(the discoverer of several compounds of antimony and also of ammonia) in the fifteenth, Paracelsus (the first public teacher of chemistry, who prepared many powerful medicines, particularly preparations of mercury) in the sixteenth, Van Helmont (who even at this early period propounded many philosophical views and was the first to distinguish various aeriform bodies by the name of Gases from common air), and Libavius in the seventeenth.

Independently of the reigning pursuit of alchemy there arose in these times many men who either devoted themselves to philosophical research like Roger Bacon and Albertus Magnus in the thirteenth century; or exposed the delusions and impostures of the alchemists, like Kircher, Conring, Guibert, Gassendi and Kepler; or were the authors of scientific works, like George Agricola (de re metallica, 1546), Lazarus Erker (Aula subterranea, 1574) and several others.

Towards the end of the seventeenth century, chemistry received an unexpected extension from the researches of Newton on attraction and light and from the experiments of Torricelli, Guericke and Boyle on the vacuum. About the same time, several salts were discovered by Glauber, phosphorus by Brandt and Kunkel, sweet spirits of nitre and several glass-fluxes by the latter, artificial volcanos by Nicholas Lemery (author of the Cours de Chimie), and boracic acid and the alum-pyrophorus by Homberg.

At the very beginning of the eighteenth century, George Ernest Stahl, by applying the already published views of Albertus Magnus and Becher (author of the Physica Subterranea) respecting that most important of all chemical processes, Combustion, to the whole collection of facts discovered by himself and others, and thus uniting them into a connected whole, laid the foundation of the first system of Chemistry. This system received the name of the Phlogistic Theory, because Stahl assumed that all combustible bodies contain one and the same principle of combustion, called Phlogiston; the escape of this substance from a heated combustible body being supposed to produce the phenomena of combustion, and its addition. to a burnt body to restore the combustibility of that body. In this system Phlogiston and the dephlogisticated metals or metallic earths (metallic oxides of the present system) were regarded as elements and displaced the four elements of the Grecian philosophers. From this time forward chemistry received more scientific cultivation, and discoveries accumulated more and more, until towards the end of the eighteenth century a reform of the system became indispensable.

In 1718, Geoffroy published the first Table of Affinities. Boerhaave published in 1732 a chemical work containing many original experiments on light, heat, &c. Hales in 1724 instituted several experiments on the air and other aeriform bodies. These experiments were however much more successful in the hands of Black, who in 1756 showed that the kind of air given out by fermenting liquids and evolved from chalk by the action of acids is different from atmospheric air; and thus directed the attention of experimenters to the more accurate investigation of aeriform bodies. Margraff, 1754-1759, first established the existence of magnesia and alumina as distinct earths, the only earths previously recognised being lime and silica; he also produced sugar from native plants, and discovered the phosphates in urine. Scheele, between the years 1773 and 1786, discovered, with very slender means, chlorine, the hydrofluoric, nitrous, prussic, tungstic, molybdic, arsenic, tartaric, citric, malic, lactic, gallic, and uric acids; also baryta, manganese

(partly), oxygen gas (discovered just before by Priestley); he also demonstrated the presence of phosphoric acid in bones, and instituted many ingenious experiments on light and heat, which led him to the adoption of a new theory of combustion. Bergman perfected the doctrine of Affinity and made several experiments on carbonic acid and other matters, besides many successful analyses of mineral substances. Cavendish, who first collected gases over water instead of receiving them in bladders, was also the first to distinguish hydrogen; he likewise discovered the formation of carbonic acid by the combustion of charcoal, the composition of water and of nitric acid (1765-1785). Priestley, who first collected gases over mercury, discovered in 1770 and the following years, Oxygen gas, protoxide of nitrogen, carbonic oxide, ammoniacal, sulphurous acid and muriatic acid gases and the gaseous fluoride of silicon : he also first observed the disengagement of oxygen gas from the green parts of plants.

Antoine Laurent Lavoisier, whose philosophical career, begun in 1770, was ruthlessly cut short in 1794 by the guillotine of Robespierre, not only perceived the defects which later discoveries had made manifest in Stahl's theory, but availed himself of these discoveries together with certain experiments of his own, made with a degree of accuracy in the determination of weights and volumes quite unknown before his time, in order to establish a theory in opposition to the phlogistic, and thence called the Antiphlogistic theory:-attempts at the establishment of such a theory had been made by Rey in 1630 and by Mayow in 1670, but without success.

