a case of a familiar and simple law, and so may that of B with C; whereas the law that A produces C may be purely empirical and apparently out of harmony with everything else. Thus in lightning it seems as if electricity had the power of creating a loud explosion; but in reality electricity only produces heat, and it is the heat which occasions sound by suddenly expanding the air. Thus thunder comes into harmony with the sound of artillery, which is also occasioned by the sudden expansion of the heated gases emitted by the powder. When chlorine was discovered it was soon found to have a strong power of bleaching, and at the present day almost all bleaching is done by chlorine instead of the sun, as formerly. Inquiry showed however that it was not really the chlorine which destroyed colour, but that oxygen is the intermediate and active agent. Chlorine decomposes water, and taking the hydrogen leaves the oxygen in a state of great activity and ready to destroy the organic colouring matter. Thus a number of facts are harmonized; we learn why dry chlorine does not bleach, and why there are several other substances which resemble chlorine in its bleaching power, for instance, ozone, peroxide of hydrogen, sulphurous acid, and a peculiar oxide of vanadium, lately discovered by Dr Roscoe. It would be impossible to understand the effect at all unless we knew that it is probably due to active oxygen or ozone in all the cases, even in the old method of bleaching by exposure to the sun *. The third and much more important case of explanation is where one law is shown to be a case of a more general law. As was explained in Lesson XXIV. we naturally discover the less general first, and gradually penetrate to the more simple but profound secrets of nature. It has often been found that scientific men were in possession of several well-known laws without perceiving the bond which connected them together. Men, for instance, had long known that all heavy bodies tended to fall towards the earth, and before the time of Newton it was known to Hooke, Huyghens, and others, that some force probably connected the earth with the sun and moon. It was Newton, however, who clearly brought these and many other facts under one general law, so that each fact or less general law throws light upon every other. The science of Electricity now harmonizes a vast series of partial laws and facts between which it was a truly difficult task to discover any resemblance. The chief properties of the magnet had been fairly known since the time of Gilbert, the physician of Queen Elizabeth; common frictional electricity was carefully studied by Otto von Guericke, Epinus, Coulomb, and others; Galvanism was elaborately investigated almost as soon as Galvani and Volta discovered the fact that the chemical action of one substance on another may produce electricity. In the early part of this century there were three distinct sciences, Magnetism, Electricity and Galvanism; now there is but one science. Oersted of Copenhagen gave in 1819 the first link between them, by pointing out that an electric current may cause movements in a compass-needle. Ampère and Faraday worked * Watts' Dictionary of Chemistry, Vọl. 1. p. 601. out the complicated relations of the three sciences, comprehending them finally in a wider science, which may be called Electro-magnetism, or we may perhaps conveniently generalize the name Electricity so as to comprehend all the phenomena connected with it. A number of minor laws and detached facts are comprehended and explained in the theory now generally accepted, that heat, electricity, light, and in fact all the phenomena of nature, are but manifestations in different forms of one same kind of energy. The total amount of energy existing in the universe is held to be fixed and unalterable, like the quantity of matter; sometimes it is disguised by affecting only the insensible molecules; at other times it is seen to produce palpable mechanical effects, as in the fall of a stone, or the expansion of steam. Now it had been previously known, ever since the time of the Greeks, that a simple lever, although greatly altering the character of force by making its action slower or faster, does not alter its amount, because the more intense the force the slower and more limited is its action. In modern times a similar truth was proved of every kind of machine; and it was recognised that, apart from friction, no kind of mechanism either creates or destroys energy. It had been independently recognised that electricity produced in the galvanic battery was exactly proportional to the amount of chemical action, and that almost any one of the forces named could be converted into any one of the others. All such facts are now comprehended under one general theory, the details of which are being gradually rendered more certain and accurate, but the main principle of which is that a certain amount of mechanical energy is equal to a certain amount of heat, a certain amount of electricity, of chemical action, or even of muscular exertion. The word hypothesis is much used in connection with the subject we are discussing, and its meaning must be considered. It is derived from the Greek words TÓ, under, and béois, placing, and is therefore exactly synonymous with the Latin word suppositio, a placing under, whence our common word supposition. It appears to mean in science the imagining of some thing, force or cause, which underlies the phenomena we are examining, and is the agent in their production without being capable of direct observation. In making a hypothesis we assert the existence of a cause on the ground of the effects observed, and the probability of its existence depends upon the number of diverse facts or partial laws that we are thus enabled to explain or reduce to harmony. To be of any value at all a hypothesis must harmonize at least two different facts. If we account for the effects of opium by saying with Molière that it possesses a dormitive power, or say that the magnet attracts because it has a magnetic power, every one can see that we gain nothing. We know neither more nor less about the dormitive or magnetic power than we do about opium or the magnet. But if we suppose the magnet to attract because it is occupied by circulating currents of electricity the hypothesis may seem a very improbable one, but is valid, because we thus draw a certain analogy between a magnet and a coil of wire conveying electricity. Such a coil of wire attracts other coils exactly in the way that one magnet attracts another; so that this hypothesis enables us to harmonize several different facts. The existence of intense heat in the interior of the earth is hypothetical in so far as regards the impossibility of actually seeing and measuring the heat directly, but it harmonizes so many facts derived from different sources that we can hardly doubt its existence. Thus the occurrence of hot springs and volcanoes are some facts in its favour, though they might be explained on other grounds; the empirical law that the heat increases as we sink mines in any part of the earth's surface is stronger evidence. The intensely heated condition of the sun and other stars is strongly confirmatory as showing that other bodies do exist in the supposed condition of the earth's interior. The cool state of the earth's surface is perfectly consistent with its comparatively small size and the known facts and laws concerning the conduction and radiation of heat. And the more we learn concerning the way in which the sun's heat is supplied by the fall of meteoric matter, the more it is probable that the earth may have been intensely heated like the sun at some former time, although for an immense period it has been growing slowly colder. A supposition coinciding with so many facts, laws, and other probable hypotheses, almost ceases to be hypothetical, and its high probability causes it to be regarded as a known fact. Provided it is consistent with the laws of thought there is nothing that we may not have to accept as a probable hypothesis, however difficult it may be to conceive and understand. The force of gravity is hypothetical in so far that we know it only by its effects upon the motions of bodies. Its decrease at a distance harmonizes exactly indeed with the way in which light, sound, electric or magnetic attractions, and in fact all influences which emanate from a point and spread through space, decrease; hence it is probable that the law of the inverse square is absolutely true. But in other respects gravity is strongly opposed to all our ideas. If sound could travel to the sun as rapidly as in the earth's atmosphere it would require nearly fourteen years to reach its destination; were the sun and earth united by a solid continuous bar of iron, a strong pull at one end would not be felt at the other until nearly three years had passed. Light indeed comes from the sun in rather more than eight minutes; but what |