and arranged the whole within the limits of the moderatesized yet comprehensive and interesting volume with which he favoured the scientific world some months ago.

Ozone, or, as it has been variously called, electrified oxygen, allotropic oxygen, nascent oxygen, and active oxygen, each of which names has a special propriety of its own, was first brought before the notice of philosophers by Van Marum in 1785, who observed that oxygen through which electric sparks had been passed acquired thereby a peculiar odour, and also the power of acting upon metallic

mercury.

At the beginning of the present century Cavallo observed that oxygen thus treated acquired the power of purifying decomposing organic matter, and he used it to correct the fœtor of foul ulcers. In 1826 Dr. John Davy recognised the existence of this principle in the air, and in 1839 Schönbein found that an odour resembling that described by Van Marum was emitted at the positive pole during the decomposition of water by voltaic electricity. The same distinguished chemist showed that this odour was due to a peculiar body which he called "ozone" from its most striking characteristic, and not merely to the action of electricity on the sense of smell, as was previously supposed by some, or to the influence of minute particles of gold or platinum detached by the electric current, as had been maintained by De la Rive, which latter hypothesis Schönbein proved untenable by showing that ozone can be prepared by simply exposing phosphorus to the action of oxygen in the presence of moisture. At first, however, Schönbein inclined to the erroneous supposition that ozone was a body which, together with hydrogen, entered into the composition of nitrogen, and so little was its real nature understood at this time, that Osann regarded it as a compound containing nitrogen. In 1845, however, Marignac and De la Rive refuted both these theories, by showing that ozone could be prepared from water under circumstances which excluded the possibility of nitrogen being present, and they expressed their conviction (which coincided with that of Berzelius) that ozone is simply an allo

tropic form of oxygen. This view was strenuously opposed by Schönbein, who next maintained it to be a compound of oxygen and hydrogen containing one more atom of the former than exists in Thenard's peroxide, and therefore to be represented O.N.HO, or N.N.H2Og. He afterwards changed his mind, and declared ozone to be identical with peroxide of hydrogen, in which opinion he was supported by Professor Williamson. The researches of Becquerel and Frémy in 1852 proved that oxygen can be wholly converted into ozone, even when no hydrogen is present, by means of causing the ozone to be absorbed, pari passu with its formation, by mercury or iodide of potassium, thereby confirming the doctrine of Marignac, De la Rive, and Berzelius, that ozone is merely an allotropic form of oxygen. But now a new question was started; were there not two kinds of ozone? or, rather, was not the term ozone applied to two wholly distinct substances? Baumert replied in the affirmative, maintaining that the ozone resulting from the electrolysis of water is some oxide of hydrogen, while that obtained from oxygen by means of phosphorus or the passage of the electric spark is simply an allotropic modification of that element. This theory, however, was overthrown in 1856 by Andrews and Tait, whose conclusions were confirmed by Soret and vou Babo, who clearly demonstrated the identity of ozone, however prepared.

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During these investigations the truth of the common opinion that a smell resembling that of sulphur, or, at least, a peculiar odour sui generis nearly allied to it, is really perceptible during electrical disturbances of the atmosphere (and also that this odour is caused by the formation of ozone), was triumphantly demonstrated by the two following facts: (1) Buchwalder, whose servant had been killed at his side by lightning in a small tent in which they were seated, and who had noticed a peculiar and very powerful odour at the time of the accident, happening to enter Schönbein's laboratory one day when the great chemist was experimenting with ozone, remarked that the odour which filled the apartment was identical with that

which he had perceived at the time of the unfortunate occurrence just described; and (2) Schönbein recognised the identity of the odour with that of which he had himself been sensible during a thunderstorm on the Jura.

The ultimate identity of ozone and oxygen having been established, the question next arose, what particular molecular or atomic arrangement, or what other allotropic modification of the element is it which causes it to manifest itself under these different conditions ? In 1858 Clausius suggested that in ordinary oxygen the atoms might be combined in pairs as molecules, while in ozone they might exist in a free state. This somewhat improbable hypothesis he subsequently abandoned. But in the same year Schönbein led him to adopt a theory which, although hardly in accordance with subsequent discoveries, well deserves to be mentioned at some length, on account of its extreme beauty and ingenuity, and also as having been the means of calling the attention of the scientific world to the subject of antozone.