The long established fact that combustible bodies, such as metals, do not lose but gain weight when burnt (an attempt at explanation had been made by supposing a fixation of the fiery particles to take place) was placed in a clearer light by Lavoisier, who showed that this increase of weight is exactly equal to the weight of the oxygen gas absorbed by the combustible body in the act of burning. On the other hand, he showed that in the conversion of a burnt body into a combustible body, a decrease of weight takes place in spite of the phlogiston which was supposed to be absorbed. He denied the existence of phlogiston, regarded combustion as the combination of a combustible body with oxygen, accompanied by a development of light and heat, and the conversion of a burnt into a combustible body as resulting from the separation of oxygen. To him also we are indebted for the discovery that the diamond is carbon, that carbonic acid is a compound of carbon and oxygen, that water is decomposed by red-hot iron, that the earthy matter deposited when water is heated in glass vessels is derived from the glass itself-as well as many well devised investigations on heat, respiration, transpiration. &c.

From this time forward Chemistry advanced with twofold rapidity, partly through the zeal of the adherents of the new theory, partly through that of their opponents, who, while they endeavoured to overthrow it by new researches, really contributed to its establishment and extension. Berthollet who in 1787 was the first to attack the new theory acquired great reputation by his researches upon Chlorine and upon the doctrine of affinity. Fourcroy undertook, particularly in connexion with Vauquelin, more exact analyses of organic substances and several other investigations. The latter in the course of his numerous analytical researches discovered chromium, glucina, and several vegetable substances. Klaproth, to whom mineral analysis is most deeply indebted,

discovered zirconia, titanium, uranium and tellurium. Richter in his Stoichiometry, laid the foundation of the doctrine of definite and simple proportions by weight according to which bodies combine. Prout investigated with great accuracy the chemical relations of several metals, and successfully combated Berthollet's doctrine of affinity. Smithson Tennant was the first who separated carbon from carbonic acid: he also discovered osmium and iridium. Wollaston discovered palladium and rhodium in platinum ore.

The discovery of Galvanic electricity by Galvani, to which Volta (the inventor of one of the most useful forms of the Eudiometer) gave increased power by means of his Pile, furnished the chemist with a new means of decomposition, which not only served to confirm Lavoisier's theory of the compound nature of water, but also, in the hands of the highly-gifted Sir Humphrey Davy, to resolve the hitherto undecomposed alkalis and earths into peculiar metals and oxygen. Besides this, Davy enriched the science by his investigations on galvanic electricity, flame, and the compounds of chlorine. At the same time also, Gay-Lussae and Thénard investigated the nature of the chlorine compounds and of the metals which form the bases of the alkalis, and were the first to effect the complete separation of fixed organic substances into their elements. Gay-Lussac also extended the science of Heat, particularly as relates to evaporation, established the Law of Volumes, which governs the combination of elastic fluids, and made a most accurate investigation of the chemical relations of iodine, the discovery of which had just before been made by M. Courtois.

The highest degree of exactness in the investigation of the proportions by weight in which bodies combine was attained by Berzelius, whose researches in this department of the science led him to adopt and confirm the almost forgotten views of Richter. By a vast number of experiments he ascertained the proportions by weight in which all the elementary bodies combine one with another. By improvements in Klaproth's methods of mineral analysis, he succeeded in decomposing several new minerals, and thereby established the existence of cerium, selenium, thorium, and partly also of lithium, as new elements. At the same time also, but with less reference to direct experiment, Dalton developed Richter's doctrine in the form of the new atomic theory: he likewise made some most able investigations upon heat, particularly on the formation of vapour.

Within the last few years, the electro-chemical phenomena have been most successfully investigated by Faraday, De Larive, Becquerel, and others; and the field of organic chemistry has, by the labours of Berzelius, Liebig, Wöhler, Mitscherlich (the founder of the theory of Isomorphism), H. Rose, Mulder, Chevreul, Dumas, Pelouze, Laurent, Malaguti, and many others,-been enlarged to such an extent, that it has become almost a new science.

COHESION.

Cohesive Power. Attraction of Aggregation. Cohäsionskraft. Zusammenhäufende Verwandtschaft. Cohæsio. Attractio aggregationis,

Cohésion.

The attraction which bodies of the same nature exhibit towards each other when brought into immediate contact. The consequence of this action is the combination of a number of homogeneous bodies into a whole of the same nature. The product so formed is called an Aggregate.

The combination may be destroyed by a mechanical pressure or thrust strong enough to overcome the cohesion.

The force of cohesion varies with the temperature and with the nature of the body.

It is a general rule that the cohesion of a body diminishes as its temperature increases. A heated liquid forms smaller drops than a cold one (sulphur alone forms an exception to this rule, increasing in consistence as its temperature rises); hard bodies when heated generally become softer, sometimes even fluid, provided no chemical changes take place in them.

