Christian Friedrich Schoenbein to Faraday   25 June 1858

My dear Faraday,

These last six months I have been rather busily working on oxygen, and flatter myself not to have quite in vain maltreated my favourite; for I think I can now prove the correctness of that old idea of mine, according to which there are two kinds or allotropic modifications of active oxygen, standing to each other in the relation of + to -, i.e. that there is a positively-active and a negatively-active oxygen, - an ozone and an antozone, which on being brought together neutralize each other into common or inactive oxygen, according to the equation <oring+>O + <oring->O = O.

The space allotted to a letter being so small, I cannot enter into the details of my late researches, and must confine myself to some general statements, which I hope, however, will give you a clear notion of the nature of my recent doings. Having written a paper on the subject, that will before long be published in the transactions of the Academy of Munich¹, I shall not fail to send it to you as soon as possible.

Ozonized oxygen, as produced from common oxygen by the electrical spark or phosphorus, is identical with that contained in a number of oxy-compounds, the principal ones of which are the oxides of the precious metals, the peroxides of manganese, lead, cobalt, nickel and bismuth, - permanganic, chromic and vanadic acids; and even the peroxides of iron and copper may be numbered amongst them.

The whole of the oxygen of the oxides of the precious metals exists in the ozonic state, whilst in the rest of the oxy-compounds named, only part of their oxygen is in that condition. I call that oxygen negatively-active, or ozone par excellence, and give it the sign <oring->O, on account of its electromotive bearing. Though generally disinclined to coin new terms, I think it convenient to denominate the whole class of the oxy-compounds containing <oring->O “ozonides.” There is another less numerous series of oxy-compounds in which part of their oxygen exists in an opposite active state, i.e. as <oring+>O or antozone, wherefore I have christened them “antozonides.” This class is composed of the peroxides of hydrogen, barium, strontium, and the rest of the alkaline metals; and on this occasion I must not omit to add, that what I have hitherto called ozonized oil of turpentine, aether, &c., contain their active oxygen in the <oring+>O state, and belong therefore to the class of the “antozonides.”

Now, on bringing together (under proper circumstances) any ozonide with any antozonide, reciprocal catalysis results, the <oring->O of the one and the <oring+>O of the other neutralizing each other into O, which, as such, cannot be retained by the substances with which it had been previously associated in the <oring->O or <oring+>O condition. The proximate cause of the mutual catalysis of so many oxy-compounds depends therefore upon the opposite states of the active oxygen contained in those compounds.

I will now give you some details on the subject.

1. Free ozonized oxygen = <oring->O, and peroxide of hydrogen = HO + <oring+>O, or peroxide of barium = BaO + <oring+>O (the latter suspended in water), on being shaken together destroy each other, HO + <oring+>O or BaO + <oring+>O being reduced to HO or BaO, and <oring+>O and <oring->O transformed into O.

2. Aqueous permanganic acid = Mn²O³ + 5<oring->O, or a solution of permanganate of potash mixed with some dilute nitric acid, is almost instantaneously discoloured by peroxide of hydrogen or peroxide of barium, the nitrate of the protoxide of manganese being formed in the first case, and in the second, besides this salt, the nitrate of baryta. It is hardly necessary to state, that in both cases the <oring->O of the permanganic acid and the <oring+>O of the peroxides of hydrogen or barium are disengaged as O.

3. An aqueous solution of chromic acid containing some nitric or sulphuric acid and peroxide of hydrogen, are rapidly transformed into the nitrate or sulphate of oxide of chromium, HO, and inactive oxygen, which is of course disengaged. A solution of chromic acid mixed with some nitric acid and BaO⁴ gives a similar result, nitrate of baryta and oxide of chromium being formed, and O disengaged.

4. If you add to a mixture of any peroxide salt of iron and the red ferro-sesquicyanuret of potassium (both substances dissolved in water) some peroxide of hydrogen, prussian blue will be thrown down and inactive oxygen set free. On introducing into a mixture of nitrate of peroxide of iron and the ferro-sesquicyanuret of potassium the peroxide of barium, a similar reaction takes place, prussian blue, hydrate of baryta, &c. being formed, and inactive oxygen eliminated. From these facts it appears that, under certain conditions, even peroxide of iron and HO⁵ or BaO⁶ are capable of catalyzing each other into FeO and HO, or BaO and O.

5. Under certain circumstances PbO⁷ or MnO⁸ are soluble in strong acetic acid, as you may see in one of my papers joined to this letter; now if you add to such a solution HO⁹ or BaO¹⁰, the peroxides will be reduced to HO or BaO, and PbO or MnO, inactive oxygen being disengaged.

6. It is a well know fact that the oxide of silver = Ag<oring->O, or the peroxide of that metal Ag<oring+>O¹¹, and the peroxide of hydrogen = HO + <oring+>O, catalyze each other into metallic silver, water and inactive oxygen. Other ozonides, such as PbO + <oring->O or MnO + <oring->O, on being brought in contact with HO + <oring+>O, are transformed into PbO or MnO, HO and O. Now the peroxide of barium = BaO + <oring+>O, acts like HO + <oring+>O. If you pour water an intimate mixture of AgO, or AgO¹² and BaO¹³, a lively disengagement of inactive oxygen will ensue, AgO, AgO¹⁴ and BaO¹⁵ being reduced to metallic silver and baryta. In concluding the first part of my letter, I must not omit to state the general fact, that the oxygen disengaged in all cases of reciprocal catalysis of oxy-compounds, behaves in every respect like inactive oxygen.

