Christian Friedrich Schoenbein to Faraday   22 March 1845

Bâle | March 22, 1845.

My dear Faraday,

I am inclined to believe that I have at last succeeded in solving the problem regarding the nature of ozon and as you were kind enough to pay some attention to my researches on that subject, I flatter myself, you will peruse with interest the following lines. Having failed in all my numerous attempts at isolating the odoriferous principal and not being able of attaining from those experiments any other results than what consisted in simple effects of oxidation, I was induced to reexamine the circumstances under which Ozon is generated. Resuming the very first idea I started about the nature of the substance five years ago and according to which ozone was to be considered as a peculiar compound consisting of oxigen and hydrogen, I recommenced my researches with ascertaining the part which water acts in the formation of our principle. I found out that it is most abundantly produced, more abundantly indeed than by any other means known to me, by putting phosphorous in immediate contact both with water and atmospheric air at a temperature of about 30˚c. Under these circumstances the air of a very large bottle may be very richly charged with ozon even within a very few minutes. In exposing phosphorous to the action of common air being deprived of its humidity as completely as possible and suffering the other circumstances to remain the same, hardly a trace of ozon makes its appearance, however long the contact of phosphorous with air may last. In making the experiments just mentioned with a gazeous mixture of Oxigen and Carbonic Acid Gas exactly the same results were obtained, ozon was abundantly produced in it when water happened to be present and the temperature to be sufficiently high, whilst the formation was stopt in the dry mixture. Already five years ago I ascertained the following points 1.) that by heating the points from which electricity is passing into the air the peculiar electrical smell is prevented from making its appearance 2.) that under these circumstances the electrical spark loses its polarizing power 3.) that heated plates of platinum are not polarized by the brush issuing from cold points of emission 4.) that the voltaic current is not capable of disengaging ozon out of water being heated to a certain degree 5.) that plates of platinum being heated to the same degree do not assume the negatively polar state when placed within an atmosphere of voltaic ozon. Presuming, the effects described might be due to a decomposition of ozon carried about by the action of heat I made (with the view of proving the correctness of this conjecture) the following experiments. In the usual way the air of a large bottle was abundantly charged with ozone, and an apparatus arranged in such a manner as to allow a current of that air passing through a narrow tube into the atmosphere. After having ascertained that the air issuing from the said tube enjoyed all the properties belonging to ozon, I applied heat to part of the tube and no sooner had the latter assumed a certain temperature, than the peculiar smell disappeared on a sudden and with it also all the other properties of ozon. In letting cool down again the tube to a certain degree of temperature, the air issuing from it reassumed its peculiar smell and along with it all its chemical and voltaic properties. It is hardly necessary to state that voltaic ozon undergoes the same change under the circumstances last mentioned and therefore it appears to be an undoubted fact that ozon, in whatever way it may have been produced, is destroyed, by heat i.e. decomposed (most likely into water and oxigen)[.]

A few days ago M. Marignac a very clever Chymist of Geneva and a friend of mine communicated to me the results of his experiments lately made on ozon and they are indeed such as to completely square with mine. In treating powder of silver with large quantities of ozonized air he could obtain nothing but pure oxide of silver and in decomposing iodide of potassium by ozon the Chymist of Geneva got (like myself) iodate of potash, some muriate and free iodine; making use of acidulated water containing a trace of nitrogen and having this fluid electrolyzed, M. Marignac and M. de la Rive obtained ozon as long as the current passed through it, provided the liquid was kept cool during its electrolysis. M. Marignac found out that oxigen of itself, is not capable of producing ozon with phosphorous, and that the latter gas must contain either nitrous acid gas or nitrogen or hydrogen in order to generate that odoriferous substance. From the results obtained both by myself and M. Marignac it appears that nitrogen has directly nothing to do with the formation of ozon, that the latter is no elementary body and that it is produced, if water and oxigen happen to come into contact with one another under peculiar circumstances. This formation of ozon which takes place during the electrolysis of water is easily accounted for, but as to the way in which phosphorous or an electrical discharge determines the atmospheric oxigen to unite with the water there into ozon we do not yet know any thing about it.

My ozon seems to differ very widely from Thenard's¹ peroxide of hydrogen², the latter being inodorous, readily soluble in water, communicating positive polarity to platinum and suffering decomposition by silver, without oxidizing that metal; (according to the statement of Thenards) whilst ozon has a peculiar and very strong smell, is scarcely soluble in water, makes platinum assume negative polarity and oxidizes silver.

Ozon being generated in the atmosphere when the latter happens to be acted upon by common electricity, it is to be presumed that the former always contains more or less ozon according to circumstances and that on this account a series of chemical actions taking place within the common air may be traced to the presence of ozon; for instance the slow oxidation of metallic and other substances, the bleaching of vegetable colours &c. I think I can even prove the correctness of these conjectures by direct experiment[.] Paste of starch mixt up with some iodide of potassium being over long exposed to the action of atmospheric air that happens to be contained within a bottle remains colourless, whilst the same paste turns blue rather rapidly when placed into the open & freely circulating air. I have also ascertained the fact that a solution of iodide of potassium undergoes a slow change by being put in contact with the open air for under these circumstances small quantities of iodate of potash are formed and traces of iodine eliminated. Now ozon acting exactly in the same way upon the said paste of starch as well as upon the saline solution just mentioned, we can hardly account for the facts before us in another manner than by admitting the presence of some traces of that highly oxidizing agency in the atmosphere. I entertain therefore little doubt that ozon will turn out after all to be a substance destined to act rather an important and general part in the oeconomy of nature.

As to the isolation of ozon, I think the only way of effecting it is, to let pass large quantities of air being richly charged with that compound through tubes which are kept as cold as possible; for the facility with which Ozon is decomposed leaves very little hope for isolating it by chemical means. Before concluding allow me to say yet a word or two about the phenomenon so long known and so ill understood regarding the Shining of phosphorous within humid atmospheric air and the oxidation of that b<ody> into phosphoric acid. From the facts above mentioned it appears, <that> by some action or other exerted by phosphorous on the atmospheric oxigen and water, these two matters unite into ozon and that the latter reacts upon phosphorous in such a way as to oxidize it into phosphoric acid. From that oxidation proceeds no doubt the principal part of light emitted during the slow combustion of phosphorous within humid atmospheric air, for as soon as water is excluded the shining of phosphorous is nearly, and the formation of ozon entirely stopt and at the same time the oxidation of phosphorous becomes exceedingly slow. The phosphorous acid always mixt up with phosphoric acid, is most likely due to the oxidation of phosphorous brought about by free atmospheric oxigen. If you think the contents of this letter interesting enough as to be worthy of the notice of the Royal Society you will oblige me by your communicating them to that learned body³. My best compliments to Mrs. Faraday and my most friendly solicitations to Yourself.

Yours most truly | C.F. Schoenbein

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