rately. Every pair of molecules in were, a private communication, apart to all other molecules. Another unperience pointed out by Newton is body does not in the least depend arches in Electricity,' vol. i. pp. 133, 134. - 162, &c. Prop. vi. Corollary i. 31 ife, we Oxygen ild then ition of e agent. of atmo e air was en might Lavoisier s, it is a deviated e, and that esults com 'n well said y consists in t simplicity, arately into st service in of matter, by most powerful the other arFaraday's most Istrated had it anent magnet, out shaking the lucing currents electro-magnet is can be brought erely touching a Drove the rotation oy the fact that a he electric current Kerr, p. 103. tion, pp. 1, 2. upon its form or texture. It may be added that the temperature, electric condition, pressure, state of motion, chemical qualities, and all other circumstances concerning matter, except its mass, are indifferent as regards its gravitating power. As natural science progresses, physicists gain a kind of insight and tact in judging what qualities of a substance are likely to be concerned in any class of phenomena. The physical astronomer treats matter in one point of view, the chemist in another, and the students of physical optics, sound, mechanics, electricity, &c., make a fair division of the qualities among them. But errors will arise if too much confidence be placed in this independence of various kinds of phenomena, so that it is desirable from time to time, especially when any unexplained discrepancies come into notice, to question the indifference which is assumed to exist, and to test its real existence by appropriate experiments. Simplification of Experiments. One of the most requisite precautions in experimentation is to vary only one circumstance at a time, and to maintain all other circumstances rigidly unchanged. There are two distinct reasons for this rule, the first and most obvious being that if we vary two conditions at a time, and find some effect, we cannot tell whether the effect is due to one or the other, or to both jointly. A second reason is that if no effect ensues we cannot safely conclude that either of them is indifferent; for the one may have neutralized the effect of the other. In our logical formulæ, A (B+ b) is identical with A (see vol. i. p. 112), and B may be indifferently present or absent; but A (BC + be) is not identical with A, and none of our logical processes enabled us to make the reduction. If we want to prove that oxygen is necessary to life, we must not put a rabbit into a vessel from which the oxygen. has been exhausted by a burning candle. We should then have not only an absence of oxygen, but an addition of carbonic acid, which may have been the destructive agent. For a similar reason Lavoisier avoided the use of atmospheric air in experiments on combustion, because air was not a simple substance, and the presence of nitrogen might impede or even alter the effect of oxygen. As Lavoisier expressly remarksf, 'In performing experiments, it is a necessary principle, which ought never to be deviated from, that they be simplified as much as possible, and that every circumstance capable of rendering their results complicated be carefully removed.' It has also been well said. by Cuviers that the method of physical inquiry consists in isolating bodies, reducing them to their utmost simplicity, and in bringing each of their properties separately into action, either mentally or by experiment. The electro-magnet has been of the utmost service in the investigation of the magnetic properties of matter, by allowing of the production or removal of a most powerful magnetic force without disturbing any of the other arrangements of the experiment. Many of Faraday's most valuable experiments would have been frustrated had it been necessary to introduce a heavy permanent magnet, which could not be suddenly moved without shaking the whole apparatus, disturbing the air, producing currents by differences of temperature, &c. The electro-magnet is perfectly under control, and its influence can be brought into action, reversed, or stopped by merely touching a button. Thus Faraday was enabled to prove the rotation of the plane of circular polarized light by the fact that a certain light ceased to be visible when the electric current f Lavoisier's Chemistry,' translated by Kerr, p. 103. of the magnet was cut off, and vice versa the light appeared when the current was re-made. These phenomena,' he says, 'could be reversed at pleasure, and at any instant of time, and upon any occasion, showing a perfect dependence of cause and effect h' Another elegant experiment by Faraday illustrates the maintainance of similar conditions. He proved that liquids may conduct electricity when solids will not, by putting the poles of a battery in melted nitre, when a strong current was shown to exist by the galvanometer. But as soon as the nitre was allowed to solidify, the current ceased. Everything else remaining the same, the current existed when the nitre was liquid, and not when the nitre was solidi. It was Newton's omission to obtain the solar spectrum under the simplest conditions which prevented him from discovering the dark lines. Using a broad beam of light which had passed through a round hole or a triangular slit, he obtained a brilliant spectrum, but one in which many different coloured rays overlapped each other. In the recent history of the science of the spectrum, one main difficulty has consisted in the mixture of the lines of several different substances, which are usually to be found in the light of any flame or spark. It is seldom possible to obtain the light of any element in a perfectly simple manner. Angström greatly advanced this branch of science by examining the light of the electric spark when formed between poles of various metals, and in the presence of various gases. By varying the pole alone, or the gaseous medium alone, he was able to discriminate correctly between the lines due to the metal and those due to the surrounding gas k. hExperimental Researches in Electricity,' vol. iii. p. 4. kPhilosophical Magazine,' 4th Series, vol. ix. p. 327. Failure in the Simplification of Experiments. In some cases it seems to be impossible to carry out the rule of varying one circumstance at a time. When we attempt to obtain two instances or two forms of experiment in which a single circumstance shall be present or absent, it may be found that this single circumstance entails one or more others. Benjamin Franklin's experiment concerning the comparative absorbing powers of different colours is well known. 'I took,' he says, 'a number of little square pieces of broadcloth from a tailor's pattern card, of various colours. They were black, deep blue, lighter blue, green, purple, red, yellow, white, and other colours and shades of colour. I laid them all out upon the snow on a bright sunshiny morning. In a few hours, the black being most warmed by the sun, was sunk so low as to be below the stroke of the sun's rays; the dark blue was almost as low; the lighter blue not quite so much as the dark; the other colours less as they were lighter. The white remained on the surface of the snow, not having entered it at all.' This is a very elegant and apparently simple experiment; but when Leslie had completed his series of researches upon the nature of heat, he came to the conclusion that the colour of a surface has very little effect upon the radiating power, the mechanical nature of the surface appearing to be more influential. He remarks that the question is incapable of being posi tively resolved, since no substance can be made to assume different colours without at the same time changing its internal structure.' More recent investigation has shown that the subject is one of considerable complication, because the absorptive power of a surface may be different according to the character of the rays which fall upon it ; Inquiry into the Nature of Heat,' p. 95. VOL. II. D |