Bodies may be divided according to their cohesive power into four classes. (1.) Imponderable Bodies, as light, heat, electricity and magnetism. In these the repulsive force the antagonist of the attractive predominates, and none of the phenomena which they exhibit are such as to make it probable that a mutual attraction or cohesion is exerted between their particles. (2.) Ponderable Elastic Fluids, viz. Gases and Vapours. In these, the ponderable matter is rendered so bighly elastic by its combination with heat that, generally speaking, all their known properties may be explained without supposing them to be possessed of cohesion, unless perhaps Faraday's experiments (Qu. J. of Sc. 3, 354, also Ann. Chim. Phys. v. 298) on the different velocities with which different gases flow through narrow tubes may be explained by supposing different degrees of cohesion to exist in the various gases. (3.) Liquids. These may be regarded as combinations of ponderable matter with smaller quantities of heat, sufficient to produce a remarkable diminution of the cohesion peculiar to solids, but not to occasion any sensible repulsive power. (4.) Solids. In these the cohesive power exhibits itself in its highest degree.

Cohesion of Liquids.

This force shows itself in the tendency towards the spherical form, inasmuch as the effort of all the several particles of a liquid mass to approach each other as closely as possible must result in the assumption of the spherical form. Since, however, the liquid mass is subject to the influence of other forces, gravitation and adhesion for instance, the spherical form can never be perfect: e. g., mercury upon glass, water upon glass which has been smeared with fat or lycopodium.

The cohesion of liquids is also shown by a certain resistance which they offer to the force of gravitation, when, in order to obey that force, they would be obliged to divide themselves into very small parts,-supposing that the cohesion is not at the same time opposed by adhesion or agitation. Thus mercury will not run through fine muslin, nor water through a metal sieve covered with fat or lycopodium.

The cohesion between the particles of a liquid being small and equal in all directions, the slightest force is sufficient to disturb them. The motion of one part does not produce an equal and parallel motion in all the rest of the mass, whereas in solid bodies the application of a slight force produces, not a disruption of the parts, but an equal and similar motion in the whole mass.

Different liquids exhibit different degrees of cohesion: the cohesive power is for the most part very nearly proportional to the density.

Cohesion of Solids.

This force shows itself particularly in three ways.

1. In an individual portion of a solid body.

In this case cohesion shows itself by the resistance which, up to a certain limit, it opposes to any mechanical force tending to separate the particles. Hence the force which is exactly sufficient to overcome the cohesion of a body affords a measure of that cohesion. (On the tenacity of metals, vid. Morveau, Ann. Chim. 71, 194; also Gilb. 34, 209.)

2. When separate masses of like nature come into close contact with one another.

If the surfaces are uneven, the points of contact are infinitely small, and the cohesive force imperceptible. But if the bodies touch each other by large flat surfaces, e. g., well polished glass or metal plates, the number of points of contact is greatly increased, and the cohesion becomes very evident. If the surfaces touched at all points, the cohesion of the bodies would be as powerful in this direction as in any other.

3. When bodies pass gradually from the liquid to the solid state. In this case the particles unite.

Or,

(1.) In masses of indefinite external form, which refract light singly in every direction, are equally easy or equally difficult to separate in all directions, and when broken exhibit a conchoidal fracture; Amorphous bodies, such as glass and gum.

(2.) In masses of geometrical shape, bounded by flat surfaces, and having solid and dihedral angles of constant magnitude for the same matter and form,-having as great a degree of transparency as is consistent with the nature of the matter of which they are formed, for the most part, refracting light doubly in certain. directions and finally, presenting greater facilities of separation or cleavage in certain directions lying in determinate planes than in others; Crystals.

Many bodies are more disposed to the amorphous, others to the crystalline form: a great many may however be obtained in one state or the other, according to circumstances.

CRYSTALLIZATION.

To enable a body to crystallize, it must first be brought into the liquid or gaseous state. This is effected either by elevation of temperature, in consequence of which the body melts (Copper, Bismuth) or sublimes (Sal-ammoniac, Iodine),-or else by causing the body to combine with another ponderable body, which either at the ordinary or at a somewhat higher temperature is liquid or gaseous (salts dissolved in water, sulphur in sulphuret of carbon, sulphate of baryta in oil of vitriol, camphor, benzoic acid, &c. in alcohol, iodine in hydriodic acid gas):-the causes which have brought the body into the fluid state must subsequently be removed.

To

1. If heat has been the only liquefying power, or if it has served to render a solid more soluble in a fluid, cooling must be resorted to. obtain definite crystals from a melted mass (sulphur or a metal for instance), it is allowed to cool to a certain extent only, and the yet remaining liquid portion, which if left in contact with the first formed crystals would unite with them into an amorphous mass, poured out. The vapour of sulphur, iodine or sal-ammoniac is conducted into a colder part of the apparatus, where it deposits crystals. A warm solution of various salts in water, of sulphur in sulphuret of carbon, of camphor, &c.