Christian Friedrich Schoenbein to Faraday   April 1862¹

I have been busily occupied with my favorite study, and have found out several new facts regarding the allotropic states of oxygen, their changeability one into another, and nitrification, and I am inclined to believe that the results obtained are not quite void of scientific interest.

After many fruitless attempts at isolating ozone from an “ozonide”, I have at last succeeded in performing that exploit; and have also found out simple tests for distinguishing with the greatest ease ozone from its antipode, “antozone”. As to the production of ozone by purely chemical means, the whole secret consists in dissolving pure manganate of potash in pure oil of vitriol and introducing into the green solution pure peroxide of barium, when ozone mixed with common oxygen will make its appearance, as you may easily perceive by your nose and other tests. By means of the ozone so prepared, I have rapidly oxidized silver at the temperature of -20°C., and by inhaling it produced a capital “catarrh.”

Regarding nitrification, the most important fact I have discovered is the generation of nitrite of ammonia out of water and nitrogen, i.e. atmospheric air, which is certainly a most wonderful and wholly unexpected thing. To state the fact in the most general manner, it may be said that the salt mentioned is always produced if water be evaporated in contact with atmospheric air. This may be shown in a variety of ways. Let, for instance, a piece of clean linen drenched with distilled water dry in the open air, moisten it then with pure water, and you will find that the liquid wrung out of the linen and acidulated with dilute sulphuric acid (chemically pure) will strike a blue colour with starch-paste containing iodide of potassium,- by the by, the most delicate test for the nitrites. It is therefore a matter of course that shirts, handkerchiefs, table-cloths, in fact all linen, &c., must contain appreciable quantities of nitrite of ammonia; and if the chemistry of England be not entirely different from that of Switzerland, you will find the same thing at the Royal Institution. The purest water, suffered to evaporate spontaneously in the open air, will after some time have taken up enough nitrite of ammonia (continually being formed at the evaporating surface) to produce the nitrite reaction. If you make use of water holding a little potash, or any other alkali, in solution, the same results will be obtained, i.e. the nitrite of that base will be formed (of course in small quantity). The most convenient way of performing the experiment is to moisten a bit of filtering-paper with a dilute solution of chemically pure potash, &c., and to suspend it for twenty-four hours in the open air. On examining the paper it will be found to contain a perceptible quantity of a nitrite, which by a longer exposure of course increases. But you may still more rapidly convince yourself of the correctness of my statements, if you heat pure water to a temperature of 50° or 60° C. in a porcelain basin, and suspend over the evaporating surface bands of filtering-paper soaked with a weak solution of potash, soda, or the carbonates of these bases. Within a very short time (in ten minutes or so) there will enough of the nitrite accumulate in the paper to produce the reactions of the salt. I enclose a bit of paper treated in that way for a couple of hours, and by laying it upon a watch-glass and pouring over it acidulated starch-paste containing iodide of potassium, you will perceive the effect produced. The fact which I have ascertained, that the purest water mixed with a little chemically pure sulphuric acid or potash and kept for some time evaporating in the open air at a temperature of 50° or 60° C. (the loss of the liquid being now and then restored) contains, in the first case a perceptible quantity of ammonia, and, in the second case, of nitrous acid, may now be easily accounted for. You know that about eighteen months ago I found that, during the slow combustion of phosphorus in moist atmospheric air, very perceptible quantities of nitrite of ammonia are formed, and drew from that fact the inference that the salt is engendered by 3 equivalents of water combining directly with 2 equivalents of nitrogen. Now there is to me hardly any doubt that the production of that nitrite is due to the evaporation of water taking place about the phosphorus, whose temperature, in consequence of its burning state, proves to be higher than that of the surrounding medium, and the fact alluded to must therefore be considered only as a particular case of a general rule. The same remark applies to the formation of nitrite of ammonia which takes place during the rapid combustion of charcoal, &c. in atmospheric air. Combustion, as such, has, I believe, nothing to do with that formation. I must not omit to tell you that by means of a large copper still, properly heated, and taking care not to introduce too much water into the vessel at once, I can prepare in a very short time several pints of water with which the reaction of nitrite of ammonia may be produced in the most striking manner. I hope before long to have an opportunity of sending you some of this water.

I cannot finish my letter without saying a word or two about nitrification in general, a fact hitherto so much enveloped in obscurity. I think the matter is now clear enough. The evaporation of water is continually going on in the atmosphere, and along with it the generation of nitrite of ammonia. Now, this salt being put in contact with the alkaline bases or their carbonates, nitrites of potash and the other alkalies are formed, which afterwards gradually become oxidized into nitrates. In our rainy countries these salts are washed away almost as soon as formed, and carried into the springs, rivers, &c.; and there is therefore no accumulation of them as in the East Indies, &c.

That the formation of our nitrite out of water and nitrogen is a fact highly important for vegetation need hardly be stated. Indeed each plant, by continually evaporating water into the atmosphere, becomes a generator of nitrite of ammonia, preparing, if not all, at least part of its nitrogenous food, and the same thing takes place in the ground on which it stands. I am therefore inclined to think that our friend Liebig is right in asserting that no plant wants any artificial supply of ammonia, or of matters producing that compound, there being enough of it offered by natural means. Having communicated the results of my researches on the subjects mentioned above to the Academy of Munich², I hope they will soon be published³.