DISCOVERIES. BY MEANS OF THE GALVANOMETER, ETC. 485

polarised by a voltaic current, and was thus enabled to accumulate the power of a small battery. It was by means of a voltaic battery current that Nobili, of Reggio, in 1826, discovered electro-chromy. Becquerel, in 1829, invented the first double fluid battery, having a porous diaphragm. Daniell, in 1836, invented his constant battery, by the use of which, in the same year, W. de la Rue discovered that copper electro-deposited from a solution of cupric sulphate produced an exact copy in reverse of the surface upon which it was formed. Grove's battery was invented in 1839; Smee's in 1840; and the latter was immediately used by its inventor to discover a large number of new facts in electrolysis, and to deposit many of the metals. Golding Bird, also, had already, in 1837, used the battery current to discover that even a feeble current was sufficient to deposit potassium and sodium into mercury. In the year 1834, Faraday, by means of his voltameter, was enabled to discover the great principle of definite electro-chemical action.

By means of a magnetic needle Ampére, previous to September 18, 1820, discovered that the current in a voltaic pile influences a magnet in the same way as that in the connecting wire; and, by means of the needle and current, was enabled to invent a galvanometer. Schweigger, of Halle, invented his improved galvanometer during the same year. By inventing also a suitably formed helix of insulated copper wire, Ampére, previous to November 6, 1820, was enabled to imitate perfectly the action of a magnet by means of an electric current. Snow Harris, in 1831, first used the bifilar suspension for needle magnetometers. Pouillet first described his invention of an astatic needle in the year 1832, and thus rendered the galvanometer capable of detecting more feeble currents. It was by means of a combination of a thermo-pile, gal

vanometer, and plates of mica that Forbes discovered the polarisation of heat-rays, both by reflection and refraction, after Berard, Melloni, and Nobili had failed; but was unable (in the year 1835), by a thermo-pile and galvanometer, to detect any heating effect in the rays of the moon, even when the rays were concentrated 3,000 times. Gauss, in 1836, invented his combination of a bifilar suspended magnet, theodolite, and scale, and employed it as a magnetometer to discover variations of terrestrial magnetism. In 1843, Wheatstone appears to have invented his rheostat (and Jacobi conceived a similar idea), and also his electric balance, by means of which many discoveries in electric conduction resistance were subsequently made. It was by means of those instruments, &c., that Matthiessen, in 1858, discovered the electric conduction resistance of nearly all the metals, also of coke, graphite, selenium, phosphorus, &c.; and, in 1859, that of numerous alloys; and, in 1860, found the effect of metals and metalloids on the conducting power of copper, and that a minute propor-tion of arsenic diminishes it very greatly. Still more recently, many new facts have been found respecting the accumulation and transmission of electricity by means of those beautiful instruments, the reflecting electrometer, reflecting galvanometer, electric replenisher, syphon recorder, &c., invented by Sir W. Thomson.

CHAPTER LIII.

DISCOVERY BY INVESTIGATING LIKELY CIRCUMSTANCES.

THIS method is one of the most successful, and includes a number of more special ones, such as investigating neglected truths and hypotheses; anomalous, peculiar, or unexplained truths; peculiar facts observed in manufac

DISCOVERY BY INVESTIGATING NEGLECTED TRUTHS. 487

turing and other operations; examining exceptional, extreme, and conspicuous instances, common but neglected circumstances, peculiar minerals, rare substances, residues of manufacturing processes, the ashes of peculiar or rare plants and animals, &c.

