against each other. The theory is consistent with the experimental laws of gases, and gives an insight into their behaviour when subjected to various physical conditions. The first to treat the theory with exactitude was Clausius. He was followed by Maxwell, and by Boltzmann. The three contemporaneously rapidly developed it. Although the theory is on a firm basis it fails to account for the diversity of values of the ratio of the two specific heats in various groups of gases.

An important addition was made to our knowledge of the constitution of bodies when Andrews published his classical researches in 1869, showing the existence of a critical point in gases. He showed that at any temperature there is a pressure at which a gaseous body is in a state of transition, being neither liquid nor gas, but in an intermediate state, such that any increase of pressure will cause some liquefaction, and any diminution of pressure will be followed by a return to the gaseous state.

Much of Maxwell's time was given up to the study of composition of colours. He invented the colour-box for analysing and combining the different colours of the spectrum. Prior to this time, the three primary colours were thought to be red, blue, and yellow. With his colour-box Maxwell showed that this is not the case, but that the three primary colours are red, green, and blue. He showed also that a mixture of blue and yellow produces pink, and not green as is commonly supposed. For these researches on light the Royal Society awarded him the Rumford Medal in 1860. The same year Maxwell was appointed Professor of Physics at King's College, London. In 1866 he retired to his estate at Glenlair. In 1870 he published his 'Theory of Heat," an elementary text-book without a parallel. The following year he accepted the Cavendish Professorship of Physics at Cambridge, and shortly afterwards he published his treatise on "Magnetism and Electricity," which is a mathematical treatment of Faraday's method. All electrical phenomena are explained by reference to energy stored up in the surrounding medium.

Maxwell called himself the interpreter of Faraday's views: but he was more than this; he built up a mathematical theory of magnetism and electricity which will be a lasting monument to his genius. He also propounded his electromagnetic theory

of light, in which he supposes that electrical energy is propagated by vibrations of the same æther which is supposed to transmit energy in the form of light. His theory supposes, in fact, that electricity and light are simply different aspects of the same phenomenon-a vibrating æther. In recent years Hertz, a pupil of Helmholtz, has, in a series of brilliant experiments, gone far towards verifying the results of Maxwell's theory of light. Electric waves have been obtained, and have been shown to be capable of reflection and refraction in exactly the same way as waves of light.

Maxwell died in 1879, and the scientific world lost its most brilliant genius.

Kelvin.

One of the first to appreciate Joule's researches was Professor Lord William Thomson (afterwards Sir William Thomson, and, still later, Lord Kelvin). He was educated at Glasgow and Cambridge, and was Second Wrangler in 1845. In 1846 Thomson brought out his theory of "Electric Images," and in the same year was appointed to the chair of Natural Philosophy in the University of Glasgow. Like Maxwell, his original contributions to physical science began when he was still a boy in his teens. As a theorist he has done more than anyone to develop the principle of conservation of energy. In 1849 he published a dynamical theory of heat, based upon the researches of Joule. Three years later he deduced the principle of "Dissipation of Energy," or the tendency of the available energy of a system to diminish while the total energy remains unaltered.

It is as an inventor of electrical instruments that Lord Kelvin stands pre-eminent. Many of the most delicate instruments for the measurement of electrical quantities owe their origin to his inventive genius. In 1858 he brought out his mirror galvanometer, which is capable of detecting excessively small electric currents. Among his more important inventions are a mariners' compass, protected so as to be unaffected by the presence of iron in the body of the ship; his siphon recorder, an instrument for recording telegraphic messages; the electric balance, in which the strength of an electric current is measured by balancing the force of attraction of two coils through which it flows against the weight of a given mass; the absolute electrometer, by means of which the difference of potential of two discs

is found in absolute measure; the quadrant electrometer, for comparative measure of differences of potential, and other instruments, too numerous to mention.

