-stances from which it originated. Whether the Greeks invented, or received from the Egyptians, the doctrine concerning the transmutation of metals, or whether the Arabians were the first who professed it, Dr. Watson thinks it is uncertain ; but Boerhaave produces a passage from Æneas Garæus, implying that the Greeks were in possession of the art before any traces of it can be discovered among the Arabians.— "Such," says he, "as are skilled in the ways of nature can take silver and tin, and, changing their former nature, make them into gold." In the seventh century, the Arabians were eager in their search after gold; and the same agent, it was imagined, that was the object of discovery, as capable of converting the baser into the precious metals, would, by its operation on the animal system, prevent or remove disease, and confer immortality. This notion was fondly adopted and ever after it became conjoined with the original object of alchemy. Amidst the darkness of the middle ages, these delusive labours continued to be prosecuted with increased ardour; and it is thought that during this period, extending from the eleventh to the sixteenth century, the greatest number of real chemical discoveries were made. In the thirteenth century gunpowder and phosphorus were known to Roger Bacon. He was followed by Arnoldus de Villa Nova, Raymond Lully, Isaac and John Isaac Hollandus, and Basil Valentine, "of whose history," says Mr. Murray, "so little is known, that it is doubtful whether some of the names are not fictitious; nor is it easy to assign, with precision, the discoveries they made." Some of them are very important, viz. of the two fixed alkalies, and several of the acids. The properties of some of the neutral salts were also subjects of investigation; the semi-metals, as they were till lately denominated, were distinguished from the metals in use, and many metallic preparations were introduced; the instruments of chemical analysis, and the methods of conducting chemical operations, had received material improvements; and the chemical arts had slowly advanced to greater perfection. Paracelsus introduced a number of metallic preparations into medical practice, and laid the foundation of chemical pharmacy. He thus began to draw off the attention of the alchemists b 2 from

from their wild projects, and directed it to an object beneficial in itself, and unconnected with concealment and imposture. To Van Helmont scientific chemistry is probably more indebted than to any of the alchemists: his inquiries were in general directed to more useful objects, and conducted with more just views. He successfully investigated the properties of several of the gases produced in chemical operations, and his researches may be traced in the experiments of Boyle and. Hales. From this period alchemy began to decline, and chemistry to rank high as a science, calculated at once to enlighten the understanding, and conduce in a high degree to improvements in the arts of life. The illustrious Bacon first assigned it this rank; and gave it, in his survey of human knowledge, its peculiar characters. He reviewed its history, exposed the errors of its professors, and suggested a number of investigations which he supposed might be successfully prosecuted.

The institution of the Royal Society in London, and of the different other learned societies in Europe, aided the progress of chemistry in common with the other experimental sciences. Many of its processes were improved, facts were multiplied, and a constant accession was made to the stock of knowledge. Kunckel, Homberg, the Lemerys and Geoffroys distinguished themselves in these labours. "Boyle is justly celebrated for the extent and importance of his researches, particularly in pneumatic chemistry; and in the same department Hooke and Mayow so far exceeded the knowledge of their times, that their discoveries could not be duly appreciated; and were allowed to fall into oblivion, to be noticed in a succeeding age with admiration, as anticipations of some of the most important truths which the combined efforts of modern chemistry have established." To sir I. Newton the science of chemistry is indebted for the generalization of its phænomena, or the discovery of the principles on which its operations depend, and the observations of some of the laws by which it is regu lated. He noticed a number of cases of chemical combination and decomposition, and referred the facts to a species of at traction exerted between the minute particles of matter, dif. ferent in its laws from that attraction by which their masses

approach

approach each other. The relative forces of the different attractions were afterwards investigated, and reduced to a tabu, lar form by Geoffroy.

