That a gaseous element can exist in an allotropic condition was first clearly shown by a careful study of the properties of ozone. Although discovered by Schönbein in 1840, chemists were for a long time unable to determine its true nature, and it was not until seven years later that Marignac[1] succeeded in proving that it was oxygen in an allotropic condition. Marignac’s work was confirmed by De la Rive, and subsequently the elaborate researches of Andrews and Tait, and Soret, as well as those of von Bato and Claus have established beyond all question that ozone is an allotropic modification of oxygen, and that its density is one and a half times that of ordinary oxygen.
The possibility of the existence of allotropic modifications of oxygen having been thus established it is not surprising that attempts should have been made to find other forms in which this element might occur. As early as 1855 Houzeau[2] stated that when barium superoxide was decomposed with concentrated sulphuric acid, at low temperatures, a colorless gas was evolved which oxidized metals and ammonia. It had a penetrating odor and possessed the power of bleaching litmus paper, and liberated iodine from potassium iodide. By heating the gas to a temperature of 75°C it was completely converted into ordinary oxygen. He calls the gas nascent oxygen and further states that it is probable that whenever oxygen is set free from any of its compounds at low temperatures it is in the nascent or active state.
Clausius[3] at one time supposed that free atoms of oxygen might exist in an uncombined state, and his hypothesis on the nature of ozone was that this substance consisted of a mixture of molecules and free atoms of oxygen. In a later paper[4], however, he abandoned this view and regarded ozone as consisting of one or more atoms of oxygen feebly united, (lose verbunden) with molecules of ordinary oxygen.
The idea that a third form of oxygen existed also obtained support from the fact that certain organic substances when exposed to the light in the presence of oxygen or air, acquire oxidizing properties. In 1850 Schönbein[5] stated that ether turpentine, and oil of lemons if allowed to stand in diffused light in contact with the air acquires the power of decomposing potassium iodide, and decolorizing indigo. In a subsequent paper[6] he shows that methyl and ethyl alcohols, tartaric and citric acids and even sulphuretted and arsenuretted hydrogen in the presence of sun light can decolorize indigo. These studies led Schönbein to publish a theory on the different modifications of oxygen. In this paper[7] he states that besides ordinary oxygen there are two other conditions in which it may exist one of these is ozone, or positively electrified oxygen, the other antozone or negatively electrified oxygen. The union of ozone and antozone gives ordinary oxygen. He also stated that antozone was formed by the action of light on turpentine and air, and subsequently in 1862[8] he claimed that antozone was identical with the gas obtained when barium superoxide is treated with acids. Meissner[9] also supported the views of Schönbein and claimed that antozone was formed in two ways:—1st, By treating barium dioxide with concentrated sulphuric acid, 2nd, By the electrification of oxygen; being produced simultaneously with the ozone.
These statements remained unquestioned for a number of years and are found in the text books of the period, (for example Graham-Otto, and Gorup-Besauez) but in 1870 Engler and Nasse[10] undertook a thorough investigation to determine whether antozone existed. By treating barium dioxide with strong sulphuric acid they find a gas to be given off which is a mixture of ozone and hydrogen dioxide, and they also show that the stronger the acid the greater the quantity of ozone produced. Secondly, Meissner had stated that by the electrification of oxygen ozone and antozone were formed. The evidence of the existence of antozone being this; when the ozonized oxygen was passed through a solution of potassium iodide to destroy the ozone, the residual gas gave white fumes when brought into contact with water, and after a time hydrogen dioxide could be detected in the water. Schönbein and Meissner held that the ozone having been destroyed by the potassium iodide the antozone passed on and oxidized the water to hydrogen dioxide. Now Engler and Nasse show that when ozone is decomposed by easily oxidisable substances in the presence of water hydrogen dioxide also is formed, and it was the vapor of this compound which had been regarded as antozone. It is known that ozone cannot oxidize water, but that it is to a slight extent oxidized when other oxidisable substances are present is not surprising, as other phenomena of a similar kind are known. Thus when nitric acid acts on an alloy of silver with gold or platinum, containing a certain proportion of silver, some of the gold or platinum are dissolved although by themselves they are insoluble. When ammonia burns some of the nitrogen as well as the hydrogen is oxidized. Engler and Nasse therefore conclude that there is no basis for the assumption of a third form of oxygen having the properties attributed to antozone.
Berthelot[11] and Houzeau[12] conclude from their investigations that the oxidizing properties which turpentine and other organic compounds acquire under certain conditions is due to the formation of unstable oxygenated compounds which readily decompose giving up oxygen.
Fudakowski[13] has described experiments showing that benzene can become active, i.e. acquire oxidizing properties, but states that he is unable to explain the phenomenon. Loew[14], however, believes active turpentine to contain atomic oxygen or antozone in solution.
After Engler and Nasse had demonstrated the nonexistence of antozone all discussion on the subject ceased for a number of years, and it was not until 1878 that Hoppe-Seyler[15] again opened the question. In studying the processes of putrefaction he observed that free hydrogen is given off in those cases in which oxygen is not present, and that whenever oxygen has access to decaying liquids, not only is all the hydrogen oxidized but energetic oxidation processes are observed as well. The simplest explanation of this seemed to be that the nascent hydrogen has the power of splitting up the oxygen molecule, uniting with one atom and setting the other free, and these free atoms he imagined brought about the strong oxidations which take place in decaying bodies.
To test this hypothesis he made experiments with palladium hydrogen. Graham has described the energetic reducing power of this compound but that it can also cause oxidations Hoppe-Seyler showed by bringing some strips of palladium charged with hydrogen into a solution of indigo in the presence of air. The solution soon became yellow and after a time the indigo was completely destroyed. If palladium hydrogen be brought into a neutral solution of potassium iodide and starch, the liquid becomes blue in a few minutes, after which the starch is slowly destroyed. In a similar way benzene was oxidized to phenol. “These experiments and others of a similar nature,” he asserts, “admit of no explanation other than that the active hydrogen renders the oxygen active, and since the former unites with oxygen we cannot well conceive of the process without supposing that the hydrogen in uniting with one atom of the molecule O2 sets the remaining atom free, thus making it active.” “Just as the hydrogen atom cannot exist in a free state so the active oxygen, if no oxidizable material is present, unites with water to form hydrogen dioxide, or with inactive oxygen to form ozone.”
This theory has been taken up and developed by Baumann, who in 1881 published a paper[16] entitled “Contribution to the knowledge of Active Oxygen.” The paper begins with the statement that besides ordinary inactive oxygen and ozone there is a third modification known as active or nascent oxygen. He states that this active oxygen cannot be isolated, and its formation can only be recognized by its action on other bodies. “Active oxygen (O) is the most powerful oxidizing substance known and can unite with inactive oxygen (O2) to form ozone (O3). The production of ozone is always preceded by the formation of active oxygen,” but he states “active oxygen can be formed under conditions when no ozone can be formed, this is the case when easily oxidized substances are in contact with the active oxygen in such a way that the latter is completely consumed in oxidizing those substances.” “Thus, for instance, ozone is formed when oxygen is rendered active by the slow combustion of moist phosphorus in air, but no ozone is formed if the atmosphere surrounding the phosphorus contains the vapor of alcohol, ether and similar substances.” The fallacy of this reasoning becomes apparent on referring to the work of Müller[17] on the luminosity of phosphorus who shows that substances which prevent the luminosity also prevent its oxidation and if the phosphorus is not oxidized we have no reason for assuming the formation of active oxygen.