observation, that though its form is irregular, yet, internally, cavaties are found in it having the forms of the cube and octohedron. When newly broken it possesses great metallic lustre, but soon becomes black if exposed to the influence of the atmosphere. It yields a white smoke before the blow-pipe, having an arsenical odour.

Arsenic has also been met with in veins of mountains in several localities in Europe, accompanying_cobalt, copper, iron, silver, and several other metals. It is found in Germany chiefly combined with cobalt ores. Arsenic is also united with, and contaminates several of the other metallic ores, in England.

Arsenical pyrites, from which the white oxide is obtained, occurs abundantly in Cornwall and Devonshire; it is also found at Alva, in Stirlingshire; and in Norway, Sweden, Bohemia, Hungary, Saxony, and other localities on the Continent.

There are two sulphurets of arsenic. The first, consisting of 65 parts (in 100) of arsenic and 35 parts of sulphur, and known by the name of realgar, is of a scarlet or dark orange colour, tasteless, and not so poisonous as arsenic itself. It is met with in Bohemia and Saxony, and is much used as a pigment, and in the manufacture of fireworks. The second sulphuret contains about 58 per cent. of arsenic, and 42 per cent. of sulphur; it is called orpiment, and is found in Hungary, China, and South America. Realgar is always found in primitive mountains, whereas orpiment generally occurs in secondary mountains. Orpiment may be formed artificially by pouring a solution of sulphuretted hydrogen into water in which arsenic has been dissolved; it possesses a fine lemon colour, and is largely employed in dyeing and calico-printing. The pigment called king's yellow is made from yellow orpiment. Both king's yellow and the white oxide of arsenic are used in colouring glass; but glass prepared with any of the compounds of arsenic, in large quantities, should never be used for domestic purposes.

Arsenic, in its pure metallic state, is a metal of a greyish white colour and strong metallic lustre, somewhat resembling polished steel; but, if exposed to the atmosphere, it soon loses its brilliancy, becoming first yellow and afterwards almost black; it undergoes no change, however, when kept under water. It is, of all metals, the most

brittle, and may readily be pulverized by trituration in a mortar. If a piece of arsenic be broken, it exhibits a laminated texture. Besides being the most brittle, this metal is, at the same time, one of the softest of that class of elementary substances to which it belongs. Arsenic has no smell while cold, but when heated to 356 degrees Fahrenheit, it becomes volatilized without being melted, the vapour possessing a strong smell not unlike that of garlic, and proving exceedingly poisonous, when inhaled, even in the minutest quantity. When sublimed, it is converted into the protoxide which crystallizes in tetrahedrons. If its crystals be heated without the contact of the atmosphere, they immediately change into a colourless gas, which condenses again into crystals. If a piece of arsenic be thrown upon bot iron, it will ignite and burn with a blue flame and a dense whitish smoke, emitting a very strong powerful odour resembling that of garlic. The specific gravity of arsenic is about 5.884; some, however, make it only 5.682.

Oxygen and arsenic very readily unite with each other. Two combinations of these bodies are known; namely, arsenious acid, and arsenic acid; the former, which is the white arsenic of commerce, may be produced by exposing the metal to a moderate heat, by which it is volatilized, united with oxygen, and converted into the white oxide; it is a hard, brittle substance, with a glassy texture. Notwithstanding this acid is one of the most virulent poisons; when administered judiciously in small doses, it is a medicine of great value for certain complaints, such as ague, for example, forming, as it does, a notable ingredient in what are termed ague-drops. By mixing this oxide with black flux, and exposing it to heat, it may be again reduced to a metallic state. It is a compound of 752 arsenic, and 24.8 oxygen; its specific gravity varies from 3.7 to 5. It is very readily soluble in water-in fact, remarkably so. Its taste is acrid and sweet; it sublimes at 383 degrees Fahrenheit, and it crystallizes in tetrahedrons. It destroys the magnetic property of all the metals which are susceptible of that property.

This oxide is chiefly obtained from the cobalt works of Bohemia and Saxony, where zaffra is made; cobalt ores containing a large proportion of arsenic. It is originally

prepared in cakes, brittle, white, rather sweetish in taste, and more or less translucent; these cakes undergo sublimation, when intended for medicinal purposes, in order that the arsenious acid may be separated from the sulphur or other impurities which the cakes may contain. It is generally sold by chemists in the form of a white, gritty powder.

Arsenic acid, the peroxide of arsenic, which, according to the manner in which it is prepared, is either in the form of a white solid mass or of the consistence of jelly, was, it is said, first known to Scheele; it contains 65.4 per cent. of arsenic, and 34.6 per cent. of oxygen. It may be procured by dissolving the white oxide in nitro-muriatic acid, evaporating to dryness, and applying sufficient heat to drive off the acids. Thus obtained, it is a white, solid mass, almost tasteless, slowing dissolving in cold, but rapidly in warm water, and having a specific gravity of 3.391. When melted at a red heat it is converted into glass. It possesses all the properties of an acid, and, like arsenious acid, is very poisonous.