There is another set of chemical phaenomena, in my opinion, closely connected with the polar states of the active oxygen contained in the two opposite classes of peroxides. You know that a certain number of oxy-compounds, for instance the peroxides of manganese, lead, nickel, cobalt, bismuth, silver, and also permanganic, chromic, and vanadic acids, furnish with muriatic acid chlorine, whilst another set, such as the peroxides of barium, strontium, potassium &c., are not capable of eliminating chlorine either out of the said acid or any other chloride. This second class of oxy-compounds produces, however, with muriatic acid, the peroxide of hydrogen; and it is quite impossible in any way to obtain from the first class of the peroxides HO¹⁶, or from the second chlorine.

You are aware that, from reasons of analogy, I do not believe in the doctrine of chlorine, bromine, &c. being simple bodies, but consider those substances as oxy-compounds, analogous to the peroxides of manganese, lead, &c., in other terms, as “ozonides.” Chlorine is therefore to me the peroxide of murium = MuO + <oring->O, hydrochloric acid = MuO + HO, and, as already mentioned, the peroxide of barium = BaO + <oring+>O, that of hydrogen = HO + <oring+>O, and the peroxide of manganese = MnO + <oring->O. Proceeding from these suppositions, it is very easy to account for the different way in which the two sets of peroxides are acted upon by muriatic acid.

From reasons as yet entirely unknown to us, HO can be chemically associated only with <oring+>O, and with no other modification of oxygen, to constitute what is called the peroxide of hydrogen; and in a similar way MuO (the hypothetically anhydrous muriatic acid of older times) is capable of being united only to <oring->O to form the so-called chlorine, which I denominate peroxide of murium. If we cause MuO + HO to react upon BaO + <oring+>O, MuO unites with BaO, and HO with <oring+>O; but if you bring together MuO + HO with Mn + <oring->O, part of MuO is associated to MnO, another part to <oring->O, water being eliminated, according to the equation

2(MuO, HO) + MnO + <oring->O = MuO, MnO + MuO, <oring->O + 2HO.

As you will easily perceive, from these views it would follow that, under proper circumstances, two opposite peroxides, on being intimately and in the right proportion mixed together and acted upon by muriatic acid, could yield neither chlorine nor peroxide of hydrogen, but mere inactive oxygen. If somewhat dilute muriatic acid be poured upon an intimate mixture of five parts of peroxide of barium and two parts of peroxide of manganese, the whole will be rapidly transformed into the muriates of baryta and protoxide of manganese, the active oxygen of both the peroxides being disengaged in the inactive condition, and not a trace of free chlorine making its appearance. The same result is obtained from dilute hydrobromic acid.

Another consequence of my hypothesis is this: that an intimate and correctly proportioned mixture of two opposite peroxides, such as the peroxide of barium and that of lead, on being acted upon by any oxy-acid, cannot produce the peroxide of hydrogen; or, to express the same thing in other terms, muriatic acid must act upon the said mixture exactly in the same way as the oxy-acids do; and that indeed is the case. Mixtures of the peroxides just mentioned and acetic or nitric acids, are readily converted into the acetates or nitrates of baryta and protoxide of manganese, the active oxygen of both the peroxides being of course disengaged in the inactive condition.

Before I close my long story I must mention one fact more, which, in my opinion, is certainly a very curious one. If you mix an aqueous and concentrated solution of bromine with a sufficient quantity of peroxide of hydrogen, what happens? A very lively disengagement of inactive oxygen takes place, the colour and the odour of the bromine solution disappear, the liquid becomes sour, and on adding some aqueous chlorine to it, bromine reappears. From hence we are allowed to conclude, that, on bringing bromine in contact with peroxide of hydrogen, some so-called hydrobromic acid is produced. The hypothesis at present prevailing cannot account for the formation of that acid otherwise than by admitting that bromine takes up the hydrogen of HO¹⁷, eliminating the two equivalents of oxygen united to H. I, of course, take another view of the case, bromine is to me an ozonide like peroxide of lead, &c., i.e. the peroxide of bromium = BrO + <oring->O. Now HO + <oring+>O and BrO + <oring->O catalyze each other into HO, BrO, and inactive oxygen, BrO + HO forming hydrobromic acid, or what might more properly be called hydrate of bromiatic acid.

You see that I am growing more and more hardened in my heretical notions, or to speak more correctly, in my orthodox views; for it was Davy who acted the part of a heretic in overthrowing the old, venerable, true creed¹⁸. Indeed the longer I compare the new and old doctrine on the nature of chlorine, &c. with the whole material of chemical facts bearing upon them, the less I am able to conceive how Davy could so lightly and slightly handle the heavy weight of analogies which, in my opinion, speak so very strongly and decisively in favour of Berthollet’s¹⁹ views²⁰. There is no doubt Sir Humphry was a man of great genius, and consequently very imaginative; but I am almost inclined to believe that, by a certain wantonness, or by dint of that transcendent faculty of his mind, he was seduced to conjure up a theory intended to be as much out of the way and “invraisemblable” as possible, and serve nevertheless certain theoretical purposes; and certainly, if he entertained the intention of solving such a problem, he has wonderfully succeeded. But what I still more wonder at is both the sudden and general success which that far-fetched and strained hypothesis met with, and the tenacity with which the whole chemical world has been sticking to it ever since its imaginative author pleased to divulge it: and all this could happen in spite of the fact that the new doctrine, in removing from the field of chemistry a couple of hypothetical bodies, was, for analogy’s sake, forced to introduce fictitious compounds, not by dozens only, but by hundreds, - the oxy-sulphion, oxy-nitrion, and the rest of those “nonentia.” But enough of this subject, upon which I am apt to grow warm and even angry. Although the results I have obtained from my recent investigations cannot but induce me to begin another, and, I am afraid, endless series of researches, I shall for the present cut short the matter and indulge for some time in absolute idleness.

I am, my dear Faraday, | Yours most truly, | C.F. Schönbein.

Bâsle, June 25, 1858.