The discoveries of Volta, Ohm, Young, Oersted, Ampère, Faraday, Joule, Helmholtz, and Maxwell opened out wide fields

for scientific research, and almost all subsequent researches have been merely extensions and developments of the principles which these philosophers brought to light. Accurate measurements of the various physical qualities have been made, and refined instruments for delicate observations invented. In all branches of physical science measuring instruments have been brought to a wonderful degree of delicacy and perfection. The galvanometers now in use are a billion times as sensitive as the

old detectors of Ampère and Schweigger. Tyndall found it difficult to find instruments sufficiently sensitive to measure radiant heat in large quantities. Instruments are now in use by which the heat radiating even from the moon can be detected.

Outlook.

There are many scientific problems still unsolved, waiting The for another Faraday or Joule to come with heaven-born genius, and still further unravel Nature's mysteries. Will such problems as "What is æther, and in what way is it related to matter?" or, "What is the true nature of gravitation?" ever be solved? Problems like these are now occupying the minds of our greatest men of science. They may be solved in the near future; they may be beyond the powers of human understanding.

STEELE.
Chemistry

Applica

Manufac

THE opening of the College of Chemistry in October, 1845, ROBER under A. W. Hofmann, marks in some respect an era in English chemistry, and is remarkable as an illustration of the way in and its which scientific researches which seem to be without any prac- tions, tical bearing often develop into the most important practical 1846-1885. consequences. Since the discovery of benzene by Faraday in 1826 a number of allied substances had been discovered by various workers. Indigo, coal-tar oil, and other substances had been distilled, and under various names a substance ultimately called aniline had been obtained from them. Before coming to Chemical England Hofmann had proved the identity of these products, ture. and other chemists had shown that aniline could be prepared from benzene. In this country Hofmann continued his researches on aniline, and discovered a number of bodies which, like it, could be considered as substitution derivatives of ammonia, and the methods he used have been of the greatest value in the development of the coal-tar industry. Among Hofmann's earliest pupils and assistants was W. H. Perkin. Under the direction of Hofmann, Perkin set to work on some bodies, anthracene and naphthalene, which have since become the starting-point for the production of very important colouring matters; but it was in attempting to carry out the artificial formation of quinine that Perkin discovered the colouring matter since so well known as mauve: this was in 1856. In 1857 it was first used commercially, and in 1862 a large

The Constitution

cal Compounds.

proportion of the colouring matters in use were aniline dyes. Yet in 1858 the following words could be used in an important text-book:"The compounds of aniline are to be reckoned by hundreds, but they are not the subjects of manufacture: they are not articles of commerce; they are of no use in the arts; they are applied to no purpose in domestic economy." It would be long to trace out in detail the progress of this industry. The next important step was taken in 1868. Two German chemists in that year succeeded in producing for the first time the colouring matter alizarin (the dyeing matter of madder) from anthracene. Aware of the importance of this discovery, Perkin at once set to work, and, by the aid of his former knowledge of anthracene and its derivatives, discovered another method of producing the substance. Before the end of 1869 he had produced one ton of artificial alizarin; in 1870, 40 tons; in 1871, 220 tons, and so on. Twenty years after the birth of the industry the annual value of the colours produced amounted to over £3,000,000 sterling (1878).

Up to this point little was known of the constitution of of Chemi- chemical compounds. The type theory had proved of considerable assistance, but science was as far as ever from understanding what part organic radicals took. The next advance in theory was due to due to Frankland (1825-1899), afterwards Sir Edward Frankland, and Kolbe (1818-84). Bunsen had discovered a remarkable compound in which arsenic entered into the composition of an organic body. Extending these researches, Frankland described a series of compounds in which tin took part in the formation of an organic body, and was led to deduce the doctrine of valency-i.e. that the combining powers of any element were fixed or satisfied always by the same number of atoms. Kolbe's work after this principally lay in the discovery of the constitution of organic compounds. In 1858 Cannizzaro, an Italian chemist, published a paper which finally formulated the opinions of chemists on the methods employed for obtaining the atomic weights of the elements. The greatest achievement of the doctrine of valency was the explanation of the structure of benzene and the so-called aromatic compounds by Kekulé (b. 1829) in 1865. From his formula Kekulé was able to predict the number of isomeric compounds that could be produced from benzene and its derivatives,