of the sticks. The jar should be loosely covered, and the air renewed by blowing into it occasionally. It should also be placed in a situation where no injury would ensue if the phosphorus were to take fire and crack the jar. Of course ozone will be perceived in abundance in the air of the jar, and in the course of a few days the water will be highly charged with phosphorous and phosphoric acids, and will have absorbed a considerable amount of oxygen from the air. If a test-tube or small cylinder be filled with this water, a little powdered binoxide of manganese thrown into it, and the tube then quickly closed and inverted with its mouth under water, brisk effervescence will ensue from the escape of oxygen which will collect in the tube, and may be recognised by the usual test with a partly extinguished match. The binoxide of manganese does not appear to be decomposed in this experiment, the whole of the oxygen being derived from the binoxide of hydrogen dissolved in the water, which is immediately decomposed, by contact with the binoxide of manganese, into water and free oxygen. If a solution of permanganate of potash (KO.Mn2O), which owes its red colour to the permanganic acid (Mn,O,), be poured into a cylinder partly filled with the liquid, it will cause a rapid evolution of oxygen, derived not only from the binoxide of hydrogen, but from the permanganic acid, the red colour of which disappears, because it becomes reduced to a lower oxide of manganese.

The usual method of preparing binoxide of hydrogen in a pure state, consists in decomposing the binoxide of barium with diluted hydrochloric acid, under certain precautions to avoid the decomposition of the very unstable binoxide of hydrogen. Its formation is represented by the equation BaO2 + HCl HO2+ BaČl. The chloride of barium is removed from the solution by the cautious addition of sulphate of silver, which precipitates the barium as sulphate of baryta, and the silver as chloride of silver, thus, BaCl + AgO. SO, = AgCl + BaO. SO3. The precipitates are allowed to subside, and the clear liquid evaporated in the exhausted receiver of the air-pump over a dish of oil of vitriol to absorb the water, which evaporates much more rapidly than the binoxide. The pure binoxide of hydrogen is a syrupy liquid of sp. gr. 1-453, with a very slight chlorous odour. Its most remarkable feature is the facility with which it is decomposed into water and oxygen.* Even at 70° F. it begins to evolve bubbles of oxygen, so that it can scarcely be prepared in hot weather. At 212° it decomposes with violence. The mere contact with certain metals, such as gold, platinum, and silver, which have no direct attraction for oxygen, will cause the decomposition of the binoxide of hydrogen, without any chemical alteration of the metal itself. It was noticed above that the binoxide of manganese decomposes it without undergoing any apparent change. The most surprising effect is that which takes place with oxide of silver. If a drop of binoxide of hydrogen be allowed to fall upon oxide of silver, which is a brown powder, decomposition takes place with explosive violence and great evolution of heat, the oxide of silver losing its oxygen, and becoming grey metallic silver. The oxides of gold and platinum are acted upon in a similar manner.

These very extraordinary changes, which were formerly described as catalytic actions, are now generally accounted for by the hypothesis that the oxygen in the oxide of silver, &c., exists in a condition different from that of the second equivalent of oxygen in the binoxide of hydrogen, and that these two conditions of oxygen have a chemical attraction for each other, similar to that which exists between different elements. If the oxygen in the oxide of silver be represented as electronegative oxygen (see 22), as its relation to the metal would lead us to expect, and the second atom of oxygen in the binoxide of hydrogen be represented as electropositive oxygen, the mutual decomposition of the two compounds might be represented by the equation,

AgO + HOQ = Ag + H0 + 0Q.

Molecules-Molecular formula.-This would lead to the belief that oxygen in its ordinary condition, as it exists in the atmosphere, is really an oxide of oxygen, consisting of two atoms of oxygen in opposite states,

The presence of a little free acid renders it rather more stable, whilst free alkali has the opposite effect. A solution of peroxide of hydrogen, containing a little hydrochloric acid, is now sold for medicinal and photographic uses.

† Such inexplicable changes as this are sometimes included under the general denomination of catalysis, or decomposition by contact.

52

CONSTITUTION OF OZONE.

and that the smallest particle of oxygen which can exist in the separate state is really composed of two atoms. This smallest particle of free oxygen would be appropriately termed a molecule of oxygen, whilst an atom of oxygen would be defined as the smallest particle which can exist in a state of combination. If the atomic weight of oxygen were taken to be 16, the molecular weight would be 32. It will be seen hereafter that there are reasons for extending this view to the constitution of some other elements, and an opinion has been propounded, somewhat in advance of existing experimental evidence, that direct combination of elements is really a double decomposition where the corresponding atoms are exchanged. According to this view, the formation of water by the combination of hydrogen with oxygen would be expressed by the equation,

and the molecule of water, or the smallest particle capable of existing in a free state, or of resulting from chemical action, would be represented by H2O, and would weigh 18 parts (H= 1), or if = 16 parts of oxygen, by H Ꮎ.

