research, as those discoveries, the materials of his induction, were the undivided property of others!

The capital discovery of Black, thus early made, and to any share in which no one has ever pretended, was that the causticity, as it was formerly termed upon a false theory, of the alkalis and alkaline earths, was owing to the loss of a substance with which they had been combined, and that their reunion with this substance again rendered them mild. But the nature of this substance was likewise ascertained by him, and its detection forms by far the most important part of the discovery, for it laid the foundation of chemical science. He found that it was a permanently elastic fluid, like air in some of its mechanical qualities, those of being transparent or invisible, and incondensable, but differing entirely from the air of our atmosphere in its chemical properties. It was separated from alkaline substances by heat, and by the application of acids, which, having a stronger elective affinity for them, caused it to be precipitated, or to escape in the aëriform state; it was heavier than common air, and it gave a slight acidulous flavour to water on being absorbed by it; hence the inference that it was an acid itself. A short time afterwards (in 1757) he discovered that this peculiar air is the same with that produced by the fermentation of vegetable substances. This he ascertained by the simple experiment of partially emptying in a brewer's vat, where the fermenting process was going on, the contents of a phial filled with lime-water. On shaking the liquid that remained with the air that had entered, he found it become turbid, from the lime having entered into union with the air, and become chalk. The same day he discovered by an experiment, equally simple and equally decisive, that the air which comes from burning charcoal is of the same kind. He fixed a piece of charcoal in the broad end of a bellows nozzle, unscrewed; and putting that in the fire, he inserted the other end in a vessel filled with lime-water.

The air that was driven through the liquid again precipitated the lime in the form of chalk. Finally, he ascertained by breathing through a syphon filled with lime-water, and finding the lime again precipitated, that animals, by breathing, evolve air of this description.

The great step was now made, therefore, that the air of the atmosphere is not the only permanently elastic body, but that others exist, having perfectly different qualities from the atmospheric air, and capable of losing their elasticity by entering into chemical union with solid or with liquid substances, from which being afterwards separated, they regain the elastic or aëriform state. He gave to this body the name of fixed air, to denote only that it was found fixed in bodies, as well as elastic and separate. He used the term "air" only to denote its mechanical resemblance to the atmospheric air, and not at all to imply that it was of the same nature. No one ever could confound the two substances together; and accordingly M. Morveau, in explaining some years afterwards the reluctance of chemists to adopt the new theory of causticity, gives as their excuse, that although this doctrine "admirably tallies with all the phenomena, yet it ascribes to fixed air properties which really make it a new body or existence" ("forment réellement un nouvel etre").*

In order to estimate the importance of this discovery, and at the same time to show how entirely it altered the whole face of chemical science, and how completely the doctrine was original, we must now examine the state of knowledge which philosophers had previously attained upon the subject.

It has often been remarked that no great discovery was ever made at once, except perhaps that of logarithms; all have been preceded by steps which conducted the discoverer's predecessors nearly, though not quite, to the same point. Some may possibly think

Supplement to the 'Encyclopédie,' vol. ii., p. 274, published in 1777.

that Black's discovery of fixed air affords no second exception to this rule; for it is said that Van Helmont, who flourished at the end of the sixteenth and beginning of the seventeeth century, had observed its evolution during fermentation, and given it the name of gas silvestre, spirit from wood, remarking that it caused the phenomena of the Grotto del Cane, near Naples. But though he as well as others had observed an aëriform substance to be evolved in fermentation and in effervescence, there is no reason for affirming that they considered it as differing from atmospheric air, except by having absorbed, or become mixed with, certain impurities. Accordingly, a century later than Van Helmont, Hales, who made more experiments on air than any other of the old chemists, adopts the commonly received opinion that all elastic fluids were only different combinations of the atmospheric air with various exhalations or impurities;* and this was the universal belief upon the subject, both of philosophers and of the vulgar.

