covers its former bulk and loses its peculiar ozone qualities. The reason the electrical discharge should be a silent one is because the electrical spark produces too much heat, which destroys a considerable portion of the ozone, and thus prevents any considerable accumulation. At p. 388 Siemen's apparatus is described (an induction apparatus), by which large quantities of oxygen may be converted into ozone.

2. This peculiar condition of oxygen is obtained by acting either on potassium permanganate (KMnO,) or upon baric dioxide (BaO,) with strong sulphuric acid.

3. A stick of phosphorus scraped clean under water, and then exposed in a bottle containing moist air, produces ozone.

4. When water is decomposed in the apparatus called a voltameter, the mixed gases give the peculiar odour and are found to contain a certain quantity of ozone.

The best tests for this condensed form of oxygen are potassium iodide, and starch paste painted with a brush on paper, or paper dipped in a solution of sulphate of manganese, MnSO,+5H.Ó. The first turns blue from the liberation of iodine and the formation of a blue compound of starch and iodine; and the second indicates the presence of ozone by the formation of the brown hydrated peroxide of manganese.

Schonboein, who first directed attention to this allotropic condition of oxygen, directs the test-paper to be made of a fixed strength, by dissolving one part of pure potassic iodide in 200 parts of distilled water, which is then to be thickened by heating it with 10 parts of starch. This solution is to be applied to bibulous paper, which, when dry, should be kept in a stoppered bottle covered with tin foil, in order to exclude the light.

It is known that sea-air contains ozone, whilst the same air, having passed over or through a town, is supposed to be deprived in a great degree of this agent, which is considered to have purifying and health-giving powers. The absence of ozone from the air is said to be prejudicial to health, and during the prevalence of cholera it was thought to be due in some degree to the absence of this condition of oxygen.

Dr. Daubeney found, in the three winter months commencing with January, at Torquay, that the south-west and westerly winds were most fully charged with ozone, whilst the north winds showed the least. On the contrary, at Oxford during the summer months of the same year, the easterly winds were most charged with ozone, and the north-westerly the least. These indications clearly pointed to the influence of the sea in augmenting the amount of ozone at Torquay, whilst the more central inland position of Oxford caused the difference between the maximum and minimum indications to be much less apparent than at the sea-side. Daubeney also found that plants growing in the sunshine liberate a body that affects the starch test like ozone, and hence inferred that this remarkable property of plants might have something to do with the maintenance of the healthiness and purity of the air.

Ozone possesses most energetic qualities. It sets free iodine from its combination with the metals. Black sulphide of lead or plumbic sulphide (PbO) is attacked by ozone; the black stain disappears, and both the sulphur and the lead oxidize, the white sulphate of lead (PbSO.) being produced. Ozone is a powerful bleaching agent: it irritates the lungs if inhaled in any quantity; and this is not surprising when it is remembered that both cork and caoutchouc are not proof against its oxidizing power. Silver, which resists common

oxygen, and is so often used in chemical processes for this reason, is converted into peroxide when exposed to the action of ozone, provided that the presence of moisture is secured.

Ozone is now introduced into certain chemical processes, and is likely to take a very prominent place as one of the oxidizers, the wealth-producers in refining sugar, bleaching calico, &c.

NITROGEN.

Symbol, N. Atomic weight, 14.

Nitrogen is described in an old work as “a simple oxidable body, by some chemists called azot, from its property of destroying life. This name appears improper, since several other gases have the same effect on animals." It was discovered by Dr. Rutherford in the year 1772, and was called Nitrogen from νιτρον nitre, and γεννάω to generate.

It would be difficult to speak of nitrogen without alluding to the important part it takes in the composition of the atmosphere. Rodwell says, from the continued observation that the cessation of breathing was the cessation of life, the belief became prevalent that the soul passed from the body with the last expiration of air; hence the expressions, "Efflare animam," "Exhalare animam," "Expiram animam." Again, πvevua, spiritus, anima, have each the triple meaning of soul, breath, wind. There is also a Hebrew word having the same meaning as the Greek pneuma, viz., soul, breath, wind. The most convenient mode of preparing nitrogen is by removing the oxygen from atmospheric air, and when this is done it is found that it constitutes four-fifths of any given bulk. A variety of processes may be employed for this purpose, but the simplest plan is to place some dry phosphorus in a German porcelain cup, and having properly supported it on the shelf of the pneumatic trough, a long unstoppered gas-jar graduated into five equal parts is placed over the whole; a heated wire is now inserted, and directly it touches the phosphorus, the latter ignites, and the stopper of the jar is quickly inserted. At first expansion occurs, and therefore the depth of water should be adjusted beforehand, so as to allow the heated air to increase in volume without bubbling out and escaping from the bottom of the jar.

If the above manipulations are skilfully performed, very little air is lost: the phosphorus burns, producing the white fumes of phosphoric pentoxide, which gradually subside, and are dissolved, by the water. The residual gas, when cold, is found to be equal to four-fifths of the original bulk, one-fifthviz., the oxygen-being removed by combining with the phosphorus.

The same result is more accurately obtained by thrusting up into a graduated tube containing atmospheric air a piece of phosphorus supported in a coil of platinum wire: after two or three days the phosphorus may be removed, and the remaining gas is found to be nearly pure nitrogen; and if 100 measures of air be used, 20 will be removed and 80 left.

