Chemists distinguish various kinds of affinity. Single affinity is that in which two bodies unite to form a binary compound. These may be two simple substances, as when oxygen and iron unite to form oxide of iron, or sulphur and copper to form sulphuret of copper; or two compounds may unite, as ulphuric acid and oxide of iron, to form sulphate of iron. But a simple substance does not often unite with a compound one; thus sulphuric acid does not unite with iron, nor does sulphur unite with oxide of copper. If we oxidise the iron, the acid unites with it readily; and if we deoxidise the copper, the sulphur will unite with it. There are, however, exceptions to this rule.

Affinity is also spoken of as being elective. Thus if dilute nitric acid be poured upon a mixture of lime and magnesia, the acid will unite with the lime in preference to the magnesia. Hence it is said, that nitric acid has a greater affinity for lime than for magnesia, and the fact may be shown in another way. If lime water be added to a solution of nitrate of magnesia, nitrate of lime is formed and magnesia is thrown down as an insoluble precipitate. This is a case of single elective affinity. There is another mode by which compounds are formed, namely, by double decomposition or double elective affinity. This is very useful in obtaining compounds which could not otherwise be procured, in many cases, without great difficulty. For example, nitrate of baryta and sulphate of soda are salts soluble in water. If the solutions of these salts be mingled together, two new compounds are formed, one of which is soluble and the other not. The change that takes place will be understood from the following diagram, in which the

substances before being mixed are shown in the parallel lines, and after mixture in the diagonal lines. The nitric acid quits the baryta to unite with the soda, forming nitrate of soda, which remains in solution; and the sulphuric acid quits the soda to unite with the baryta, forming sulphate of baryta, which is an insoluble precipitate. Some of the processes of the dyer depend upon such a case as this, which may be still more completely illustrated by the following diagram, where the solutions before being mixed are placed on the outside of the perpendicular lines, their component parts are shown within them, and the new compounds are stated on the out

side of the horizontal lines.

It was formerly supposed that the relations of affinity were fixed and constant between the same

Professor Fownes remarks, that "the order pointed out in these lists, is now acknowledged to represent the order of precedence for the circumstances under which the experiments were made, but nothing more. So soon as these circumstances become changed, the order is disturbed. The ultimate effect, indeed, is not the result of the exercise of one single force, but rather the joint effect of a number, so complicated and so variable in intensity, that it is seldom possible to predict the consequences of any yet untried experiment."

There is an extensive class of chemical actions which have been grouped together under the general title of disposing affinity. A familiar example of this occurs in the preparation of hydrogen gas from zinc or iron and dilute sulphuric acid. A piece of polished zinc or iron remains perfectly bright under water for any length of time, and does not show any tendency acid be added, the water begins to be decomposed, to decompose it. But if a small quantity of sulphuric the metal, and this is dissolved by the acid as fast as hydrogen is freely evolved, a film of oxide forms on it is formed. This appears to be the use of the acid in this experiment, but it is difficult to explain why the oxide should be produced when the acid is present and not otherwise.

Chemical affinity is promoted by many circumstances, especially by diminishing the cohesive attraction of the bodies to be acted on. Thus a lump of marble in dilute hydrochloric acid slowly wastes away, but by reducing the marble to powder, the action is very

(1) Fownes' Manual of Chemistry, 1848

Many bodies will only combine in what is called the nascent state, or at the moment of separation from combination. Thus carbon and nitrogen will not combine with gaseous hydrogen; but when these bodies are simultaneously liberated from some previous combination they unite readily, as when organic matters are destroyed by heat or by putrefactive fermentation.