It has been suggested that ozone is really the negative atom of oxygen detached

from the positive atom or antozone associated with it in the molecule ( f), and

this view is supported by the circumstance, that binoxide of hydrogen appears to be formed in all cases where ozone is produced by slow oxidation in the presence of water, making it appear probable that the latter (HO) combines with the antozone to form binoxide of hydrogen (HOP) whilst the ozone O is eliminated in the free state. The production of ozone in the electrolysis of water (see 23), appears also to be attended by that of binoxide of hydrogen. Upon this view of the nature of ozone, however, it would not be easy to explain the contraction which pure dry oxygen has been found to suffer during partial conversion into ozone by the action of the electric discharge, or the circumstance, that when a mixture of oxygen and ozone so produced is exposed to the action of mercury, no diminution of volume is observed, although the metal removes the ozone, combining with it to form an oxide of mercury. Both these, however, would be explicable on the theory that ozone is really formed by a coalition of atoms of oxygen, to produce a compound which may be represented as binoxide of hydrogen (HÖÖ), in which the hydrogen is replaced by oxygen, forming (000); then, just as two volumes of hydrogen combining with one volume of oxygen contract to two volumes of steam, it might be supposed that two volumes of oxygen combining with one volume of oxygen, would contract to two volumes of ozone, and just as on decomposing two volumes of steam with a metal, two volumes of hydrogen are left, so on decomposing the two volumes of ozone with a metal two volumes of oxygen would be liberated, and no contraction observed. * Reasoning analogically from the properties of binoxide of hydrogen, this theory would also enable us to explain the easy reconversion of ozone by heat, and by the action of binoxide of manganese.† The occurrence of binoxide of hydrogen in so many cases of the production of ozone would also favour this view.

Some recent experiments have indicated that the specific gravity of ozone is just what this theory would require, that is, half as great again as that of ordinary oxygen, or 1.66. + Finely divided platinum, which causes decomposition of binoxide of hydrogen, has also been found to destroy ozone.

CARBON.

50. This element is especially remarkable for its uniform presence in organic substances. The ordinary laboratory test by which the chemist decides whether a substance under examination is of organic origin, consists in heating it with limited access of air, and observing whether any blackening from separation of carbon (carbonisation) ensues.

Few elements are capable of assuming so many different aspects as carbon. It is met with transparent and colourless in the diamond, opaque, black, and quasi-metallic in graphite or black lead, velvety and porous in wood-charcoal, and under new conditions in anthracite, coke, and gas-carbon.

In nature, free carbon may be said to occur in the forms of diamond, graphite, and anthracite (the other varieties of coal containing considerable proportions of other elements).

Apart from its great beauty and rarity, the diamond possesses a special interest in chemical eyes, from its having perplexed philosophers up to the middle of the last century, notwithstanding the simplicity of the experiments required to demonstrate its true nature. The first inkling of it appears to have been obtained by Newton, when he perceived its great power of refracting light, and thence inferred that, like other bodies possessing that property in a high degree, it would prove to be combustible ("an unctuous substance coagulated"). When this prediction was verified, the burning of diamonds was exhibited as a marvellous experiment, but no accurate observations appear to have been made till 1772, when Lavoisier ascertained, by burning diamonds suspended in the focus of a burning-glass, in a confined portion of oxygen, that they were entirely converted into carbonic acid gas. In more recent times this experiment has been repeated with the utmost precaution, and the diamond has been clearly demonstrated to consist of carbon in a crystallised state.

A still more important result of this experiment was the exact determination of the composition of carbonic acid, without which it would not be possible to ascertain exactly the proportion of carbon in any of its numerous compounds, since it is always weighed in that form.

The most accurate experiments upon the synthesis of carbonic acid have been conducted with the arrangement represented in fig. 47.

