I doubt not you have found before this, as I have done, that the substance we mistook for _sulphuretted_ hydrogen is _telluretted_ hydrogen, very soluble in water, combinable with alkalies and earths, and a substance affording another proof that hydrogen is an oxide. I have met with another analogous compound, that of _boracium_ with hydrogen, which possesses very similar properties. I find that taking _ammonium_ as the basis of hydrogen, according to the ideas which I stated, all the compounds will agree with the suppositions that I mentioned to you, _viz._ eight cubic inches of hydrogen, two of oxygen, ammonia; four and two, water; four and four, nitrogen; four and six, nitrous oxide; four and eight, nitrous gas; four and ten, nitric acid. Where the multiples are not in geometrical order, the decomposition is most easy, _i.e._ in nitrous oxide and nitric acid; more easy in water than in ammonia; but most difficult in nitrogen, where there is probably the most perfect equilibrium of affinities. I have kept charcoal white hot by the Voltaic apparatus, in dry oxymuriatic acid gas for an hour, without effecting its decomposition. This agrees with what I had before observed with a red heat. It is as difficult to decompose as nitrogen, except when all its elements can be made to enter into new combinations. I find the radiation, _in vacuo_, from ignited platina, is to that in air as three to one:--so much for Leslie's hypothesis. A little electrical machine acts with a repulsion as two, in a vacuum equal to five inches of mercury; as thirty, in common air; as thirty, in oxygen; as twenty-nine or thirty, in hydrogen; and as forty-five, in carbonic acid. I showed this experiment, made with every precaution, to Mr. Cavendish, Dr. Herschell, Dr. Wollaston, and Warburton: so much for the theory, that electricity is dependent upon oxidation. I do not think our worthy friend Pepys will resist any longer. Pray let me know what you have been doing. I hope you will not suffer these beautiful and satisfactory experiments of the capacities of metals to remain still. Write me a letter as egotistical as the one I have given you. You are pledged to do good and noble things, and you must not disappoint the men of science of this country. With kindest remembrances to your excellent father, and with hopes that we shall soon meet, I am, my dear friend, Very faithfully and affectionately yours, H. DAVY.