Another mode of preparing nitrogen is by passing air over finely divided metallic copper at a red heat. The experiment conducted by Dumas and Boussingault was performed by them with great precautions in the exact analysis of air, and they found that 100 parts, by weight, of air from which the aqueous vapour, carbonic anhydride, and ammonium had been remove

contained 77 parts of nitrogen and 23 parts of oxygen; or, more precisely, taking the average of a number of experiments,—

Nitrogen has no colour, taste, or smell. It is lighter than oxygen gas, and slightly lighter than atmospheric air: 100 cubic inches at 60° F., 30 in. ba.., weigh 30119 grains. A lighted taper immersed in this gas is immediate extinguished, no incandescent snuff remaining. It must not, however, be supposed that nitrogen is poisonous: it simply destroys life in the absence of oxygen gas, and cannot be poisonous, or we could not continue to breathe air, which is a mechanical mixture of the two gases always maintained in the same relative volumes by one of those wonderful conservative powers of Nature represented by the vegetable kingdom. Although the two gases differ in weight, they never separate; and by the law of the universal diffusion of gaseous bodies, they have the power of incorporating perfectly with each other; and this property of gases in general, and specially in this case, has, no doubt, the most important bearing on the purity and healthiness of the air. No combination occurs between the oxygen and nitrogen contained in atmospheric air, although it is quite possible to conceive that where ozone is produced in hot climates with certain peculiarities of soil-as in Spain, Egypt, and India --that there the nitrogen is attacked by the condensed oxygen ozone, and nitrates produced.

Professor Graham has shown that the velocity of the diffusion of the various gases is in the inverse ratio of the square roots of their densities; and, as before stated, this principle of diffusion explains why the composition of 100 parts by volume of the air may be taken at an average as follows:

Although this article on Chemistry must necessarily be confined to certain limits, and therefore the various compounds formed by the combination of the different elements cannot all be considered, exceptions to this rule must occa sionally be made; and having considered the chemical nature of air, it would be hardly possible to avoid making some remarks on the constitution of water, the more so as this is the chief source from which hydrogen is obtained.

Water is presented to us in nature having different degrees of purity; hence we speak of hard or soft water. The former may contain calcium carbonate and sulphate, magnesium carbonate and sulphate, sodium sulphate and chloride, and many other substances, in considerable quantities, especially if the water flowing into the well be derived chiefly from surface drainage. When the water-such as rain-water-has been collected after several hours' rain, it is almost in a state of purity, containing then only certain gaseous matters in solution: such water is usually called soft, because it is free from the salts already mentioned. If the rain-water be collected after a long drought, it may then contain nitrates and salts of ammonium, and, if near the sea-side, would always contain sodium chloride or common salt.

River-water usually comes under this denomination, because it contains a less proportion of saline matters in solution: it is not, however, so good to drink as spring-water, because it frequently occurs that rivers receive the sewage of large towns, and hence the water contains organic matter in solution, and, should the water be taken whilst this organic matter is undergoing decomposition, very serious consequences may result to the person drinking it. It is now, however, a rule in sanitary matters to endeavour to divert the sewage from our noble rivers when possible, and with the help of proper filters the Thames water is now potable and wholesome.

All rivers flow into the sea, hence sea-water contains a larger quantity of sodium chloride, and many other salts, in solution, likewise organic matter; but, curious to say, it remains in a uniform condition so far as the quantity of saline matter is concerned, and the specific gravity varies little, the mean being 1,027, pure water being 1,000.

When sea-water or any other hard water is placed in a still and boiled, the earthy or saline matters are left behind, and, the steam only being condensed, pure water is obtained.

FIG. 475.-The Still placed on a common fire or fitted to a proper furnace.

The operation of distilling may be performed on a very small scale by using a little flask fitted into a bent tube, which is placed in a basin containing cold water (Fig. 476). Very convenient little tin or copper stills are made by Mr. How, of Foster Lane, Cheapside: they are heated by a Bunsen burner, and will supply a sufficient quantity of fresh distilled water for any analytical operations con

It is invaluable as a solvent, and no laboratory is complete without a proper supply of it stored in well-stoppered bottles.

Pure water is the standard to which the specific gravities of other liquids and solids are referred.

Under the ordinary pressure of the air, it boils at 212° F. If, however, it be confined in a very strong wrought-iron vessel, such as the apparatus called a

Ů

FIG. 479. A Papin's Digester with Safety Valve.

Papin's digester, the boiling-point is raised, and, as the steam does not escape, the solvent powers of the water are greatly increased. Water has in this way been raised to the temperature of 419° F., and Muschenbroek stated that he had made water hot enough in a Papin's digester to melt tin.

The chemical composition of water may be determined analytically or synthetically. In the article on voltaic electricity it has been shown that water, when subjected in a proper manner to a current of electricity, is decomposed into oxygen and hydrogen, and if they are collected in separate tubes, the latter, eliminated at the platinode or negative platinum plate, is found to be double the volume of the oxygen set free at the zincode or positive plate. Thus the composition of water is shown to be in the proportion of two measures of hydrogen with one of oxygen. According to modern views, the atomic constitution of water is represented by the formula H.O

-18, instead of, as formerly, HO=9.