conditions, of assuming such totally different habits and properties, that they appear to be entirely different substances. Thus, the element sulphur, as usually seen, is a primrose-yellow, opaque, solid substance, extremely brittle, and readily dissolved by carbon disulphide. Under certain circumstances it may be made to appear a totally different thing; it is then a translucent ambercoloured substance, soft and elastic like indiarubber, and insoluble in carbon disulphide; it is still sulphur, and sulphur only. Phosphorus, again, as usually known, is a nearly colourless, translucent, wax-like solid, which melts at a temperature only slightly above that of the hand, and which takes fire a few degrees higher; it is also extremely poisonous. Under special influences phosphorus can be made to assume the following properties :-A dark reddishbrown powder, resembling chocolate, which may be heated to 250° without taking fire, and which is non-poisonous. The substance is still phosphorus, and phosphorus only. This property possessed by certain of the elements of appearing in more than one form, of assuming, as it were, an alias, is called allotropy; the more uncommon form being spoken of as the allotropic modification, or the allotrope of the other.

From a study of the best known instances of this phenomenon, it is believed that allotropy, in all cases, is due to a difference in the number of atoms of the element that are contained in the molecule. In the case of ozone, which is

the allotrope of oxygen, this is known to be the case. Ordinary oxygen molecules consist of two atoms, while the molecule of ozone is an aggregation of three oxygen atoms.

OZONE.

Molecular symbol, Og. Molecular weight=48. Density=24.

History. When an electrical machine is in operation a peculiar and characteristic smell is noticed in its vicinity. The same smell is sometimes observed in and about buildings, or other objects, when struck by lightning. In 1785 it was observed by Van Marum that when electric sparks were passed in oxygen, the oxygen acquired this peculiar smell. Schönbein (1840) showed that the oxygen obtained by the electrolysis of water also contained this substance having a smell, and he gave to it the name ozone, signifying a smell. Schönbein made a careful study of the substance, and found that it might be obtained by various other methods. The more recent work of Andrews, Soret, and Brodie has brought our knowledge of the constitution of ozone to its present state.

Occurrence.-Ozone is present in the atmosphere in extremely small quantities (see Atmospheric Ozone).

Modes of Formation.-(1.) Mixed with an excess of oxygen, ozone is best obtained by exposing pure dry oxygen to the influence of the silent electric discharge. This may be effected by means of the instrument shown in Fig. 36, known as "Siemens' ozone tube."

It consists of two concentric glass tubes, A and B. Tube A is coated upon its inner surface with tinfoil, which is brought into metallic contact with the binding screw D, as shown in the figure. Tube B is coated upon the outer surface, also with tinfoil, which is in metallic connection with binding screw C. These two surfaces of tinfoil are connected by means of their respective binding screws

to the terminals of a Ruhmkorf coil, and the slow stream of oxygen which is admitted at E, and which passes along the annular space between the two tubes, is there exposed to the action of the silent electric discharge. A small portion of the oxygen so passing becomes converted into the allotropic modification, and the mixture

?

FIG. 37.

of oxygen and ozone issues from the narrow tube at the opposite end of the apparatus.

For general purposes of illustration, a very simple arrangement may be substituted for the above. It consists, as shown in Fig. 37, of a straight length of narrow glass tube having a piece of platinum wire down the inside, which passes out through the walls of the tube near to one end, and is there sealed to the glass. A second

platinum wire is coiled round the outside of the tube, and these two wires are connected to the induction coil. On passing a slow stream of oxygen through the tube, the issuing gas will be found to be highly charged with ozone.

(2.) Ozone is also formed when an electric current is passed through water acidulated with sulphuric acid. Thus, in the ordinary electrolysis of water the oxygen evolved from the positive electrode is found to contain ozone in sufficient quantity to be readily detected, both by its odour and by other tests.

(3.) During many processes of slow oxidation at ordinary temperatures, ozone is formed in varying quantities. Thus, when phosphorus is exposed to the air an appreciable amount of ozone is formed. One or two short sticks of freshly scraped phosphorus are for this purpose put into a stoppered bottle containing air, and allowed to remain for a short time, when the air will be found to contain ozone.

(4.) Ozone is also formed during the combustion of ether upon the surface of red-hot platinum. When a spiral of platinum wire is warmed in a gas-flame, and while hot is suspended over a small quantity of ether contained in a beaker, the mixture of ether vapour and air undergoes combustion upon the surface of the platinum, which continues in an incandescent state so long as any ether remains. During this process of combustion a considerable quantity of ozone is formed. (See also Peroxide of Hydrogen.)

