simpler, atoms; it was also necessary to find means of determining the number of atoms of each kind which unite to form a more complex atom. A general method for solving these two problems was given to chemistry in 1811-12 by an Italian physical chemist named Avogadro, who brought into science the notion of a second order of minute particles, supplementing the conception of atom by that of molecule.

It is not possible in this brief sketch to indicate the many new fields of investigation which were opened, and made fruitful, by the Daltonian atomic theory. From the many workers who used this theory as a means for pressing forward along new lines of enquiry, two may be selected, since their work is typical of much that was done in chemistry during the first half of the nineteenth century.

Alexander Williamson strove to make chemists realise the need of using the Avogadrean molecule as well as the Daltonian atom. By his work on etherification, and by other experimental investigations, as well as by reasoning on his own results and those obtained by other chemists, Williamson demonstrated the fruitfulness of the notion of the molecule. He endeavoured to determine the relative weights of molecules by purely chemical methods. These methods proved to be less satisfactory, and much less general, than the physical method which had been described by Avogadro.

The conception of equivalency, that is, equal value in exchange, of determinate weights of different homogeneous substances, has been helpful in chemistry. In 1852, Edward Frankland applied the notion of equivalency to the atoms of elements, that is, homogeneous substances which have not been separated into unlike parts. He arranged the elements in groups, the atoms of those in any one group being of equal value in exchange, inasmuch as each of these atoms combines with the same number of other atoms to form molecules.

When Frankland's conception had been developed, and the method of determining the equivalency of atoms made more definite and more workable, a vast new field of enquiry was opened, a field which has proved remarkably fruitful both in purely scientific work, and in applied chemistry. It is not an exaggeration to say that the great industry of making aniline

colours is an outcome of the notion of atomic equivalency introduced by Frankland into chemical science.

The words element and principle were used by the alchemist as nearly synonymous; both words were used vaguely. The meaning given to the term element, by Lavoisier, towards the end of the eighteenth century-a definite kind of matter which has not been decomposed, that is, separated into unlike parts— was elucidated, and confirmed as the only fruitful connotation of the term by the work of Sir Humphry Davy on potash and soda in 1808.

Humphry Davy was the most brilliant of English chemists. He was the friend of Wordsworth and Sir Walter Scott. Lockhart says that the conversation of Davy and Scott was fascinating and invigorating. Each drew out the powers of the other.

I remember William Laidlaw whispering to me, one night when their "rapt talk" had kept the circle round the fire until long after the usual bedtime of Abbotsford-"Gude preserve us! this is a very superior occasion!"

Davy sent an electric current through pieces of potash and soda; the solids melted, and "small globules, having a high metallic lustre, and being precisely similar in visible characters to quicksilver, appeared." By burning the metal-like globules, Davy obtained potash and soda. Making his experiments quantitative, weighing the potash and the soda before passing the current, and the potash and soda obtained by burning the metal-like products of the first change, he proved that potash and soda, which, at that time, were classed with the elements, are composed each of a metal combined with oxygen. The new metals-potassium and sodium-are soft and very light, and instantly combine with oxygen when they are exposed to the air.

Everyone had been accustomed to think of a metal as a heavy, hard solid, unchanged, or very slowly changed, by exposure to air. Had chemists strictly defined the term metal, they could not have allowed the bases of potash and soda (as Davy called the new substances) to be included among metals. Happily, the definitions of natural science are not as the

definitions of the logician; they are descriptive summaries of what is known, and suggestive guides to further enquiry.

As every attempt to separate potassium and sodium into unlike parts failed, Davy put them into the class elements; he said "Till a body is decomposed, it should be considered as simple.'

In 1810, Davy investigated a substance concerning the composition of which a fierce controversy raged. Oxymuriatic acid was said by almost all chemists at that time to be a compound of oxygen with an unknown base. No one had been able to get oxygen from it, or to isolate the base supposed to be a constituent of it. By putting away, for the time, all hypotheses and speculations, and by conducting his experiments quantitatively, Davy showed that oxymuriatic acid is not an acid, but is a simple substance, that is, a substance which is not decomposed in any of the changes it undergoes. He proposed to name this simple substance chlorine; a name, Davy said, "founded upon one of its obvious and characteristic properties -its colour." Davy remarked-"Names should express things not opinions.

