hydrogen. All the changes we can effect consist in the separation of atoms bound together before and in the union of those previously separated.

The atomic theory while highly serviceable has always been subjected to severe criticism. In 1840, for example, Dumas declared that it did not deserve the confidence placed in it, and that if he could he would banish the word "atom," convinced that science should confine itself to what could be known by experience. As late as 1852 Frankland says:

I had not proceeded far, in the investigation of the organo-metallic compounds before the facts brought to light began to impress upon me the existence of a fixity in the maximum combining value or capacity of saturation in the metallic elements which had not before been suspected. . . . It was evident that the atoms of zinc, tin, arsenic... had only room, . . . for the attachment of a fixed and definite number of the atoms of other elements.

Independent researches have, in combination with the older chemical theories, introduced so much definiteness into this line of thought that 'the Newtonian theory of gravitation is not surer to us now than is the atomic or molecular theory in chemistry and physics - so far, at all events, as its assertion of heterogeneousness in the minute structure of matter, apparently homogeneous to our senses, and to our most delicate direct instrumental tests.' - Kelvin, 1886.

The three main criticisms of the atomic theory are:

(1) that it is based on inference, not on direct observation; and is therefore only a provisional hypothesis; (2) that it takes no account of chemical forces - "affinity"; (3) that it overemphasizes analysis.

The idea of "atomicity" and "valency". was not possible without the clear notion of the "molecule" as distinct from the "atom." This idea had lain dormant in the now celebrated but long forgotten law of Avogadro, which was established in 1811 almost immediately after the appearance of Dalton's atomic theory.

It had been known since . . . Boyle and Mariotte that equal volumes of different gases under equal pressure change their volumes equally if the pressure is varied equally, and it was also known...

that equal volumes of different gases under equal pressure change their volumes equally with equal rise of temperature. These facts suggested to Avogadro, and almost simultaneously to Ampère, the very simple assumption that this is owing to the fact that equal volumes of different gases contain an equal number of the smallest independent particles of matter. This is Avogadro's celebrated hypothesis. It was the first step in the direct physical verification of the atomic view of matter. Merz.

Until the middle of the

SYNTHESIS OF ORGANIC SUBSTANCES. nineteenth century there was an apparently fundamental separation between organic and inorganic nature. Since then they have been brought together by the general laws of energy and to some extent by the principles of evolution, as will appear in the following chapter. In 1828 Wöhler (of Göttingen) had indeed succeeded in preparing urea out of inorganic materials, a discovery which disproved such difference as was hitherto considered to exist between organic and inorganic bodies.

A PERIODIC LAW AMONG THE ELEMENTS. With gradually increasing knowledge of the fundamental constants of chemistry

the atomic weights attempts were naturally made to connect these with the chemical and physical properties of the corresponding elements valence, affinity, specific gravity, specific heat, etc. In 1869-71 Mendeléjeff, a Russian chemist, succeeded in establishing remarkable relations between these data, and on tabulating them enunciated his Periodic Law, which has resulted in the discovery of several new and hitherto unsuspected elements. As the existence of the planet Neptune (page 341) had been predicted to fill an apparent gap in a system, so Mendeléjeff under the periodic law was able to predict the existence of other and missing elements in the series of chemical elements. And just as the prediction of Adams and Leverrier was fulfilled by the actual discovery of Neptune, so the prophecy of Mendeléjeff was justified by the discovery of gallium in 1871, scandium in 1879, and germanium in 1886. Furthermore, the periodic law enabled Mendeléjeff to question the correctness of certain accepted atomic weights, and here, also, he was justified by subsequent redeterminations.

It may be questioned whether the celebrated periodic law of Newlands, Lothar Meyer and Mendeléjeff, which has brought some order into the atomic and other numbers referring to the different elements, and has even made it possible to predict the existence of unknown elements with definite properties, stands really in a firmer position than the once well-known but now forgotten law of Bode, according to which the gap in the series which gives the distances of the planets from the sun indicated the existence of a planet between Mars and Jupiter.

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CHEMICAL STRUCTURE. Crystallography-a science of the nineteenth century-established an important connection between chemistry and geometry. Haüy made mineralogy as precise and methodical as astronomy. . . . He was to Werner and Romé de l'Isle, his predecessors, what Newton had been to Kepler and Copernicus."

