sical and chemical series and tables of constants, is in this case of great assistance, by enabling us to compare the phenomena in groups instead of instance by instance. Every force has its own set of laws, and the presence of each force may be inferred from those laws, and vice versâ. As the number of essentially different forms of energy which produce all the varied phenomena on this globe is a very small one, their characters definite, and their modes of operation in nearly all cases also well-defined, we can generally soon determine to which of the forces the new phenomena are not essentially related, and at once dismiss them from our consideration. For example, if the phenomena take place in inanimate substances, we can at once dismiss from consideration the vital and nervous powers, and all their modes of action and relations. The phenomena of chemistry also are usually of so distinct a kind, taking place according to the law of definite proportion by weight, that we can in most cases soon decide whether chemical force is involved in the case or not; if there is no permanent change of property or alteration of weight of the substances, chemical action has not occurred. Following up this process of exclusion, we usually soon find one or at most two forces to which only the phenomena can be related or due. By now remembering the various chief ways in which those forces act, and the kind or class of substances in which each of them is manifested, we are often enabled to judge which is likely to be the cause. For instance, if the substance employed contains iron, and is attracted or moved by a magnet, we consider it probable that the attraction is magnetic ; but if the substance is non-metallic, and is first attracted into contact with and then repelled by an electrified body, we conclude that the attraction is electric; and if the phenomenon is electric, we can usually tell whether it is static or dynamic,

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and thus still further narrow the extent of uncertainty. This is only a crude illustration, by means of familiar phenomena, of the very much more difficult and tedious process employed in actual research.

Or we may think of the most frequently occurring causes of similar phenomena. For instance, if it is a case of motion, we think of ordinary mechanical causes, vibration of the room, currents of air, &c., as being the most frequent and therefore the most probable; then of motion produced by expansion by heat, or by electric or magnetic attraction, &c. As the most probable cause of an event usually is that circumstance which in the greatest number of cases accompanies it, we also observe what are the conditions and circumstances most frequently present; and if we perceive that a similar force is active, or similar conditions exist in the phenomena under investigation as in others which are known, we must at once infer that such a force or conditions may be the cause, and we devise hypotheses which agree with this. If a phenomenon have one antecedent which appears to be the only invariable one, that one is probably the cause; different antecedents may, however, produce the same effect.

In some cases, however, we arrive at this stage of the enquiry at once and without sensible effort, because the phenomena belong obviously to some particular force; and the next step is to discover the way in which the particular form of energy operates to produce the effect. To ascertain this, we think of each general mode or principle of action, and each qualitative order and series of constants of the particular force, in order to exclude those which evidently have no relation to the effect, and to find a case of resemblance. If a resemblance is found, concordant hypotheses must be imagined, and suitable ways of testing them invented; but if no resemblance

can, after close scrutiny, be thus found, the case is a difficult one. Either scientific knowledge is not sufficiently advanced to afford the true explanation, or the discovery of it lies beyond our mental power; or we have met with a phenomenon of a new and unknown kind.

If in any case, after all our test experiments and conclusions, the assumed cause is found not to be the true one, we must imagine others and test them until the true one is discovered; and if we experience great difficulty in finding it, we may then make radical changes or even haphazard experiments. Such changes and experiments may also be made when any inscrutable interference or even too great harmony of effects occurs; in the latter case because such harmony is extremely suspicious, and strongly indicates an uniform error. Any circumstance also, no matter how small or apparently insignificant it may be, which we meet with in a research, and which we cannot at all reconcile with the other circumstances present, or with the known principles of science, is always worthy of the deepest study and investigation, because it often contains the true explanation, or else an anomalous and therefore important truth.

A supposed cause should always be tested in as many ways as possible, aud those conditions selected which yield the most conclusive result. We should try (if we can) the effect of presence, absence, direction, and quantity of each condition. Previous to or whilst varying the conditions of an experiment it is often of service to form a written list of the chief variations which can be made, something like the following, but much more fully, and more adapted to the special case:-Change the forces employed, considering each force in succession; change the strength of the forces; vary their direction; employ different substances, considering the probable effect of different classes

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of substances, and of individual substances, and selecting the most likely. Use different quantities and proportions of the substances. Change the materials of the apparatus. Consider the influence of volume, weight, specific gravity, elasticity, width, length, depth, thickness, shape, &c., of the substances employed. Try substances of different degrees of transparency to light, conductivity for heat and electricity, magnetic capacity, solubility in acids, &c. Considerable assistance in forming the list may be obtained by consulting the index of a suitable book on science.

In these processes it is entirely by means of a knowledge of their effects that causes are discovered. The discovery of causes by such means is called induction, and consists in imagining hypothetical explanations of phenomena already known, including new results and observations, deducing consequences which must occur if those explanations are true, and testing by suitable experiments or observations whether those consequences really take place. In proving a supposed cause we must be able to deduce new effects, which must follow if the assumed cause be the true one.

One phenomenon may be invariably connected with another, either as cause or effect, as a necessary condition, or as a mere coincidence. In the discovery of causes we must remember that a phenomenon A may be related to another one, B, in several different ways in different cases. 1. B may be a direct result of A; thus the fall of a body to the earth is a direct result of the universal force of gravity. 2. B may be an indirect result of A, some other phenomenon or intermediate cause (perhaps of an obscure kind) intervening between them; thus I found that if a metallic ball was placed upon two horizontal metal rails, insulated from each other, and an electric current passed

from one rail through the ball to the other, the ball rotated, and ran round the circular railway as long as the current continued. In this case the electric current was the cause of the motion, but only indirectly, the direct cause being expansion produced by the heat of electric conduction-resistance. The current produced heat at the points of contact of the ball and rails, and the heat produced expansion of those parts into small prominences. During the fraction of time required to produce these effects, the started ball, by its momentum, moved forward a minute distance, and thus the prominences were always a little behind the centre of gravity of the ball, and pushed the ball forwards. 3. The phenomenon B may be the only direct result of A; this is a rare case: or 4. It may be, as commonly occurs, only one of the direct results, others being coincident with it; thus an electric current passed through an iron wire not only produces a molecular change, sound, and heat in the wire, but also magnetic influence around it. 5. B may be composed of two portions, one of which is directly due to A, and the other to a second and coincident cause A'; thus the heat produced by burning a jet of hydrogen is partly due to the chemical union of the two substances, and partly to the latent heat set free by the condensation of the product of combustion. 6. Or B may be an effect (usually one of several) of the combined action of two conditions, neither of which alone is capable of producing it in the slightest degree; thus a flame applied to hydrogen alone or to oxygen alone produces no explosion, but if applied to a mixture of the two, produces it. 7. B may also be a result of many concurring conditions, and does not take place in the least degree unless they are all present. Instances of this kind occur more frequently in the concrete sciences of animal life than in the simpler ones relating to inorganic matter; for in