Helmholtz. The problem of the effects produced by the translation of electric charges, raised by the same investigator, was solved by the researches of the present Cavendish professor at Cambridge, George Francis FitzGerald of Dublin, and others: in the mathematical development of the theory, which now proceeded apace, they, again, took a prominent part. In 1883, FitzGerald explained a system of magnetic oscillators by which radiant energy could be obtained from electrical sources, thus confirming Maxwell's theoretical conclusion that light was an electromagnetic phenomenon. Some of Maxwell's assumptions on which he had based his theory still remained unconfirmed; but, a year or two later, the theory was placed on a firmer experimental basis by Hertz. The results, incidentally, led to the introduction of wireless telegraphy.

The question of the conduction of electric discharges through liquids and gases had been raised by Faraday. It was now taken up seriously, and various types of rays, cathode rays, Röntgen rays, etc., were discovered. These researches led to new views on the constitution of matter. The investigations began with a theory of electrons, and, finally, led to the view that every so-called atom is formed by a combination of two elements in varying proportions, and that, possibly, these two elements are to be identified with forms of electricity-one of the most far-reaching hypotheses propounded in recent times.

The efforts to extend the theory of the electromagnetic field to cases where heavy masses are in motion introduces the difficult question as to whether the ether round and in bodies is affected by their motion, and to this theory of relativity much attention is now being paid.

One of the striking features of the Victorian period has been the equipment of large laboratories where experiments can be carried out by students with an accuracy wholly impossible in former days. Two of the earliest of these were built at Oxford and Cambridge, the former known as the Clarendon laboratory in 1872, the latter, known as the Cavendish laboratory, being the gift of the seventh duke of Devonshire. In the latter, Clerk Maxwell taught and has been succeeded by professors not less distinguished. The existence of such laboratories in seats of learning has profoundly affected the teaching of the subject by training large numbers of com

petent observers, besides calling forth in ever widening circles an intelligent interest in physical studies.

It is not, we think, too much to say that the work in physics of the Victorian period has completely revolutionised the subject, and, both on its theoretical and practical sides, far exceeds in value that previously done in any period of similar extent. The theory of gravitation was the great achievement of the Newtonian school. In the following century, physical optics and, later, the nature of ether attracted most attention from philosophers, while practical men developed the steam-engine and studied the theory of heat. The Victorian age has seen electricity raised to the rank of an all-embracing science, and applied to innumerable industrial uses-powerengines, lighting, heating, telegraphy, telephones. Other important scientific and industrial applications relate to photography and spectrum analysis; the development of the turbineengine; the invention of the internal combustion-engine, with its numerous uses in transport on land and water; the introduction of submarine boats, and heavier-than-air flying-machines; and the use of wireless telegraphy. In this chapter, however, a bare reference to these practical applications must suffice.

B. CHEMISTRY

Chemistry has always busied itself with the changes of material things. By working in metals and precious stones, by making colours, by producing things used by artists to give delight to themselves and others, by fashioning natural materials into things useful to men, by concocting potions which had strange effects on the bodies and minds of those who swallowed them, by doing these things and things like these, chemists slowly amassed much knowledge, knowledge, however, which was fragmentary and disconnected. The strange changes which chemists discovered impelled the more ardent and adventurous among them to dream of the possibility of finding a universal medicine which should put an end to disease and suffering and enable the adept to bring all imperfect things to a state of perfection. The history of alchemy is the history of a particular branch of the universal quest, the quest of the unchanging.

In the later years of the eighteenth century, between 1770 and 1790, chemistry passed, at a bound, from being an empirical art to becoming a science. The man who made the great transformation was Antoine Laurent Lavoisier. With the work of the master we are not concerned here.

From the time of Lavoisier to our own day, chemistry has progressed, in the main, along four lines. For some years, chemists concentrated their attention on one definite class of material changes, the changes which happen when substances are burned in the air. The knowledge which was gained of the changes of composition and of properties during combustion incited and guided chemists to a searching examination of the distinctive properties of many different substances, and this examination brought about the clarifying of the conception of definite kinds of matter, and the application of this conception to the opening of many paths of chemical enquiry. While these advances were being made, a quiet member of the Society of Friends presented chemistry with a marvellously delicate and penetrative instrument for furthering accurate knowledge of material changes. John Dalton made what seemed a small addition to the Greek atomic theory, an addition which changed an interesting speculation into a scientific theory. As the century went on, chemists began to elucidate the connections between chemical events and physical phenomena. The science of physical chemistry began.

