Christian Friedrich Schoenbein to Faraday   19 November 1847

My dear Faraday I am very sorry indeed to learn from your kind letter¹ that the state of your health is not such as your friends so ardently wish it to be. I think turning your back to noisy smoky London and living in some retired quiet corner blessed with a pure atmosphere is by far the best you can do. And the winter past I should in your place quit England for a time and take up a temporary abode at some spot on the beautiful lake of Como, or at Meran or somewhere there about. Placing yourself in the midst of a serene, grand scenery will afford your mind a beneficial excitement and inhaling a pure balmy air will strengthen your frame and recall the elasticity of your spirits. Beautiful as England is in many respects, nature there is too tame and uniform the sky too pale, the air too thick to suit your present condition. Excuse my acting the part of a medical adviser but as my council comes from the conviction that it is the best which can be given to you I am sure you will not take it ill.

You know my heterodoxical notions regarding the nature of Chlorine which after the old creed I hold to be an oxy-compound similar in constitution to the peroxides of hydrogen, manganese &c. Now those notions are the source of all the experiments I have made these many years and if I have been fortunate enough to ascertain some interesting facts, I owe it entirely to my strange hypothetical views and to reasonings founded upon them. You are also aware that Ozone bears in many respects a very close resemblance to Chlorine, Bromine and Jodine and the strongest atmosphere of ozone being almost instantaneously destroyed by powder of Charcoal, I was curious to see how Chlorine Bromine &c are acted upon by powdered charcoal and my experiments have led to results of which I shall give you a summary account.

1. The strongest atmosphere of Chlorine on being shaken with powder of common charcoal is almost instantaneously destroyed at the common temperature as well as at 212˚.

2. A current of Chlorine passing through a tube filled with powder of charcoal is readily taken up much heat being disengaged from the latter. The charcoal thus treated does not exhibit the odour of Chlorine, even if considerably heated, but emits fumes of muriatic acid and yields the same acid to water. The freshly prepared chlorified charcoal has however like Chlorine the power of discharging the colour of an Indigo solution and decomposing jodide of potassium, but in leaving it for some time in contact with water or atmospheric air it looses that property.

3. The strongest aqueous solution of Chlorine, if shaken with a sufficient quantity of charcoal powder quickly looses its yellow colour, smell, bleaching power &c., muriatic acid being produced. Powder of charcoal is also capable of completely destroying the bleaching power of aqueous solutions of hypo chlorites for instance that of the common Chloride of lime. The same effect is produced by Charcoal upon what Berzelius considers as deutochloride of manganese and which is obtained by treating peroxide of manganese with muriatic acid at the common temperature. Charcoal transforms the solution of that compound into that of the common protochloride of manganese.

4. The densest atmosphere of Bromine Vapour most rapidly and completely disappears even at a temperature of 212˚ when brought in contact with powder of Charcoal, and liquid Bromine on being mixt up with the same powder is rendered so latent that the mixture may be heated to 212˚ without yielding a trace of bromine; at a higher temperature however some Bromine is given off. The brominiferous charcoal has the power of discharging the colour of Indigo solution and decomposing jodide of potassium. The strongest aqueous solution of Bromine on being shaken with powder of charcoal becomes colourless, looses its smell, bleaching power &c.

5. Charcoal powder causes rapidly the disappearance of the densest vapour of Jodine even at a temperature of 212˚ and an intimate mixture of 9 parts of Charcoal and one part of jodine exhibits not the slightest smell and does not yield a trace of vapour of jodine even at the boiling point of water, at a considerably higher temperature however some Jodine vapour is disengaged. The colour of an aqueous solution of Jodine is quickly discharged by powder of charcoal.

6. A colourless mixture of one part of hyponitric acid and 9 parts of water on being mixt up with charcoal powder gives rise to a most lively and abundant disengagement of deutoxide of azote, no carbonic acid being produced under these circumstances. Monohydrate of nitric acid on being put in contact with charcoal powder even at a temperature of 0˚ F is partly decomposed, hyponitric acid being eliminated but no carbonic acid produced. You know that I consider that monohydrate as NO<4> + HO<2> and hold the opinion that on mixing hyponitric acid and water together two compounds are formed: NO<4> + HO<2> and NO<2> + HO<2>. Now as to the decomposition of what they call monohydrate of nitric acid effected by Charcoal I am inclined to ascribe it to the well known power of that substance of composing the peroxide of hydrogen, and agreeably to the same hypothesis I account for the disengagement of deutoxide of azote out of the mixture before mentioned. HO<2> united to NO<2> is decomposed by Charcoal into water and oxigen the latter being thrown upon some NO<2> + HO<2>, to form NO<4> + HO<2> and the NO<2>, being freed from HO<2>, set free. - It seems to me that the facts above mentioned are not due to the well known power of charcoal of absorbing gazeous bodies, but to something else, of which we have not yet got a clear notion and I am inclined to think that the cause which makes charcoal act upon Chlorine, Bromine, Jodine in the manner described is the same that gives to charcoal the power of destroying Ozone, Thenard's² peroxide of hydrogen, permanganic acid, monohydrate of nitric acid, what they call aqueous hyponitric acid, solutions of the hypochlorites &c, without producing carbonic acid. But what that cause is I am far from being prepared to say. At any rate it is a fact worthy of consideration, that all the substances that are so peculiarly acted upon by charcoal bear the same electro motive character; they are electro-negative bodies. Before I conclude allow me to mention to you another fact which I ascertained some time ago and will interest you. If paste of starch being mixt up with so much jodide of lead as to give the former a lively yellow colour be spread over a band of white paper and exposed to the action of direct solar rays, it suddenly turns its colour becoming green in the first instance and dark blue within a very few seconds. This change of colour effected by direct solar light is almost as instantaneous as that brought about by a strong atmosphere of chlorine or ozone and of course due to an elimination of jodine from the jodide of lead.

I am not aware of any other substance being so suddenly and perceptibly affected by solar light as the said paste proves to be, and on that account I think it might be used as a means for examining more closely the relative chemical powers of the different species of rays of which white solar light is made up.

Living at this present moment amidst the clamour of civil war³ and writing under the impression of extraordinary events, I am sure you will be indulgent to me as to the great imperfection of this letter. Pray present my best compliments to Mrs. Faraday and believe me my dear Faraday

Your's most truly | C.F. Schoenbein

Bâle November 19, 1847.

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