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.
The genius displayed by Mr. Knight in investigating the phenomena of vegetable nature, and in applying the knowledge thus acquired to objects of practical improvement, excited in Davy, as might have been expected, feelings of the highest admiration; and when, in addition to such claims, he was the acknowledged patron and hospitable friend of the angler, the reader will readily imagine the warmth of feeling with which our philosopher cherished his friendship.
On commencing the present work, I applied to Mr. Knight for any assistance he might be able to afford me, in aid of so arduous a labour; and he very kindly returned an answer, from which I extract the following passage.
"My late lamented friend, Sir Humphry Davy, usually paid me a visit in the autumn, when he chiefly amused himself with angling for grayling, a fish which he appeared to take great pleasure in catching. He seemed to enjoy the repose and comparative solitude of this place, where he met but few persons, except those of my own family, for we usually saw but little company. He always assured me that he passed his visits agreeably, and I had reason to believe he expressed his real feelings.