Among those who investigated the phenomena of combustion in the eighteenth, and early nineteenth, century, Priestley and Cavendish are pre-eminent. Black was the first chemist to make an accurate, quantitative examination of a particular, limited, chemical change, and, by so doing, to give clearness to the expression "a homogeneous substance." The atomic theory was Dalton's gift to science. From the many chemists who amplified the work of Dalton, and used the conceptions of atom and molecule to connect and explain new classes of chemical facts, Williamson and Frankland may be selected as the representatives. As workers in the borderland between chemistry and physics, Graham and Faraday are specially to be remembered. The investigations of Davy touched and illuminated every side of chemical progress.

Besides these men, who greatly enriched and advanced the

science of chemistry in the period under review, there were many workers whose contributions cannot be considered here. References are given in the bibliography to the writings of some of them.

Joseph Priestley was a man of many gifts and a very versatile mind. When a youth at an academy, he tells us that he "saw reason to embrace what is generally called the heterodox side of almost every question." When about twenty-eight years of age, he taught, in a school at Warrington, languages (he had a great natural gift of tongues), oratory and criticism, elocution, logic, natural phenomena, civil law and anatomy.

In the seventies of the eighteenth century, Priestley turned his attention to different kinds of airs. He obtained and partially examined many gases, but rarely troubled about separating them completely from impurities. In August, 1774, Priestley obtained a large lens with which he concentrated the sun's rays on whatever substance happened to come to his hand, with the object of finding what air could be extracted from it. When he thus heated mercurius calcinatus per se (now called oxide of mercury), he obtained an air in which a candle burned with a "remarkably vigorous flame." This result, he says, "surprised me more than I can well express." The new air was subjected to many tests; it always behaved in a very unexpected manner. He placed a mouse in his new air; the mouse remained lively, and the air did not become "noxious." The results of other experiments caused Priestley to lie awake through the night "in utter astonishment." last, he concluded that the new air was "between four and five times as good as common air." He regarded the new air as a very superior kind of common air.

At

Priestley thought alchemically, not as a chemist. To the alchemist, the properties of things were external wrappings which might be removed from one thing and put round another, without affecting the essential substance of either thing, which substance it was the business of properties to hide from the uninitiated. Priestley thought of different airs as identical, or nearly identical, in substance, and only apparently different because of superficial differences in the mantles, the properties, by which the essential substance was concealed. When he obtained the air from burnt mercury, he thought he

had removed from common air something which made it "noxious, vitiated, depraved, corrupt. He had not learnt, what Black's experiments, made twenty years before 1774, might have taught him, that each particular, material thing is known only by its properties. Priestley's forced explanation of the facts which he himself discovered helped to convince investigators that the notion of identity of substance hidden under differences of properties is a great hindrance to the acquirement of accurate knowledge of natural

events.

Priestley could not get over his astonishment at the behaviour of the new air. In science, one does well to be astonished; but, to astonishment one must add investigation, to investigation, reasoning, and, to reasoning, more investigation. Stopping at astonishment, Priestley made his facts square with the theory that dominated him, the theory of phlogiston. The phlogisteans taught that something, which they had named phlogiston, the principle of fire, rushes out of a burning substance as it burns. Phlogiston was never captured. Priestley held that the elusive phlogiston is a great corrupter of your airs or gases. He supposed that he had deprived common air of this depraving principle; he named his new gas dephlogisticated air. He invented many very ingenious hypotheses to account for facts observed by himself. Had he made a few accurate quantitative experiments, he might have broken the toils of his favourite theory.

The French chemist Lavoisier saw the importance of Priestley's discovery of dephlogisticated air, and, by a series of rigidly quantitative experiments with tin and mercury, proved that, when a substance burns in air, it combines with a constituent of the air, which air-constituent is the gas prepared by Priestley. Lavoisier called his gas oxygen, because many of its compounds are acids.

Priestley's insatiable curiosity, his mental alertness, his impatience of details, were required for the advancement of chemistry, no less than the passionless determination and the scrupulous accuracy of Cavendish.

Henry Cavendish, of Peterhouse, was bred in the theory of phlogiston, as Priestley was, and remained faithful to that theory, as Priestley did. He thought of many airs, or gases, as