Christian Friedrich Schoenbein to Faraday   19 April 1844

My dear Faraday

Since I wrote you last¹ I have continued my researches on "ozone" and obtained from them results which seem to be important enough as to justify my addressing to you another letter on the subject. I have succeeded to put it beyond even a shade of doubt that the odoriferous principles being disengaged during electrical discharges in the common air, the electrolysis of water and the slow action of phosphorus upon the atmosphere are absolutely identical to one another as to their chemical nature and that my ozone, as I originally suspected it be, is really a halogenous body very closely resembling to Chlorine. The named principle has the power 1) of destroying vegetable colors, 2) of decomposing a variety of compounds which are decomposed by Chlorine, for instance sulphuretted hydrogen, Ammonia, Jodide of potassium. Water also is decomposed by Ozone in similar circumstances under which Chlorine produces that effect, ex. gr. when sulphurous acid or a number of readily oxidable matters are simultaneously acting upon water. 3) of changing the yellow ferro-cyanide of potassium into the red one. I could add many other facts more showing the chlorine-like nature of ozone, but the stated ones are sufficient to prove the correctness of my assertion. Ozone if inhaled proves very deleterious to the constitution and produces effects similar to those called forth by Chlorine. A mouse is already fallen victim to my discovery and I myself have strongly felt the powerful action of ozone upon the system. Having drawn up a paper in which I have given a detailed account of the results obtained from my researches and which I hope will soon be published I take the liberty to refer you for the sake of particular information to that memoir². I cannot however help adding that the whole body of facts which I have been lucky enough to ascertain render it highly probable if not certain that ozone is derived from azote, that is to say that the latter body is a compound consisting of ozone and hydrogen. Starting from that conclusion or if you like supposition the disengagement of Ozone taking place under circumstances apparent[l]y so widely differing from each other is very easily accounted for. 1. The disengagement of Ozone in atmospheric air by means of common electricity. If an electrical discharge takes place in the common air, the oxigen of the latter unites with hydrogen of azote and sets Ozone at liberty. Should hydro-chloric acid happen to be a constituent part of our atmosphere instead of Azote, a series of phenomena would take place at the points of emission of an electrical machine closely resembling those which we observe now at those points. A smell of Chlorine would make its appearance there, a stripe of gold held into the electrical brush would become negatively polarized, starch mixt up with jodide of potassium would turn blue, the yellow ferro-cyanide be changed into the red one, organic coloring matter be bleached, &c and the whole series of the phenomena mentioned rendered impossible to take place, if the points of emission were surrounded by an atmosp[h]ere holding some sulphuretted hydrogen, sulphurous acid, vaporous phosphorus, &c dissolved. Indeed the disengagement of ozone at those points is entirely stopt by mixing up the atmosphere with very small quantities of the gazeous substances last mentioned, as you will learn from my memoir.

2. The disengagement of ozone by the slow action of Phosphorus upon the atmospheric air. Phosphorus being simultaneously in contact with Azote and Oxigen causes the latter to unite with the hydrogen of azote, whilst another portion of oxigen combines with phosphorus to form phosphorus acid. Ozone is set at liberty part which reacts however upon phosphorus forming ozonide of phosphorus, whilst another part is thrown into the air being placed above the phosphorus. That compound being in contact with water is changed into phosphoric acid and ozonide of hydrogen i.e. Azote. The transformation of phosphorus into phosphatic acid, whilst that elementary body is acted upon by atmospheric air essentially depends upon the action mentioned. I must not omit to mention that all vaporous or gazeous substances which, when mixt up with atmospheric air prevent phosphorus from emitting light (for instance vapor of ether, alcohol carburetted hydrogen &c) do also stop the disengagement of ozone as well as the oxidation of phosphorus.

3. The disengagement of Ozone by voltaic electricity. Azote being an electrolyte like hydro-chloric acid is decomposed into its constituent parts by a current if dissolved in water i.e. rendered liquid by that agency. According to the results of my recent researches water being deprived of atmospheric air i.e. Azote does not yield the smallest quantity of Ozone at the positive electrode and acquires that property again by shaking that sort of water with atmospheric air. I may as well mention here, that water containing only very small quantities of sulphurous acid, sulphuretted hydrogen in short those substances which have the power to prevent the disengagement of Ozone near electrical points does not yield the slightest trace of ozone. From the preceding remarks you will perceive that the disengagement of Ozone brought about by electrical, voltaic and chemical means is easily and simply accounted for by supposing azote to be ozonide of hydrogen. Conclusive however as my results appear to me to be as to the compound nature of azote, I readily allow that many more experiments must be made and in particular that of isolating Ozone, before my conclusion or supposition can or will be considered as decisive. On treating my ozone with a solution of potash I obtain nitrate of potash which fact goes rather far to prove the identity of nitric and ozonic acid. You have made the same experiment in causing the electrical brush to act upon a piece of paper being impregnated with a solution of potash. You got salpetre by the electrical ozone, I by ozone being produced in the chemical way. In your experiment as well as in mine the formation of nitric acid is due to a secondary chemical action and not to the immediate or direct union of Azote and Oxigen. I think it likely that during the action of Ozone upon the alkaline solution, not only ozonate of potash is formed, but also ozonide of potassium just in the same manner as out of Chlorine and potash chlorate of potash and Chloride of potassium are produced. It is however a chemical possibility also that ozone and potash generate nothing but Ozonate of potash. If you wish to repeat my principal experiments I strongly recommend to you the use of paper being impregnated with starch and jodide of potassium. It is a test for ozone being far superior even to the most delicate galvanometer. By that means you will easily ascertain the disengagement of ozone near a piece of phosphorus if that body is put into the open air after having been a little rubbed and dried by filtering paper. Provided the slow oxidation of phosphorus be rather rapid your test-paper will not fail being turned blue in a few instants. At a low temperature no such result will be obtained. To give you a matter-of-fact proof of the bleaching power of ozone I lay by three stripes of litmus paper of which Nro. 1. was bleached by the electrical brush (produced by a four hours working of the machine), Nro. 2. by voltaic ozone and Nro. 3. by chemical ozone. As the latter one is in a more condensed state it bleaches more rapidly than the two other sorts of ozone do.

To produce a fair quantity of ozone, put a piece of phosphorus into a bottle being filled with common air and expose the whole to a temperature of 15-25˚C. Within a few minutes you will find your air charged with ozone already sufficient to turn your test paper into blue, and after an hours action the bleaching power of our atmosphere is such as to render (within a short time) a piece of litmus paper not strongly colored entirely white.

As the matter which I have now got into my hands promises to become rather a rich mine for scientific research I flatter myself that you will not think me intrusive if I take the liberty to acquaint you from time to time with my results. I trust however, that before the year will be much older, I shall have the pleasure of paying you a visit and work with you in the Royal Institution; for I have a strong mind to cross the water in the month of July. Pray let me soon hear from you and excuse my hastily and badly written letter. As you may easily imagine I am now in rather a feverish state working from morning to night in my laboratory and sleeping very little at night. Mrs. Schoenbein is quite surprized at my taciturnity and prolonged absences from home. She unites with me in kind regards to you and Mrs. Faraday and begs to be kindly remembered by you.

Your's very faithfully | C.F. Schoenbein

Bale April 19th 1844.

Do you think a paper on Ozone would prove acceptable to the Royal Society? S

Address: Doctor M. Faraday | &c &c &c | Royal Institution | London.