Affinity is in most cases greatly promoted by heat. Melted sulphur will not combine with carbon, but by raising the sulphur to the state of vapour, and bringing it in contact with red-hot charcoal, these bodies combine and form sulphuret of carbon, a limpid, colourless liquid. Charcoal requires to be made redhot before it will burn in oxygen gas, that is, before it will combine with oxygen. At ordinary temperatures, oxygen may be mixed with hydrogen and other inflammable gases without combining with them, but on the approach of flame or the electric spark, combination takes place immediately. Cold water dissolves many salts only to a limited extent, but the application of heat greatly increases the solvent power of the water. There are, however, some salts which dissolve more freely in cold than in hot water, but these form exceptions to the general rule. Heat also favours decomposition as well as affinity. For example, if mercury be heated in contact with the atmosphere, it is converted into a peroxide by combining with the oxygen of the air; now if this compound be again heated to a higher temperature than was required for its formation, it is decomposed; oxygen is given off, and the mercury returns to the metallic state. So also, if iron filings be heated to redness in a porcelain tube, and vapour of water be passed over them, the water is decomposed, its oxygen combining with the iron to form an oxide, and its nydrogen escaping from the extremity of the tube. If, on the contrary, oxide of iron be heated in a tube and a stream of dry hydrogen be passed over it, the metal is immediately reduced by the hydrogen uniting with the oxygen of the oxide, and forming water, which escapes as a jet of steam from the extremity of the tube." In these experiments," says Fownes, "the affinities between the iron and oxygen and the

"Electricity has considerable influence over chemical affinity. A mixture of oxygen and hydrogen instantly combines by passing an electric spark through it, but this is probably a mere effect of heat. By passing a series of electric sparks through a mixture of oxygen and nitrogen, nitric acid is formed. A portion of this acid also appears to be formed in the atmosphere during a thunder storm, and the action of electricity in these cases is probably more specific than that of heat. But of all the various states of electricity, galvanism is the most interesting in a chemical point of view. By means of the voltaic pile or battery, a large number of chemical compounds have been decomposed. When, for example, two platina wires are connected with the poles or electrodes of a voltaic battery, and their unconnected ends are immersed in water, oxygen gas is evolved at the positive pole, or, as it is now called, the anode,' and hydrogen gas is evolved at the negative pole or cathode. So, also, when saline solutions are submitted to the action of the battery, acids are developed at the anode, and alkaline bases at the cathode.

Now it is a law of electricity, that bodies dissimilarly electrified attract each other; and as in these decompositions oxygen, chlorine, acids, &c. invariably go over to the positive pole, and hydrogen, the metals, inflammable substances in general, and the alkalies, appear at the negative pole, it has been supposed that chemical affinity, or that force with which bodies combine, is a consequence of their being in opposite electrical states; that an acid being negative, and an alkali being positive, unite to form a salt, and that when the union is once effected, the electricity of the compound exists in a neutral state, or in a state of equilibrium. When, however, the superior force of the voltaic battery is brought to bear upon such a compound, its constituents separate and return to the original electric state which they had before combination. Hence it has been supposed, that when substances or their atoms are similarly electrified, they refuse to combine, but that they may be made to do so by communicating to them opposite electrical states. This theory has been useful to chemistry, but it is by no means certain that chemical affinity is identical with electrical attraction. According to Dr. Faraday, chemical affinity is merely a result of the electrical state of the particles of matter. He has found that when bodies are combined or decomposed by an electric current, the composition and the decomposition are always effected according to the laws of definite proportions; and

(1) That is, the surface at which the electric current enters the electrolyte.

(2) The surface at which the electric current leaves the body under decomposition

Light has also considerable influence in controlling chemical affinity. Hydrogen and chlorine gases mixed and exposed to the sun's rays combine with explosion, and form hydrochloric acid. Chlorine and carbonic oxide gases exert no action on each other until they are exposed to the light, when they combine and form phosgene gas. The beautiful chemical arts of PHOTOGRAPHY and DAGUERREOTYPE depend upon the action of light upon chemical substances.

AGATE, from axárns, a stone, said by Theophrastus to come from the river Achates, in Sicily, now called the Drillo, in the Val di Noto. It is one of the numerous forms in which silica is arranged, and contains 98 per cent. of that mineral. It is not transparent, like rock-crystal, but almost opaque, with a resinous or waxy fracture, and with various shades of colour produced by the presence of minute portions of iron. When agates are cut open they display a singular variety of forms, in some cases resembling animals and plants, in others zigzag lines like the plan of a modern fortification, and hence this variety is called Fortification Agate. These lines are the edges of successive layers or deposits of the mineral during the process of its formation. As the internal surfaces are capable of receiving a high degree of polish, agates are much valued as ornamental stones. They are extensively used at Paris and elsewhere, in the manufacture of cups, rings, seals, handles for knives and forks, sword-hilts, beads, smelling-bottles, snuff-boxes, and other articles. Burnishers are also formed of agate, for the use of the bookbinder and other mechanics. Agates are usually met with in that variety of trap-rocks called

