comprehension of their laws. He who frames a theory and is content with his own deductions from it, like Descartes, will only surprise the world with his misused genius; but the best student of science is he who with a copious store of theories and fancies has the highest power of foreseeing their consequences, the greatest diligence in comparing them with undoubted facts, and the greatest candour in confessing the ninety-nine mistakes he has made in reaching the one true law of nature.

LESSON XXXI.

EXPLANATION, TENDENCY, HYPOTHESIS, THEORY, AND FACT.

IN the preceding Lessons I have used several expressions of which the meaning has not been defined. It will now be convenient to exemplify the use of these terms, and to arrive as far as possible at a clear understanding of their proper meanings.

Explanation is literally the making plain or clear, so that there shall be nothing uneven or obscure to interrupt our view. Scientific explanation consists in harmonizing fact with fact, or fact with law, or law with law, so that we may see them both to be cases of one uniform law of causation. If we hear of a great earthquake in some part of the world and subsequently hear that a neighbouring volcano has broken out, we say that the earthquake is thus partially explained. The eruption shows that there were great forces operating beneath the earth's surface, and the earthquake is obviously an effect of such causes. The scratches which may be plainly seen upon the surface of rocks in certain parts of Wales and Cumberland, are explained by the former existence of glaciers in those mountains; the scratches exactly harmonize

with the effects of glaciers now existing in Switzerland, Greenland, and elsewhere. These may be considered explanations of fact by fact.

A fact may also be explained by a general law of nature, that is the cause and mode of its production may be pointed out and shown to be the same as operates in many apparently different cases. Thus the cracking of glass by heat was explained (p. 257) as one result of the universal law that heat increases the dimensions of solid bodies. The trade-winds are explained as one case of the general tendency of warm air to rise and be displaced by cold and dense air. The very same simple laws of heat and mechanics which cause a draught to flow up a chimney when there is a fire below, cause winds to blow from each hemisphere towards the equator. At the same time the easterly direction from which the winds come is explained by the simplest laws of motion; for as the earth rotates from west to east, and moves much more rapidly at the equator than nearer the poles, the air tends to preserve its slower rate of motion, and the earth near the equator moving under it occasions an apparent motion of the wind from east to west.

There are, according to Mr Mill, three distinct ways in which one law may be explained by other laws, or brought into harmony with them.

The first is the case where there are really two or more separate causes in action, the results of which are combined or added together, homogeneously. As was before explained, homogeneous intermixture of effects (p. 252) means that the joint effect is simply the sum of the separate effects, and is of the same kind with them. Our last example of the trade-winds really comes under this case, for we find that there is one law or tendency which causes winds to blow from the arctic regions towards the equator, and a second tendency which causes then to blow

from east to west. These tendencies are combined together, and cause the trade-winds to blow from the NorthEast in the northern hemisphere, and from the South-East in the southern hemisphere. The law according to which the temperature of the air is governed in any part of the earth is a very complicated one, depending partly on the law by which the sun's heating power is governed, partly on the power of the earth to radiate the heat away into space, but even more perhaps on the effect of currents of air or water in bringing warmth or carrying it away. The path of a cannon-ball or other projectile is determined by the joint action of several laws; firstly, the simple law of motion, by which any moving body tends to move onward at an uniform rate in a straight line; secondly, the law of gravity, which continually deflects the body towards the earth's surface; thirdly, the resistance of the air, which tends to diminish its velocity.

The reader will perhaps have noticed the frequent use of the word tendency, and I have repeatedly spoken of a cause as tending to produce its effect. If the joint and homogeneous action of causes has been clearly explained, it will now be clear that a tendency means a cause which will produce an effect unless there be opposite causes, which, in combination with it, counteract and disguise that effect. Thus when we throw a stone into the air the attractive power of the earth tends to make it fall, but the upward motion we have impressed upon it disguises the result for a certain time. The interminable revolving motion of the moon round the earth is the result of two balanced tendencies, that towards the earth, and that to proceed onward in a straight line. The laws of motion and gravity are such that this balance must always be preserved; if the moon by any cause were brought nearer to the earth its tendency to fly off would be increased, and would exceed the effect of gravity until it had regained

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, Vol. 1. p. 601.