and Davy speaks of this discovery as the foundation of all that had since been done in electro-chemical science.

It is otherwise with the discovery of electro-magnetism. or the relation between the magnet and electricity. Oersted, in common with many others, had suspected the existence of some relation between these strange powers and he appears to have tried to detect its exact nature. Once, as we are told by Hansteen, he had employed a strong galvanic battery during a lecture, and at the close it occurred to him to try the effect of placing the conducting wire parallel to a magnetic needle, instead of at right angles, as he had previously done. The needle immediately moved and took up a position nearly at right angles to the wire; he inverted the direction of the current, and the needle deviated in a contrary direction. The great discovery was made, and if by accident, it was such an accident as happens only to those who deserve them, as Lagrange remarked of Newton. There was, in fact, nothing accidental, except that, as in all totally new discoveries, Oersted did not know what to look for. He could not infer from previous knowledge the nature of the relation, and it was only repeated trial in different modes which could lead him to the right combination. High and happy powers of inference, and not accident, subsequently induced Faraday to reverse the process, and show that the motion of the magnet would occasion an electric current in the wire.

Sufficient investigation would probably show that almost every branch of art and science had an accidental beginning. In historical times almost every important new instrument, such as the telescope, the microscope, or the compass, was probably suggested by some accidental occurrence or observation. In pre-historic times the germs of the arts must have arisen still more exclusively in hLife of Faraday,' vol. ii. p. 396.

the same way. Cultivation of plants probably arose, in Mr. Darwin's opinion, from some such accident as the seeds of a fruit falling upon a heap of refuse, and producing an unusually fine variety. Even the use of fire must, some time or other, have been discovered in a like accidental manner.

With the progress of any branch of science, the element of chance becomes much reduced. Not only are laws discovered which enable results to be predicted, as we shall shortly consider, but the systematic examination of phenomena and substances often leads to important and novel discoveries, which can in no true sense be said to be accidental. It has been asserted that the anaesthetic properties of chloroform were disclosed by a little dog smelling at a saucerful of the liquid in a chemist's shop in Linlithgow, the singular effects upon the dog being reported to Dr. Simpson, who turned the incident to such good account. This story, however, has since been shown to be a fabrication, the fact being that Dr. Simpson had for many years being endeavouring to discover a better anæsthetic than those previously employed, and that he tested the properties of chloroform, among other substances, at the suggestion of Mr. Waldie, a Liverpool chemist. The valuble powers of hydrate of chloral have since been discovered in a like manner, and systematic inquiries are continually being made into the therapeutic or economic value of new chemical compounds.

If we must attempt to draw any conclusion concerning the part which chance plays in scientific discovery, it must be allowed that it more or less affects the success of all inductive investigation, but becomes less important with the progress of any particular branch of science. Accident, too, may bring a new and valuable combination to the notice of some person who had never expressly searched for a discovery of the kind, and the probabilities

are certainly in favour of a discovery being occasionally made in this manner. But the greater the tact and industry with which a physicist applies himself to the study of nature, the greater is the probability that he will meet with fortunate accidents, and will turn them to good account. Thus it comes to pass that, in the refined investigations of the present day, genius united to extensive knowledge, cultivated powers and indomitable industry, constitute the characteristics of the great dis

coverer.

Empirical Observations subsequently Explained.

The second great portion of scientific knowledge consists of facts which have been first learnt in a purely empirical manner, but have afterwards been shown to follow from some law of nature, that is, from some highly probable hypothesis. Facts are said to be explained when they are thus brought into harmony with other facts, or bodies of general knowledge. There are few words more familiarly used in scientific phraseology than this word explanation, and it is necessary to decide exactly what we mean by it, since the question touches the very deepest points concerning the nature of science. Like most terms referring to mental actions, the verbs to explain, or to explicate, involve material similes. The action is ex plicis plana reddere, to take out the folds, and render a thing plain or even. Explanation thus renders a thing clearly comprehensible in all its points, so that there is nothing left outstanding or obscure.

Every act of explanation consists in detecting and pointing out a resemblance between facts, or in showing that a greater or less degree of identity exists between apparently diverse phenomena. This resemblance may be of any extent and depth it may be a general law of

nature, which explains and harmonizes the motions of all the heavenly bodies, that is, shows that there is a similar force which governs all those motions, or the explanation may involve nothing more than a single identity, as when we explain the appearance of shooting stars by showing that they are identical with portions of a comet. Wherever we detect resemblance, there is a more or less satisfactory explanation. The mind is always somewhat disquieted when it meets a novel phenomena, one which is sui generis; it seeks at once for any parallels which may be found in the memory of past sensations. The so-called sulphurous smell which attends a stroke of lightning long excited the attention and fears of men, and it was not explained, until the exact similarity of the smell to that of ozone, or allotropic oxygen, was pointed out. The marks upon a flagstone are explained when they are shown to correspond with the feet of an extinct animal, whose bones are elsewhere found. Explanation, in fact, generally commences by the discovery of some very simple resemblance; the theory of the rainbow began as soon as Antonio de Dominis pointed out the resemblance between its colours and those presented by a ray of sunlight passing through a glass globe full of water.

The nature and limits of explanation can only be fully considered, after we have entered upon the subject of generalization and analogy. It must suffice to remark, in this place, that the most important process of explanation consists in showing that an observed fact is only one case of a general law or tendency. Iron is always found combined with sulphur, when it is in contact with or included in coal, whereas in other parts of the coal strata it always occurs as a carbonate. We explain this empirical fact as being due to the ordinary reducing powers of carbon and hydrogen, which prevent the iron from combining with oxygen, and leave it open to the affinity of the sulphur.

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Although the great mass of recorded ficts must be empirical, and awaiting explanation, such knowledge is of minor value, because it does n ́t admit of extensive and safe inference. Each recorded result infirms us exactly what will be experienced again in the same circumstances, but has no bearing upon what will happen in other circumstances.

Overlooked Results of Theory.

We must by no means suppose that, even when a scientific truth is firmly in our possession, all its consequences will be foreseen. Deduction is, as I have frequently remarked, certain and infallible, in the sense that each step in deductive reasoning will lead us to some result, as certain as the law itself. But it does not follow that every mode of deducing a fact from a law, or a