Fig. 14. THE AGATE.

amygdaloid: they form in it detached rounded nodules, not cemented to the rock, but easily separable from it, having generally a thin layer of green earth interposed, and a rough, irregular exterior. Agates are also met with as loose pebbles in the beds of rivers or in gravel, and they vary in size, from that

common size is 1, 2, or 3 inches in diameter. The colours of agate may be darkened by boiling the stone in oil, and then dropping it into sulphuric acid. A little oil is absorbed by some of the layers, and this becomes blackened or charred by the acid. Fig. 14 is a copy of a good specimen of an agate with chalcedony.

There are various other siliceous stones closely allied to agate, and not to be distinguished from it in chemical composition, except as respects the colouring matter of one or two of them. These are, 1. Carnelian, so called from the Latin carnis, flesh, some of the most common varieties being of a flesh colour. There are, however, various shades of red and yellow, but the deep, clear red is the rarest, and most valuable. The colours may be deepened by exposing the stones for several weeks to the sun's rays. The chief supply of carnelians is from Japan; they are also imported from Bombay, after being collected in the province of Guzzerat; but the best varieties are said to come from the gulf of Cambay. Many antique gems are in carnelian, and the stone is now much used for seals and beads. The Japanese cut beads of it into the form of the fruit of the olive. 2. Calcedony, so called from being found at Chalcedon, in Bithynia, opposite to Constantinople. This is a gem of a uniform milky white, or pale yellow colour; it often has a wavy structure, and a peculiar blistered surface. It is found abundantly in the Faroe Islands, in Iceland, Cornwall, and many other places. It sometimes occurs in large masses, of which cups and vessels are formed. 3. Onyx. In this variety of agate, the siliceous particles are arranged in alternate flat layers, of an opaque white and translucent grey or brown colour, resembling the marks on the human nail, whence its name from the Greek word ovvg, a nail. Some of the most beautiful of the ancient cameos were executed in this material, the figure being cut out of the opaque white, while the dark parts formed the ground; or the white parts formed the ground, and the dark parts the figure. 4. Sardonyx, a variety of onyx from Sardes, in Lydia, or, as some say, from Sardo, the Greek name for Sardinia. In this stone, the opaque white alternates with a rich, deep orange brown, which, when considerably translucent, greatly adds to its value. 5. Mocha stones and moss agates are transparent varieties of calcedony, the section of which exhibits various forms produced by iron, manganese, bitumen, and chlorite or green earth, but sometimes, also, by the presence of real vegetable bodies, such as confervæ and mosses. Mocha stones are so called from having been brought from Mocha in Arabia. 6. Blood-stone, a green agate coloured by chlorite with numerous bright red spots like drops of blood. It is also called Heliotrope, and Oriental Jasper. 7. Chrysoprase, from xpúoreos, beautiful, and párov, a leek; a variety of calcedony found in Silesia; its colour, which is of an apple-green, is due to oxide of nickel. 8. Plasma, a green semitransparent calcedony, of a dark tint sprinkled by yellow and whitish dots. It is coloured by chlorite.

The figured agates of commerce are chiefly obtained from Oberstein, a small town in the valley of the Nahe, not far from Mayence. The business of cutting and polishing the agates, occupies a considerable number of the inhabitants. The surface is first coarsely ground by large mill-stones of a hard, reddish sandstone, moved by water-wheels, in numerous small mills scattered along the stream. The polish is afterwards given on a wheel of soft wood, moistened and imbued with a fine powder of hard red tripoli, found in the neighbourhood. Agates are found in many parts of Scotland, especially at the Hill of Kinnoul, near Perth; they are hence called Scotch pebbles.

In the mineralogical collection of the British Museum (Room IV.), is a specimen of Globular or Fig. 15. Egyptian Jasper, which exhibits in the two fractural surfaces a likeness of the poet Chaucer. Fig. 15 is an accurate copy of this curious specimen. In the same case [No. 24] are other specimens of siliceous minerals, the lines of which fall into the shapes of animals,

&c.