In the early years of the atomic theory Wollaston had predicted that philosophers would seek a geometrical conception of the distribution of the elementary particles in space—a prophecy first practically fulfilled by Van't Hoff's Chemistry in Space (1875).

The chemical character is dependent primarily upon the arrangement and number of the atoms, and in a lesser degree upon their chemical nature (V. Meyer). The atomic view first became a scientific instrument, when arithmetical relations of a definite and unalterable kind were proved to exist; it became a yet more useful instrument, when to the arithmetical there were added geometrical conceptions.

Merz.

PHYSICAL CHEMISTRY: ELECTROLYTIC AND THERMODYNAMIC DEVELOPMENTS OF CHEMISTRY. In the latter part of the nineteenth century much light was thrown on a wide range of physical and chemical phenomena by the study of solutions and their electrolytic behavior. Much had already been accomplished by Davy in the decomposition of substances by the electric current, leading for example to the first isolation of the elements, sodium and potassium. Faraday showed that for a given substance the amount decomposed is dependent solely on the quantity of electricity passed through and that for different substances the

amounts set free at the electrodes are proportional to their chemical equivalents. To him the name electrolysis is due. A closer study of the phenomena of electrolysis led Clausius to the hypothesis that the molecules of salts, acids, and bases, previously regarded as disintegrated only by the passage of the electric current, are already dissociated in ordinary solutions. To these electrically charged part-molecules Faraday gave the name ions. Arrhenius proved that salts in dilute solution are dissociated into their ions almost completely, instead of only very slightly as Clausius supposed. This theory of Arrhenius, known as the Theory of Electrolytic Dissociation, of which an account would be too technical for the present purpose, coordinates and correlates heterogeneous masses of chemical facts, which apparently bore little or no relation to one another, and refers them to a common cause.

During the latter part of the nineteenth century a study of the rate and equilibrium conditions of chemical reactions led by degrees to the formulation of the so-called law of mass action and to many important thermodynamic relations. Chemistry thus came to share with physics the possibility of utilizing the calculus, becoming thereby more fully a quantitative science.

REFERENCES FOR READING

MERZ, J. T. History of European Thought in the Nineteenth Century.
POINCARE, H. Science and Hypothesis.

RAMSAY, WILLIAM. The Gases of the Atmosphere and the History of Their

Discovery.

ROSCOE, H. E. John Dalton.

SODDY, F. Matter and Energy.

THOMPSON, S. P. Michael Faraday: His Life and Work.

TILDEN, W. A. Progress of Scientific Chemistry in Our Own Time.

TYNDALL, JOHN.

Faraday as a Discoverer.

CHAPTER XVII

SOME ADVANCES IN NATURAL SCIENCE IN THE NINETEENTH CENTURY. COSMOGONY AND EVOLUTION

What the classical renaissance was to men of the fifteenth and sixteenth centuries, the scientific movement is to us. It has given a new trend to education. It has changed the outlook of the mind. It has given a new intellectual background to life. Sadler.

The rapid increase of natural knowledge, which is the chief characteristic of our age, is effected in various ways. The main army of science moves to the conquest of new worlds slowly and surely, nor ever cedes an inch of the territory gained. But the advance is covered and facilitated by the ceaseless activity of clouds of light troops provided with a weapon- always efficient, if not always an arm of precision the scientific imagination. It is the business of these enfants perdus of science to make raids into the realm of ignorance wherever they see, or think they see, a chance; and cheerfully to accept defeat, or it may be annihilation, as the reward of error. Unfortunately the public, which watches the progress of the campaign, too often mistakes a dashing incursion .. for a forward movement of the main body; fondly imagining that the strategic movement to the rear, which occasionally follows, indicates a battle lost by science. - Huxley.

INFLUENCE OF EIGHTEENTH CENTURY REVOLUTIONS. - If the French Revolution had done no more than to upset as it did the social equilibrium of the centuries, its effect in stimulating inquiry and generating doubt in almost every direction could not have failed to further scientific studies and promote wholesome investigation into the fundamental relations of man and nature. But even before that revolution, some of the ablest minds in France, keenly alive to the teachings of Descartes and Newton and the lessons of seventeenth century science, had rejected the current cosmogony of Moses, although they had nothing with which to replace it. In particular, the eighteenth century questioned all custom and authority, and the theory of special creation possessed no other basis.