(5.) Ozone is formed during the liberation of oxygen in a number of the reactions by which that gas is obtained; thus, from manganese dioxide and sulphuric acid the oxygen that is evolved contains sufficient ozone to answer to the ordinary test. In the same way, by the action of sulphuric acid upon barium peroxide or potassium permanganate, this allotrope is present with the ordinary oxygen that is evolved.

Properties. As prepared by any of the methods described, ozone is always mixed with a large excess of unaltered oxygen, probably never less than about 80 per cent. of the latter gas being present. Even in this state of dilution it has a strong and rather unpleasant smell, which rapidly induces headache. When inhaled it irritates the mucous membranes, and is rather suggestive of dilute chlorine.

Ozone is a most powerful oxidising substance; it attacks and rapidly destroys organic matter: on this account ozonised oxygen cannot be passed through the ordinary caoutchouc tubes, as these

are immediately destroyed by it. It bleaches vegetable colours, and most metals are at once acted upon by it. Even metals like mercury, which are entirely unaltered by ordinary oxygen, arc attacked by ozone. Its action upon mercury is so marked in its result, that the presence of exceedingly small traces of ozone can be detected by it; the mercury is seen to lose its condition of perfect liquidity, and adheres to the surface of the glass vessel containing it, leaving "tails" upon the glass. Ozone converts lead sulphide (PbS) into lead sulphate (PbSO4), and liberates iodine from potassium iodide

2KI+H2O+O3=O2+I2+2KHO.

This property is generally made use of for detecting the presence of ozone, advantage being taken of the fact that iodine, when set free from combination in the presence of starch, gives rise to a deep blue-coloured compound, the reaction being one of extreme delicacy. In order to apply this test for ozone, strips of paper are dipped in an emulsion of starch to which a small quantity of potassium iodide has been added. These papers may be dried and preserved, and are usually spoken of as ozone test papers. When one of these papers is moistened with water, and placed in air containing ozone, the iodine is liberated from the potassium iodide, and being in the presence of starch, the paper instantly becomes blue by the formation of the coloured compound of starch. It will be obvious that this method of testing for ozone can only be relied upon when there is no other substance present which is able to decompose potassium iodide; for example, when testing for ozone in the atmosphere, the presence of oxides of nitrogen or peroxide of hydrogen (both of which are capable of liberating iodine, and are liable to be present in the air) would materially vitiate the result (see also Atmospheric Ozone). The above decomposition of potassium iodide by ozone may be made use of as a test for ozone in another way, which, although less delicate, is also less likely to be vitiated by the presence of other substances. Blue litmus papers are dipped into water which has been rendered very feebly acid, and to which a small quantity of potassium iodide has been added. The papers may be dried and preserved. On moistening one of these papers with water and exposing it to ozone the iodide is decomposed as in the former case, and the potassium hydroxide which is formed, being a powerfully alkaline substance, converts the colour of the litmus from red to blue.

When heated to a temperature of about 250°, ozone is retransformed into ordinary oxygen; if, therefore, the ozonised gas obtained by means of the Siemens' ozone tube be passed through a glass tube heated by means of a Bunsen flame, the whole of the ozone will be decomposed, and the issuing gas will therefore be found to be without action upon the ozone test papers.

Ozone is also decomposed by certain metallic oxides, such as those of manganese, copper, and silver. The action appears to be one of alternate reduction and oxidation, the metallic oxide remaining unaltered at the conclusion, thus—

Ag2O+O3=Ag2+ 202.
Ag2+O3=Ag2O+O¿

The oxidising power of ozone is due to the instability of the molecule and the readiness with which it loses an atom of oxygen, leaving a molecule of ordinary oxygen, thus

The oxygen molecule is comparatively inert, but the liberated atom in its nascent state is endowed with great chemical activity. No change of volume accompanies these processes of oxidation by ozone, as the volume of the oxygen molecule (O) is the same as that of the ozone molecule (O3), the third atom of oxygen being that which enters into new combination with the oxidised substance.

Ozone is soluble to a slight extent in water, imparting to the solution its own peculiar smell. 1000 c.c. of water dissolve about 4.5 c.c. of ozone.

Under the influence of extreme cold, ozone condenses to liquid having an intense blue colour. So deep is the colour, that a layer of it 2 mm. in thickness is opaque. This liquid is obtained by passing ozonised oxygen through a tube which is cooled by being immersed in boiling liquid oxygen, which has a temperature of - 182.5°. At this temperature the ozone liquefies, but most of the oxygen with which it was mixed passes on. In a higher state of purity it has been more recently obtained by first liquefying ozonised oxygen, and then separating the more volatile oxygen by fractional distillation. Liquid ozone boils at 119°. It is described by Olszewski and Dewar as an extremely explosive substance.

Constitution of Ozone.-The fundamental difference between ordinary oxygen and its allotrope ozone lies in the fact that the molecule of the latter contains three atoms, while that of ordinary