AIR, (from the Greek and Latin aer,) a term now limited to the atmosphere, although formerly applied to various gases; thus oxygen was called vital air; hydrogen, inflammable air; carbonic acid, fixed air; ammoniacal gas, alkaline air, &c. The atmosphere or sphere of gases (drupoî) is the general term applied to the whole gaseous portion of this earth. Being much lighter than either land or water, it floats or rests upon them, and rises to the height of probably 40 or 50 miles above the sea-level. It consists essentially of two gases, OXYGEN and NITROGEN, in a state of mechanical mixture. One hundred parts by weight, contain 77 parts nitrogen and 23 parts oxygen; or by measure, 79.19 nitrogen, and 20.81 oxygen. So that if we mix 1 volume of oxygen with 4 volumes of nitrogen, we get a mechanical compound almost identical with pure atmospheric air. Oxygen is remarkable for its active properties: it promotes combustion, respiration, and other chemical changes, with great energy. Nitrogen, on the contrary, is inert; it supports neither respiration nor combustion, and its chief use in the atmosphere seems to be to dilute the oxygen. During the processes of respiration and combustion, a quantity of carbon is set free, every 6 parts of which, by weight, unite with 16 parts by weight of oxygen, and form a compound gas, called CARBONIC ACID, which is always present in the atmosphere in small but varying quantities. In 10,000 volumes of atmospheric air, the mean proportion of carbonic acid is only 5 volumes, and this varies from 6.2 as a maximum, to 3.7 as a minimum. Near the surface of the earth, the proportion of carbonic acid is greater in summer than in winter, and

during night than during day. It is also rather more abundant in elevated situations, as on the summits of high mountains, than in the plains; and, although this gas is considerably heavier than its own bulk of pure atınospheric air, (its specific gravity being about 1.52, air being 1,) yet it appears to be diffused through the whole mass.

By the evaporation of the waters of the earth, and of moist surfaces, the atmosphere is constantly supplied with a quantity of aqueous vapour. In 100 parts by weight of atmospheric air, the mean quantity of watery vapour is nearly one part and a half. The amount, however, varies with the temperature. At 50°, the mean temperature of England, the air contains th of its weight of water in an invisible state without forming cloud, mist or rain. If it contain more than this, it is precipitated in a visible form. At a higher temperature a larger quantity of vapour may remain invisible; thus, at 82°, the mean temperature of the equator, the air may contain as much as d of its weight of invisible steam; and air that contained only th would be injuriously dry, though the same air cooled down to 50°, would be at its maximum of humidity. (HYGROMETER.)

As the sea contains a little of everything that is soluble in water, so the atmosphere contains a little of everything capable of existing in the gaseous form at common temperatures. Ammonia is always present, and is supposed to be the source of nitrogen in plants; while in crowded cities, and in the neighbourhood of gas works, smelting furnaces, sewers, stagnant pools, sulphur-springs, &c. there is much local contamination of the air from the presence of different gases. Various forms of infection, malaria, and marsh-miasma, probably arise from the presence of noxious gases in the air. Berzelius states that in the first experiments which he made upon seleniuretted hydrogen, he let up into his nostrils a bubble about the size of a pea. "It deprived me so completely of the sense of smell, that I could apply a bottle of concentrated ammonia to my nose without perceiving any odour. After 5 or 6 hours I began to recover the sense of smell, but a severe catarrh remained for about 15 days." On another occasion a little of the gas accidentally escaped; it produced a sharp sensation in the nose, red eyes, and a dry and painful cough, which at length was succeeded by expectoration, tasting like the vapour from a boiling solution of corrosive sublimate. "These symptoms were removed by a blister to my chest. The quantity of seleniuretted hydrogen gas, which on each of these occasions entered into my organs of respiration, was much smaller than would have been required of any other inorganic substance whatever to produce similar effects." Dr. Prout quotes these facts to show how small a quantity of accidental ingredients diffused in the atmosphere may produce powerful effects in the human system, and may even be the origin of influenza, and other epidemic disorders.

With respect, however, to the two essential ingredients of the atmosphere, thev always exist in the