This chemist found that peroxide of lead was formed by the action of peroxide of hydrogen on plumbic acetate. He also observed that when the plumbic peroxide, thus formed, was subjected to the continued action of peroxide of hydrogen, both peroxides were reduced, with the formation of water and protoxide of lead and the simultaneous evolution of free oxygen. He sought to explain this curious phenomenon by the ingenious and probably correct hypothesis that the oxygen exists in an opposite condition of electric polarity in the two peroxides, being negative in the lead salt and positive in the peroxide of hydrogen. He found that a similar process of reduction, together with the evolution of free oxygen, ensued when permanganate of potash was treated with peroxide of hydrogen, a phenomenon which, of course, admitted of the same explanation as the preceding. This suggested to him the notion that there might exist both a negative and a positive form of oxygen, which he named respectively ozone and antozone, and which by their union constitute ordinary oxygen. ozone he conceived a positive atom of oxygen to lie between

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two negative, and in antozone a negative to lie between two positive atoms. It will be at once seen that this hypothesis involves the simultaneous production of a corresponding amount of antozone whenever ozone is formed. Schönbein, moreover, considered that one or other of the bodies is formed whenever oxygen enters into combination with any substance whatever, and he accordingly classed all compounds containing oxygen under two heads; viz. ozonides, or those which contain the negative variety of oxygen, of which the typical representatives are permanganic and chromic acids and the peroxides of manganese, silver, lead, cobalt, nickel, bismuth and iron, all of which he believed to liberate chlorine from its compounds and to turn guaiacum paper blue; and antozonides, or those which contain oxygen in the positive state, typified by peroxide of hydrogen and the peroxides of the alkalies and alkaline earths, which do not exhibit either of the last-named reactions.

Applying the results of his investigations to physiology, Schönbein maintained that the blood-corpuscles resolved the oxygen of the air into its oppositely polarized constituents ozone and antozone; adding that the former was consumed in various oxidizing processes within the body, and that the latter was in part transformed by the bloodcorpuscles into ozone, and the remainder into peroxide of hydrogen by combining with the water of the blood. This transformation of antozone into ozone by the action of the blood he supported by showing that tincture of guaiacum is only coloured blue by peroxide of hydrogen (i. e. by antozone water according to Schönbein's hypothesis), when blood- or pus-corpuscles are present. However, it must be remembered that many other substances besides ozone impart a blue colour to guaiacum. Schönbein further maintained, in support of the existence of antozone as a distinct body from ozone, that the former could be produced by rubbing a piece of Bavarian fluor spar, which, according to him, produced a different odour from that of ozone; but in this he seems to have been mistaken, since Schrötter, on the contrary, not only pronounced the odours identical, but

further showed that the product thus obtained liberates iodine from iodide of potassium.

In support of the views of Schönbein, Meissner, in 1863, showed that if a stream of electrified oxygen be passed through water, a cloud or mist will appear in the receiver into which it is conducted, the production of which he attributed to the influence of a substance which he named "atmizone," and which was afterwards shown to be identical with Schönbein's antozone. He succeeded in isolating this by passing a stream of electrified oxygen through a solution of iodide of potassium which absorbs all the ozone, while the emerging gas produced a dense white mist after being led through a vessel containing water. By these discoveries Meissner sought to explain the formation of clouds in the atmosphere, regarding these bodies as an aggregate of antozone or atmizone aqueous vapour. To

the influence of atmizone he also attributed the formation of coal-smoke and tobacco-smoke, as well as the fumes of phosphorus and gunpowder.

Those who maintained the existence of antozone as a distinct body from ozone were of opinion that the former is destitute of the power of oxidizing such bodies as phosphorus and pyrogallic acid, or of liberating iodine from iodide of potassium, while it is readily soluble in water, which it converts into peroxide of hydrogen. Ozone has diametrically opposite properties. Babo and Weltzien, however, and subsequently Nasse and Engler, observing that "atmizone" or antozone is only produced when ozone suffers decomposition from the action of water, and that ozone and autozone are not formed simultaneously when dry oxygen is subjected to the action of electricity, as must necessarily be the case on Schönbein's hypothesis of their constitution, came to the conclusion that the doctrine of the existence of any distinct substance as antozone is erroneous, and that the phenomena of which it is the supposed cause are due to the diffusion of peroxide of hydrogen through air or oxygen, thus completely overturning Schönbein's ingenious hypothesis. Brodie coincides in this view. In support of it Nasse and Engler showed