Davy thought much about the connections between chemical affinity and electrical energy, and investigated these connections by well-planned experiments. In 1807, he said -"May not the electrical energy be identical with chemical affinity?" He used the expressions-"different electrical states," and "degrees of exaltation of the electrical states," of the particles of bodies. Recent researches into the subject of chemical affinity have established the great importance of the conceptions adumbrated by Davy in these expressions.

Chemistry, the study of the changes of composition and properties which happen when homogeneous substances interact, has always been closely connected with physics, the study of the behaviour of substances apart from those interactions of them in which composition is changed. Among the earlier physical chemists, Graham occupies an important place.

Thomas Graham was a shy, retiring man, most of whose life was spent in his laboratory. There is a tradition in the Glasgow institution, where he taught chemistry in his younger days, before moving to London (in his later years he was master of the mint), that, when he came into the lecture theatre,

to deliver his first lecture to a large audience, he looked around in dismay and fled.

Graham established the fundamental phenomena of the diffusion of gases and of liquids; he distinguished, and applied the distinction, between crystalloids, solutions of which pass through animal and vegetable membranes, and colloids, which do not pass through those membranes. The investigation of the behaviour of colloidal substances has led in recent years, to great advances in the knowledge of phenomena common to chemistry, physics and biology.

Electrochemistry, the study of the connections between chemical and electrical actions, has been productive, in recent years, of more far-reaching results than have been obtained in any other branch of physical chemistry. Much of what has been done in the last half-century is based on the work of Faraday, and, indirectly, on the suggestion of Davy. Both were men of genius, that is, men who see the central position of the problem they are investigating, who seize and hold that position until the problem is solved, letting the surface phenomena, for the time, "go to the dogs, what matters?" Men of genius work from the centre outwards.

To Michael Faraday, we owe the fundamental terms of electrochemistry. The separation of a salt into two parts by the electric current, he called electrolysis; the surfaces from which the current passes into, and out of, an electrolysable compound, he named electrodes; the substances liberated at the electrodes, he called ions. Faraday measured “the chemical power of a current" by the quantities of the ions set free during a determinate period of electrolysis. Taking as his unit the quantity of electricity which liberates one gram of hydrogen from an electrolysable compound of that element, he showed that the weights of different ions liberated from compounds by unit quantity of electricity are in the proportion of their chemical equivalents. Using the language of the atomic theory, Faraday declared that "the atoms of bodies which are equivalent to each other in their ordinary chemical action have equal quantities of electricity mutually associated with them."

In 1834, Faraday said "The forces called electricity and chemical affinity are one and the same." Faraday distinguished the intensity of electricity from the quantity of it, and indicated

the meaning of each of these factors. One would not greatly exaggerate if one said that the notable advances made in the last quarter of a century in the elucidation of chemical affinity are but developments and applications of Faraday's pregnant work on the two factors of electrical energy.

The results established by Faraday have led to the conception of atoms of electricity, a conception which has been of great service in advancing the study of radioactivity. Faraday's results have also been the incentives and guides in researches which go to the root of many problems of the physical sciences, and of not a few of the biological sciences also.

At the time of the foundation of the Royal Society, chemistry was a conglomeration of more or less useful recipes, and a dream of the elixir. To-day, chemistry is becoming an almost universal science. Happily, chemists still dream.

C. BIOLOGY

Although science, during the eighteenth century, was, like many other intellectual activities in our country, more or less in abeyance, an attempt has been made, in the following pages, to carry on the subject in the present chapter from that which appeared in a previous volume (VIII) of this History.

"The Royal Society of London for Improving Natural Knowledge," one of the oldest scientific societies in the world and certainly the oldest in the empire, was formally founded in 1660, and received its royal charter of incorporation two years later. At a preliminary meeting, a list had been prepared of some forty "names of such persons as were known to those present whom they judged willing and fit to joyne in the designe," and among these names we find those of "Mr. Robert Boyle, Sir Kenelme Digby, Mr. Evelyn, Dr. Ward, Dr. Wallis, Dr. Glisson, Dr. Ent, Dr. Cowley, Dr. Willis, Dr. Wren," names whose owners have been dwelt upon in Volume VIII.

Thus, for the first time in our country, the study of science was, to a degree, organised and its advancement promoted, not only by periodical meetings where experiments were conducted and criticism freely offered, but by the collection of scientific books, which still remain at Burlington house, and of "natural