a. By examining neglected truths and hypotheses. -Important facts and hypotheses are sometimes neglected for many years, until circumstances arise to call attention to them. The facts discovered by Geber (who was born in the year 830), that iron, lead, and copper became heavier by being heated to redness and cooled in the air, so as to become oxidised; and by Boyle (during the seventeenth century), that tin behaved similarly, remained almost unnoticed, or at any rate uninvestigated and without a true interpretation, until about the year 1778, when Lavoisier inferred their true explanation, and, by means of them and similar experiments of his own, made the great discovery of the true nature of oxidation, combustion, respiration, and of chemical union in general. Avogadro's hypothesis, published in 1811, and reproduced by Ampére in 1814, asserting that equal volumes of all gases contain equal numbers of molecules, was also neglected for a long time, but has since been proved by experiment to be one of the greatest truths of chemical science.

b. By examining peculiar or unexplained truths in science.-Galileo, at the age of nineteen, in the year 1583, observing that a lamp, suspended from the roof of the cathedral of Pisa, took the same time to swing backwards and forwards whether the arc of vibration was more or less, investigated the circumstance, and found the principle of the pendulum, viz., that the period of vibration was constant, provided that the length of the string remained the same and that the arc of vibration was not very large. In the year 1589 he further observed that

a body, falling from a height, descended more and more quickly until it reached the ground; and by investigating this circumstance he succeeded in discovering the law of falling bodies, i.e., what the rate of increase of velocity of falling was for each additional second of time of descent.

Bode having, from calculations, inferred the existence of the orbit of a missing planet between those of Mars and Jupiter, it was resolved at a meeting of German astronomers at Lilienthal in Saxony, in the year 1800, to investigate this peculiar circumstance, and search for the supposed missing body. Piazzi, astronomer in the observatory at Palermo, sought for it, and during the first night of the year 1801 he observed a previously unnoticed small star in the constellation Taurus. He soon found that it changed its place. He now became ill, and no one could find the star again; but Gauss, from the facts which Piazzi had given, calculated where it should be, looked there, and found it. Thus was the discovery of Ceres made, the first of the asteroids. In 1802, Dr. Olbers, of Bremen, discovered another asteroid near Ceres, and called it Pallas. And, in 1804, Harding discovered Juno. Olbers then inferred and suggested the existence of an exploded planet, because all these asteroids or small planets were about equidistant from the sun; and in 1807 a fourth was found, which he called Vesta. And since that time additional ones have been occasionally discovered, until we now know more than 150, all moving round the sun, between the orbits of Mars and Jupiter, in the space which, according to Bode's law, ought to contain a planet. Some of these asteroids are exceedingly small, being only a few miles in diameter. Pallas is the largest yet found, and is about 600 miles in diameter.

The persistent investigation of the singular property

DISCOVERY BY EXAMINING PECULIAR TRUTHS.

489

of double refraction in a crystal of Iceland spar largely aided the discovery of the general laws of light. In the year 1669, Erasmus Bartolinus published a work on the subject, and also discovered by observation the fact that one of the images was produced according to the ordinary law of refraction, and the other according to an extraordinary and new law, and varied also in different positions of the crystal. It was by investigating peculiar phenomena that Huyghens discovered polarisation of light. He says: 'Before I quit the subject of this crystal I will add one other marvellous phenomenon, which I have discovered since writing the above; for though hitherto I have not been able to find out its cause, I will not, on that account, omit pointing it out, that I may give occasion to others to examine it.' He then states the phenomena, which are, that when two rhombohedrons of Iceland spar are in parallel positions, a ray, doubly refracted by the first, is not further divided when it falls on the second; the ordinarily refracted ray is ordinarily refracted only, and the extraordinary ray is only extraordinarily refracted by the second crystal, neither ray being doubly refracted. The same is still the case if the two crystals have their principal planes parallel, though they themselves are not parallel. But if the principal plane of the second crystal be perpendicular to that of the first, the reverse of what has been described takes place; the ordinarily refracted ray of the first crystal suffers, at the second, extraordinary refraction only, and the extraordinary ray of the first suffers ordinary. refraction only at the second. Thus, in each of these positions, the double refraction of each ray at the second crystal is reduced to a single refraction, though in a different manner in the two cases. But in any other position of the crystals, each ray, produced by the first, is doubly