Hitherto chemistry might be regarded almost as a mere collection of insulated facts without system or theory; but Joachim Beccher of Spires, who appeared about this time, in his work entitled "Physica Subterranea," has collected all the chemical phænomena which were then known, and has described them with strict accuracy. He even foretold many of the discoveries which have been successively made since he wrote; such as the existence of aëriform or gaseous substances, the possibility of reducing animal bones into a transparent glass, &c. He withdrew chemistry from the too nar row limits of pharmacy; showed its connection with all the phænomena of fermentation, putrefaction, &c.; and by his speculations gave rise to that theory which was adopted by Stahl, and from him was received as the true doctrine among all the philosophers of Europe for the greater part of a century. G. Ernest Stahl, born in 1660, and afterwards physician to the late king of Prussia, wrote a commentary upon the work of Beccher. Possessed almost from his birth with a strong passion for chemistry, he applied to it the whole force of his superior genius. He reduced to certain general principles all the facts with which the subject had been enriched. He classed his materials with admirable order and method, and expressed himself in a language less ænigmatical than that of his predecessors. The name, therefore, of this philosopher marks the commencement of a new era in the annals of che mical science, and will always be remembered in connection with the theory which it designates, and which is sometimes called the phlogistic and sometimes the Stahlian theory.Of the general operations of chemistry, combustion is undoubtedly the most important; not merely from the striking phænomena which it exhibits, and the number of bodies susceptible of it, but also from being the source of some of the most active chemical agents, particularly the class of acids. The explanation which Stahl gave of combustion, and the changes connected with it, was extremely simple. He adopted

the popular idea that combustible or inflammable bodies contain a common principle, to which their inflammability is owing. This principle he named pure fire, or phlogiston, Combustion he conceived to be merely its disengagement under the form of the heat and light which attend the process.

Chemistry from this period continued to make a silent progress, unmarked by any splendid discovery, till Dr. Black began the investigation of what peculiarly distinguishes the science in its modern form. The mention of this celebrated chemist brings us to the present reign, in the course of which his most important discoveries in chemistry were made. In 1756 he succeeded Dr. Cullen as professor of medicine and lecturer in chemistry at Glasgow. He published an essay containing experiments to investigate the nature of magnesia, quicklime, and some other alkaline substances recommended as solvents of the stone in the bladder. In the course of these experiments he demonstrated the existence of an aërial fluid, which he denominated fixed air, but which has since obtained the name of carbonic acid gas; the combination of which with alkalies and calcareous earths renders them mild; while, on the other hand, if this be taken from chalk and other like substances, what is left becomes highly caustic; a discovery which laid the foundation of the improvements since made in our knowledge of gases by Priestley, Cavendish, Lavoisier, and other chemists. After this he enriched his favourite science with his experiments on latent heat, which is found to exist in all bodies; explaining in a very satisfactory manner the connection between heat and fluidity. Thus the particles of all bodies were found to be subjected to the action of two opposite powers, the one repulsive, the other attractive, between which they remain in equilibrio. So long as the attractive force remains stronger, the body continues in a stateof solidity; but if, on the contrary, heat has so far removed these particles from each other, as to place them beyond the sphere of attraction, they lose the adhesion which they before had with each other, and the body ceases to be solid. Water gives us a regular and constant example of these facts; while below 32° of Fahrenheit it remains solid, and is called ice.

Above that degree of temperature it becomes liquid and when its temperature is raised higher than 212°, its particles assume the state of vapour or gas, and the water is changed into an aëriform fluid called steam. The same may be affirmed of all bodies in nature: they are either solid, liquid, or in the state of elastic aëriform vapour, according to the proportion which takes place between the attractive force inherent in their particles, and the repulsive power of the heat acting upon these, or, which is the same thing, in proportion to the degree of heat to which they are exposed.

Dr. Black thus directed the attention of chemists to pneumatic chemistry; and the future history of the science, till the time of Davy, is little more than a detail of the discoveries that have been made in this department, and the application to which they led. The subject was soon prosecuted by Mr. Cavendish, the son of lord Charles Cavendish, who was born in 1731. The first chemical paper of this gentleman was published in the year 1766; it was entitled "Experiments on factitious Air;" and constituted an important step in the science. Dr, Hales had demonstrated that air is given out by a vast number of bodies under peculiar circumstances. But he never suspected that any of the airs which he obtained differed from common atmospheric air. This indeed had been ever regarded as an elementary substance, to which every elastic fluid was referred. Dr. Black had demonstrated that calcareous spar and the mild alkalies contained a quantity of air chemically combined with lime and alkaline bodies. This, as we have seen, he called fixed air; and this, before he had an opportunity of examining its properties, he concluded was very different from common air. Mr. Cavendish, in this paper, demonstrates that there are two species of air quite different from one another, and also from the common atmospherical air: these are inflammable air, since named hydrogen gas, and fixed air or carbonic acid gas. He noticed a third

species of air, which was obtained by the solution of metals in nitrous acid. The inflammable air was obtained by dissolving iron, zinc, or tin, in diluted sulphuric or muriatic acids. The quantities of air differed according to the different metals used;