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Hales also, about the same time, made the same kind of experiments, and obtained similar results. Dr. Black, in the year 1755, made, by means of experiments and observations, one of the first steps towards the discovery of the properties of individual gases. He found that limestone consisted of a gas united to caustic lime, and that by the union or fixation of this gas (or 'fixed air,' as it was then called) to magnesia, lime, soda, or potash, these bodies lost their causticity, and became mild' alkalies. He, by experiment, discovered the same gas in our breath, and in fermenting beer. He further found out some of its chief properties, that it would precipitate lime-water, &c. ; but it was Bergmann who, by additional experiments, discovered it to be an acid.

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Cavendish, in the year 1766, discovered hydrogen, by adding zinc or iron to dilute sulphuric acid, and collecting the gas, and testing its properties in various ways. He found that it was combustible, burning with a blue flame, that it would not support animal life, nor the combustion of a candle flame, that it was very light, and that when mixed with air the mixture exploded, on the application of a flame. Bergmann, about the year 1770, was one of the first to employ chemical tests, and by means of them to analyse substances, mineral waters, &c. He suspected the 'fixed air' of Dr. Black to be an acid, because it united itself to lime, and he proved it by means of experiments, for he knew that dissimilar substances united chemically together. He discovered the specific gravity of that gas, and its solubility in water, and called it aerial acid,' or acid air. It was by means of well-conceived experiments upon oxide of mercury that Priestley, in 1774, discovered oxygen; and Scheele, by using black oxide of manganese, also discovered it during the same year. Wenzel made and published, in 1777, many accurate experiments of chemical

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DISCOVERY BY MEANS OF NEW EXPERIMENTS.

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analysis, and discovered that the proportions of the ingredients of substances were definite; and observing that when two neutral salts decomposed each other the resulting salts were also neutral, he was led to the discovery that the definite proportions were reciprocal. It was by means of the experiment of burning a diamond in oxygen gas that Lavoisier, about the year 1778, discovered that that gem was composed of carbon alone. By burning charcoal also in oxygen, and analysing the product, he discovered that fixed air' was composed of twenty-eight parts by weight of carbon and seventy-two of oxygen; and having found that most substances, by union with oxygen, acquired acid properties, he called 'fixed air' by the name of carbonic acid. Warltire, in 1781, by exploding a mixture of atmospheric air and hydrogen in a closed vessel, by means of an electric spark, discovered, after the experiment, what he considered to be water adhering to the inner sides of the vessel. Cavendish, by means of similar experiments made in 1784, with a mixture of oxygen and hydrogen, discovered the formation and composition of water. Fourcroy, Vauquelin, and Seguin, by means of a continuous series of such experiments with that mixture, lasting from May 13 to 22, in the year 1790, discovered that it produced a nearly equal weight of water. Ritchie also, in 1792, discovered what the proportions were of the common acids and bases which would saturate each other.

By tying an artery and vein, and observing the mechanical effects upon the two sides of their tied parts, Harvey, in the year 1619, was led to suspect, and ultimately to discover, the circulation of the blood. It, however, required nineteen years of experiment and study to enable him to trace the entire course of the blood through the whole of the human body, and completely prove his discovery. It was by means of actual experi

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ments, made during the middle of the eighteenth century, that Bonnet and Spallanzani discovered that if the horns, tails, legs, eyes, or even the head of some creatures including even garden snails-were cut off, they would grow again. The tail and legs of a salamander were removed, and reproduced themselves as many as eight times in succession. It has also been found, by means of experiments, that the more simple the structure of an animal is, the more do its several parts possess a power of independent existence, and that in the more complex animals the derangement of one part much more affects the action of the entire organism. By putting two live mice in a closed vessel filled with pure oxygen, Dr. Priestley, about the year 1774, discovered that they lived longer than in an equal volume of ordinary air; he also breathed pure oxygen, and felt benefited, and said, 'Who can tell whether this pure air may not at last become a fashionable luxury?' By confining some growing mint in a vessel of air, the oxygen of which had been converted into carbonic anhydride by combustion or breathing, he discovered the important fact that the air was again rendered fit to support combustion and life. Some of the earliest experiments to discover the effects of galvanism on animals were made by Fowler, in the year 1793. Sir Humphry Davy, by means of a series of experiments upon himself, breathing particular vapours and gases, and gradually increasing the duration of inspiration of each, discovered the intoxicating effects of nitrous oxide or laughing gas,' as it was afterwards called.

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The foregoing instances constitute only a small portion of the discoveries selected out of the multitudes which have been made by means of experiment. In nearly all of them it must not be supposed that the discoveries were completely made by experiment alone. Experiment was

DISCOVERY BY MEANS OF REPETITION OF EXPERIMENTS. 543

only one of the chief steps in the process; suggestion, imagination, observation, comparison, and inference were also employed, and constituted an important part in the operation. At the same time, a certain increase of knowledge was due in each instance to experiment and observation alone, and so far experiment alone may be regarded as a fertile source of new discoveries.

a. By making or repeating, in a modified form, experiments suggested by other persons. Sometimes one man suggests an experiment, and another carries it out, or repeats it in a modified form. Hooke proposed to observe the vibrations of a bell, by strewing flour upon it. But it was Chladni, a German philosopher, who enriched acoustics with the discovery of the vast variety of symmetrical figures which are exhibited on plates of regular forms, when made to sound.' Newton, about the year 1770, suggested, but Lagrange discovered, that the cause of the moon always presenting the same side to the earth, was the attraction of the latter upon the swelling at the lunar equator. Previous astronomers suggested, and Adams and Le Verrier confirmed by calculation, that the orbit of the November meteors extended beyond Uranus. Volta's great discovery arose from repeating Galvani's experiments and studying the results; he made his celebrated pile in the year 1800. Wartmann, in the year 1846, by repeating with rays of heat the experiment which Faraday in 1845 had made with rays of light, discovered the rotation of the plane of polarisation of heat rays by magnetism. By modifying the apparatus employed by Faraday in the liquefaction of gases, I was enabled to subject a large number of solid and liquid substances to the action of liquefied carbonic anhydride, ammonia, cyanogen, and hydrochloric acid

1 Whewell, History of the Inductive Sciences, 3rd edit. vol. ii. p. 258.

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gases, and to discover the approximate degree of electric conduction resistance of liquid carbonic anhydride. Warltire's experiment of exploding a mixture of common air and hydrogen in a closed vessel, in order to ascertain whether heat was a ponderable substance, suggested to Cavendish his experiment, by means of which he discovered the composition of water by synthesis. After Scheele had discovered that nitrate of silver turns black in the violet rays of the spectrum, Ritter, in the year 1801, by repeating the experiment, found that the greatest blackening effect took place not in the yellow or brightest part of the spectrum, nor even in the visible part at all, but at a little distance beyond the violet end. This was the origin of photography. By making some improvements in the apparatus employed by Magnus for testing the transparency of gases to radiant heat, Buff and Hoorweg were enabled to discover that air containing aqueous vapour was but little more opaque than air alone, and much less so than it had been previously supposed to be.1

b. By extending the researches of others.-Very few scientific researches are entirely novel. In nearly all of them something has already been done; and in many the original research of one man is only extended by another either by means of additional experiments of a similar kind, or by extending the same research in some particular direction; and indeed the whole. realm of new knowledge may be considered as being evolved by one vast research, of which different investigators develop particular branches. We require to stand upon the terra firma of the known, in order to be able to acquire a view of the unknown.

After one man has discovered an apparently singular property in a substance, either himself or others discover 1 See Philosophical Magazine, December 1877, pp. 423, 424.

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