Within the porcelain tube A, which is heated to redness in a charcoal fire, was placed a little platinum tray, accurately weighed, and containing a weighed quantity of fragments of diamond. One end of the tube was connected with a gas-holder B, containing oxygen which was thoroughly purified by passing through the tube C, containing potash (to absorb any carbonic acid and chlorine which it might contain), and dried by passing over pumice soaked with concentrated sulphuric acid in D and E. To the other end of the porcelain tube, A, there was attached a glass tube F, also heated in a furnace, and containing oxide of copper, to convert into carbonic acid any carbonic oxide which might have been formed in the combustion of the diamond. The carbonic acid was then passed over pumice soaked with sulphuric acid in G, to remove any traces of moisture, and afterwards into a weighed bulbapparatus H, containing solution of potash, and two weighed tubes I K, containing, respectively, solid hydrate of potash and sulphuric acid on pumice, to guard against the escape of aqueous vapour taken up by the excess of oxygen in its passage through the bulbs H. The increase of weight in H, I, K, represented the carbonic acid formed in the combustion of an amount of diamond indicated by the loss of weight suffered by the platinum tray, and the difference between the diamond consumed and the carbonic acid formed would express the amount of oxygen which had combined with the carbon. A large number of experiments conducted in this manner, both with diamond and graphite, showed that 6 parts of carbon furnished 22 parts of carbonic acid, and consumed, therefore, 16 parts of oxygen.

The ordinary mode of exhibiting the combustion of the diamond on the lecture

54

COMBUSTION OF DIAMOND.

table, consists in suspending it within a double loop of platinum wire attached to an iron wire passing through a deflagrating-collar, and heating it in a jet of oxygen

sent through a gas or spirit flame (fig. 48). As soon as it has attained a white heat, the diamond is plunged into a globe of oxygen, and after burning for a few seconds, it is withdrawn, and a little lime-water is shaken in the globe to produce the milky

deposit of carbonate of lime. It not unfrequently happens that the blowpipe flame fuses the platinum wire, and the diamond drops out before it can be immersed in the oxygen. A more convenient arrangement is shown in fig. 49. The diamond is supported in a short helix of platinum wire A, which is attached to the copper wires B B, passing through the cork C, and connected with the terminal wires of a Grove's battery of five or six cells. The globe having been filled with oxygen by passing the gas down into it till a match indicates that the excess of oxygen is

streaming out of the globe, the cork is inserted, and the wires connected with the battery. When the heat developed in the platinum coil, by the passage of the current, has raised the diamond to a full red heat, the connexion with the battery may be interrupted, and the diamond will continue to burn with steady and intense brilliancy.

Fig. 49.

To an observer unacquainted with the satisfactory nature of this demonstration, it would appear incredible that the transparent diamond, so resplendent as to have been reputed to emit light, should be identical in its chemical composition with graphite (plumbago or black lead) from which, in external appearance, it differs so widely. For this difference is not confined to their colour; in crystalline form they are not in the least alike, the diamond occurring generally in octahedral crystals, while graphite is found either in amorphous masses (that is, having no definite crystalline form), or in six-sided plates which are not geometrically allied with the form assumed by the diamond. Carbon, therefore, is dimorphous, or occurs in two distinct crystalline forms. Even in weight, diamond and graphite are very dissimilar, the former having an average specific gravity of 3.5, and the latter of 2.3. Again, a crystal of diamond is the hardest of all substances, whence it is used for cutting and for writing upon glass, but a mass of graphite is soft and easily cut with a knife. The diamond is a non-conductor of electricity, but the conducting power of graphite renders it useful in the electrotype process.

Diamonds are chiefly obtained from Golconda, Borneo, and the Brazils. They usually occur enveloped in sandstone or quartz pebbles, which appear to have been rounded by attrition in the beds of running streams. The hardness of the diamond renders it necessary to employ diamonddust for the purpose of cutting and polishing it, which is effected with the aid of a revolving disk of steel, to the surface of which the diamonddust is applied in the form of a paste made with oil. The crystal in its natural state is best fitted for the purpose of the glazier, for its edges are usually somewhat curved, and the angle formed by these cuts the glass deeply, while the angle formed by straight edges, like those of an ordinary jeweller's diamond, is only adapted for scratching or writing upon glass. The diamond-dust used for polishing, &c., is obtained from a dark amorphous diamond found at Bahia in the Brazils; 1000 ounces annually are said to have been occasionally obtained from this source. When burnt, the diamond always leaves a minute proportion of ash of a yellowish colour in which silica and oxide of iron have been detected.

Although the diamond, when preserved from contact with the air, may be heated very strongly in a furnace, without suffering any change, it is not proof against the intense heat of the discharge taking place between two carbon points attached to the terminal wires of a powerful galvanic battery. If the experiment be performed in a vessel exhausted of air, the diamond becomes converted into a black coke-like mass which closely resembles graphite in its properties.

Graphite always leaves more ash than the diamond, consisting chiefly of the oxides of iron and manganese, with particles of quartz, and sometimes titanic acid. The purest specimens are those of compact amorphous graphite from Borrowdale in Cumberland; an inferior variety, imported from Ceylon, is crystalline, being composed of hexagonal plates. Graphite is obtained artificially in the manufacture of cast iron: in some