Arsenic may be united with so large a proportion of oxygen as to become changed into a perfect acid. Arsenic, chromium, columbium, molybdenum, and tungsten (?), are the only metals, we believe, which are completely acidifiable.

This metal may be also combined with hydrogen, with chlorine, with iodine, with phosphorus, and with sulphur (as we have noticed before). Hydrogen and arsenic, when united, form a permanently elastic and invisible fluid, somewhat lighter than atmospheric air, and known as arsenuretted hydrogen gas. This gas has a foetid smell; it is incapable of supporting combustion, and consequently would destroy animal life. It is in itself combustible. Arsenic, when combined with chlorine gas, burns with much brilliancy, and the result is chloride of arsenic. Phosphorus and arsenic readily combine, and form together a brilliant black powder. Sulphur and arsenic speedily unite, when fused together, and form a red, vitrious, semi-transparent mass, which is a true sulphuret of arsenic, and is known as realgar (the composition of which has already been given). The ancients used a sulphuret of arsenic, both as a medicine and a paint; sandarache, mentioned by Aristotle in his writings, was a sulphuret of this metal. The term arsenicon,

signifying "masculine " (no doubt conferred on the mineral on account of its powers), appears to have been first used by Dioscerides at the commencement of the Christian era.

Although chloride of arsenic was made by the old chemists, and was known by them as butter of arsenic, yet the salts which have been formed with this metal are few. Arsenite of potash may be procured by boiling caustic potash with white arsenic-the white oxide of arsenic, or, more accurately, arsenious acid. The arsenite of soda may be prepared in a similar manner. Several arsenites may be obtained by double decomposition: for instance, the arsenite of copper may be formed by adding a solution of sulphate of copper to one of arsenite of potash; and the arsenite of lead by merely mixing a solution of nitrate of lead with a solution of arsenite of potash. None of the salts of arsenic (considered as a base) have been found native; but we are acquainted with a large number of native salts in which arsenic occurs as an acid; among others may be mentioned the arsenites of cobalt, copper, iron, and lime. Both the arsenious and the arsenic acids form salts with metallic oxides, the former with bases forming the class called arsenites, the latter arseniates.

Most of the other metals may easily be alloyed with arsenic; it certainly promotes their fusibility, but it destroys their malleability and renders them brittle, even in very small quantities; hence, how necessary it is, in the reduction of their ores, to free the metals from every particle of arsenic! This metal occurs in a very large number of natural alloys, but, fortunately, the property which arsenic possesses cf easy volatilization, greatly facilitates its separation from other metals.

Arsenic is employed to whiten copper. Alloyed with copper, it forms a malleable and flexible alloy, which takes a fine polish, and is therefore of great use for specula of reflecting telescopes, and other optical instruments. Arsenic is also used in making small shot, because it renders the lead more brittle, and better capable of granulating. Its oxides are used in many processes by the dyer, besides as fluxes for glass, and in several of the arts. Its sulphurets form valuable pigments of different colours, and are largely employed in calico-printing, on account of its property of deoxidizing indigo, and thereby rendering it soluble in an alkali.

This metal, upon the whole, is perhaps one of the least useful to man of all the metallic substances which he has discovered.

In its pure metallic state, arsenic has little effect upon the system; but when combined with oxygen it becomes a virulent poison.

CHAPTER VII.

BARIUM.

THE next metal which claims our attention, is known by the name of Barium. Barium was first known by Sir Humphrey Davy, who discovered it in the year 1807. It is the basis of an alkaline oxide or earth called barytes; this earth, also known as baryta, was so named from the Greek adjective barys (which signifies heavy), on account of its great density. It may be procured by strongly heating baryta in an iron tube, through which the vapour of potassium is conveyed; the barium can then be extracted from the oxide of potassium by amalgamating it with mercury, which may afterwards be driven off by means of heat. It is a malleable metal of a vellow colour. When exposed to the atmosphere it gradually absorbs oxygen it, and becomes covered with a thin coating of baryta. If it be moderately heated, in contact with the air, it burns with a deep blue flame; it melts below a red heat, and is volatilized at the highest possible temperature. It rapidly decomposes water, giving out hydrogen, and is converted into baryta. It is between four and five times as heavy as water, and has been employed in the manufacture of certain kinds of glass.

Barium and oxygen unite in two proportions.

from

The protoxide of barium, barytes, or baryta, is very abundant in nature; it is usually found mixed with carbon and sulphur, forming the veinstone in several lead mines.