1 will yet be found.1 Similarly, as the conducting powers of bodies for heat and for electricity appear to obey the same general laws, we may assume as probable that in any case where there exists an anomalous or exceptional action with regard to the one property, there may be found a similar irregular action with regard to the other. From the truthful hypothesis, that the movement of conducting bodies is retarded by magnetism, it is probable that the flow of saline liquids in vegetables and animals will be found to be retarded between the poles of a powerful magnet. The hypothesis of the intimate correlation of all the physical and chemical powers was one of Faraday's leading ideas during the whole of the latter part of his life. c. By assuming that most of the principles which operate in the simpler sciences operate also in the complex and concrete ones.-The principle of isochronism or rhythm, for example, operates in the vibrations of solids, liquids, vapours, gases, and of the universal ether which pervades all bodies and all space, also in the transmission of sound, light and radiant heat, and probably also in that of electric and magnetic induction. Guthrie has suggested 2 that it also manifests itself in the fact that colloid substances arrest colloids by contact while they are permeable (transparent, diathermanous) to crystalloid substances. The principle of action and reaction, also, we know operates between all the forces of nature, without exception. The hypothesis of uniform principles of action may further lead us to imagine the hypothesis that lines of electric force are decomposable, and to ask the question: 1 Professor Tait has since shown that this molecular and magnetic change is attended by an equally sudden change of thermo-electric capacity. 2 Philosophical Magazine, September 1876. DISCOVERY BY EXAMINING HYPOTHESES. 521 will static electric inductive influence suffer a greater alteration in passing through a first plate of intervening insulating substance than in passing through a second. and precisely similar one? We may similarly imagine that magnetism is decomposable, and be incited to enquire is magneto-electric inductive influence more intercepted by a first plate of conducting material than by a second exactly like it? and so on. Although elementary substances and their compounds are divided into electropositive and electro-negative, chemists, even at the present day, do not seem to formally recognise the existence of two kinds of chemical attraction, viz., that in metals and bases, and that in metalloids and acids, corresponding to their two kinds of electrical property. A great number of new truths will yet be evolved in the complex subject of morality by assuming that the chief rules of that subject are based upon the great principles of nature, especially that of causation. According to Dr. Adam Clarke the fundamental rules of moral conduct and righteousness are, 1. Whatever I judge reasonable or unreasonable that another should do for me, that I should in the like case do for him; and 2. We should anxiously endeavour to promote in general, to the utmost of our power, the welfare and happiness of all men.1 These rules are perfectly in accordance with the decisions of reason. The first agrees with the great principle of cause and effect, and is based upon it, for if 'in the like case' what we did for another would produce a different effect to what it would when done for ourself, the rule would be of no use. The second also agrees with the great laws of nature, for the more we obey those laws, the more do we really promote the happiness and welfare of all men.' 1 See Sedgwick, Methods of Ethics, p. 358. 1 ་ d. By assuming that certain general statements which are true of one force or substance are true to some extent of others.-By means of this method, followed by appropriate investigations, many propositions which are true of light have been found to be true of radiant heat; for instance, Faraday having, in the year 1845, discovered the magnetic rotary polarisation of light, Wartmann, in the following year, succeeded in discovering a similar action with a beam of heat. In a similar manner, properties known to exist in potassium have been predicted of and found to exist in rubidium: for instance, the carbonates of sodium and potassium are not decomposed by a red heat, neither are those of rubidium or caesium. Some of the statements which are true of chlorine have been found to be true, in varying degrees, of bromine and iodine; and some of those true of iron have been found to be true of manganese, chromium, and aluminium; and similarly with every distinct family or group of substances united by similarities. After I had found the molecular change in antimony electro-deposited from its chloride, I sought for and discovered it in that deposited from its bromide and iodide; and after having found irregular magnetic changes in iron by heat, I also found similar ones in nickel. The method itself is based upon the fact, that where any particular substance is found to possess a particular property, other substances which are similar to it in most respects may also be reasonably expected to possess it. Sir Humphry Davy having discovered that potassium might be isolated by means of electrolysis, immediately proceeded to isolate sodium and other bodies of analogous properties, and succeeded. e. By assuming the existence of converse principles of action.—Magneto-electricity and electro-dynamic induc DISCOVERY BY MEANS OF CONVERSE EXPERIMENTS. 523 tion were discovered in this way. As electric currents produced magnetism, so magnetism was assumed to be able to produce electric currents. In 1831 Faraday again sought for electro-dynamic induction, and, after some futile trials, at last found it in a form different from that in which he had looked for it. It was then seen that, at the precise time of making or breaking the contact which closed the galvanic circuit, a momentary effect was induced in a neighbouring wire, but disappeared instantly. Once in possession of this fact, Mr. Faraday ran rapidly up the ladder of discovery to the general point of view. Instead of suddenly making or breaking the contact of the inducing circuit, a similar effect was produced by removing the inducible wire nearer to or farther from the circuit; the effects were increased by the proximity of soft iron; when the iron was affected by an ordinary magnet instead of the voltaic wire, the same effect still recurred; and thus it appeared that, by making and breaking magnetic contact, a momentary electric current was produced; it was produced also by moving the magnet, or by moving the wire with reference to the magnet. Finally, it was found that the earth might supply the place of a magnet in this as in other experiments; and the mere motion of a wire, under proper circumstances, produced in it, it appeared, a momentary electric current. These facts were curiously confirmed by the results in special cases. They explained Arago's experiments, for the momentary effect became permanent by the revolution of the plate; and, without using the magnet, a revolving plate became an electrical machine; a revolving globe exhibited electro-magnetic action, the current being complete in the globe itself without the addition of any wire; and the mere motion of the wire of a galvanometer produced an electro-dynamic effect upon its needle.'1 1 Whewell, History of the Inductive Sciences, vol. iii. 3rd ed. p. 85. f. By assuming the existence of complete homologous series.-The assumption of the truth of Bode's law, and of the necessity of another planet to fill the missing space in the series, led, as we have already shown,' to the discovery of all the asteroids. That of complete homologous series in organic chemistry led to the discovery of a whole multitude of compound substances, including cyanides, ethers, alcohols, fatty acids, paraffins, compounds of monad, dyad, triad, tetrad, and hexad compound radicals; benzene, napthalene, and anthracine compounds, and numerous other substances; and the number of compound bodies which might be discovered in order to complete homologous series, even by the union of one class of substances only, has been shown by Berthelot to be immense.2 Wilde has recently, by means of the hypothesis of homologous series, predicted the future discovery of a number of new elementary substances.3 CHAPTER LV. DISCOVERY BY MEANS OF NEW EXPERIMENTS AND METHODS THIS method is also based upon the principle that new 3 See Proceedings of the Manchester Philosophical Society, April 30, 1878. |