It is now fit that we see in what manner the subject was treated by scientific men at the period immediately preceding Black's discoveries. The article Air' in the French 'Encyclopédie' was published in 1751, and

* It may safely be affirmed that Van Helmont's observation, which lay for a century and a half barren, threw no light of any value upon the subject. No one questions Newton's title to the discovery of the different refrangibility of light, and the true theory of the rainbow; yet, at the beginning of the 17th century, Antonio de Dominis, Archbishop of Spalatro, had really made an ingenious and well-grounded experiment on the similarity of the rainbow colours with those formed by the sun's rays refracted twice and reflected once in a globe filled with water. The doctrine of universal gravitation was known to both Kepler and Galileo; and Boulland (Astronomia Philolaica, lib. i., 1645), distinctly stated his belief or conjecture that it acted inversely as the squares of the distances. The famous proposition of equal areas in equal times was known to Kepler. The nearest approach to the Fluxional Calculus had been made by Harriott and Roberval and Fermat; and to take but one other example, the electrical explosion of the Leyden jar, discovered in 1747, obtained the name of the coup-foudroyant, and was by Abbé Nollet conjectured to be identical with lightning, Franklin's celebrated experiment being only made in 1752.

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written by D'Alembert himself. It is, as might be expected, able, clear, elaborate. He assumes the substance of the atmosphere to be alone entitled to the name of air, and to be the foundation of all other manently elastic bodies: "L'air élémentaire, ou l'air proprement dit," he says. He describes it as "homogène," and terms it "l'ingrédient fondamental de tout l'air de l'atmosphère, et qui lui donne son nom." Other substances or exhalations mix with it, he says, but these he terms "passagères," passing vapours, and not permanent: the air alone (that is, the atmospheric air) he calls "permanent," or permanently elastic (vol. i. p. 225). So little attention had the observation of Van Helmont respecting the Grotto del Cane excited, that we find a conjecture hazarded in the article 'Grotte' (vol. vii. p. 968), which appeared in 1756,-" peutêtre respirent ils (les chiens), au lieu d'air, des vapeurs minérales;" but this was some time after Black's discovery had taught us to distinguish such permanently elastic vapours from atmospheric air. In the article 'Fermentation' (vol. vi. p. 523) we find Van Helmont's doctrines of the connection between fermentation and digestion treated with ridicule, and those who adopted them jocularly called the "fermentateurs."

6

A few years later, however, the face of things changed. In the 'Supplement, published in 1776, we find an article on Fixed Air,' and a reference to Dr. Black's discovery; but nothing can be more indistinct than the author, M. Morveau's, ideas respecting it; for he leaves us in doubt whether it be the atmospheric air or a separate substance, and yet he states that the phenomena of fermentation and putrefaction are explained by the evolution or absorption of this air, and that mineral waters derive from its presence their flavour. An abstract of M. Venel's book had in 1765, under the head of 'Mineral Waters,' given this explanation; but instead of representing the air combined with the water as a different substance, he calls

it "véritable air et même très pure." We have, however, seen that, in the following year (1777), M. Morveau's ideas were perfectly distinct on the subject; for he treats it as a new substance, wholly different from atmospheric air. The slowness with which Black's doctrine made its way in France may be presumed from Morveau's remark on causticity, already cited, and also from this, that the article on 'Magnesia,' published in 1765, dogmatically asserts Black to be in error when he describes Epsom salts as yielding that earth, "because," says the author, "those salts are purely Seidlitian," "entièrement Seidlitiens" (vol. x. p. 858). In fact, Epsom salts, magnesia, limestone, and sea-water are the great sources from which all magnesia is obtained. The first of these substances is in truth only a combination of magnesia with sulphuric acid.

The other discoveries to which Black's led were as slowly disseminated as his own. Oxygen gas had been discovered, in August, 1774, by Priestley, and soon after by Scheele without any knowledge of Priestley's previous discovery; yet in 1777 Morveau, who wrote the chemical articles in the 'Supplement,' never mentions that discovery, nor the almost equally important discovery of Scheele, chlorine, made in 1774, nor that of azote, discovered by Rutherford in 1772, nor hydrogen gas, the properties of which had been fully investigated by Cavendish as early as 1766. Lavoisier's important doctrine, well entitled to be called a discovery, of the true nature of combustion, had likewise been published in 1774 in his 'Opuscules,' yet Morveau doggedly adheres to his own absurd theory of the air only being necessary to maintain those oscillations in which he holds combustion to consist; and finding that the increase of weight is always the result of calcination as well as combustion, he satisfies himself with making a gratuitous addition to the hypothesis of phlogiston, and supposes that this imaginary