Humphry Davy, Poet and Philosopher

by T. E. Thorpe

Davy communicated the results of his inquiries made prior to the summer of 1806 in a paper to the Royal Society, which was made the Bakerian lecture of the year.[F] It is entitled “On some chemical Agencies of Electricity,” and is divided in nine sections and an introduction. In the first section, “On the Changes produced by Electricity in Water,” he set at rest the disputed question as to the origin of the acid and alkaline matter which had been observed to form during the electrolysis of this liquid. By some these substances were supposed to be generated from pure water by the action of electricity; and M. Brugnatelli had even attempted to prove the existence of a body sui generis which he termed the electric acid. By a series of convincing experiments Davy showed that the substances were due to the presence of saline matter in the water, derived either from faulty purification, or from the solvent action of the water on the vessels, etc., with which it was in contact. Cruickshank had found that in some cases the acid was nitric acid and the alkali ammonia: these substances were shown by Davy to be due to the presence of dissolved air. When pure water, contained in vessels on which it exerted no solvent action, was “electrised” in vacuo, not a trace of either acid or alkali was produced.

[F] This lecture, which is one of the events of each session of the Royal Society, owes its origin to Mr. Henry Baker, F.R.S., a learned antiquary and naturalist, who, by his will of July, 1763, bequeathed the sum of £100 to the Society, the interest of which was to be applied “for an oration or discourse to be spoken or read yearly by someone of the Fellows of that Society, on such part of Natural History or Experimental Philosophy, at such time, and in such manner, as the President and Council of the said Society for the time being, shall please to order and appoint.” Baker died in 1774, and the bequest came into operation during the presidency of Sir John Pringle; and Peter Woulfe—one of the last of the English alchemists—was appointed to deliver the lecture, which he did for three successive years.

In the second section, “On the Agencies of Electricity in the Decomposition of various Compounds,” he begins by pointing out that in all the experiments recorded in the preceding section—that is, in all changes in which acid and alkaline matter had been present—the acid matter collected in the water round the positive pole, and the alkaline matter round the negative pole. This he shows to be true even of such sparingly soluble substances as gypsum, the sulphates of strontium and barium, and fluorspar. By connecting together cups or vessels made of the substances under investigation by a thread of well-washed asbestos, as suggested by Wollaston, he found that in all cases the acid element collected round the positive, and the earthy base round the negative pole. Basalt from Antrim, a zeolite from the Giant’s Causeway, vitreous lava from Etna, and even glass, in like manner yielded alkaline matter to water when subjected to the action of voltaic electricity. Soluble salts, such as the sulphates of sodium, potassium, and ammonium, the nitrates of potassium and barium, the succinate, oxalate and benzoate of ammonium, were similarly decomposed: the acids in a certain time collected in the tube containing the positive wire, and the alkalis and earths in that containing the negative wire. When metallic solutions, such as those of iron, zinc, and tin were employed, metallic crystals or depositions were formed on the negative wire, and oxide was likewise deposited round it; and a great excess of acid was soon found in the opposite cup.

In the next section, “On the Transfer of Certain of the Constituent Parts of Bodies by the Action of Electricity,” he points out that the observations of Gautherot and of Hisinger and Berzelius rendered it probable that the saline elements evolved in decompositions by electricity were capable of being transferred from one electrified surface to another, according to their usual order of arrangement, but that exact observations on this point were wanting. He connected a cup of gypsum with one of agate by means of asbestos, and filling each with purified water, he inserted the negative wire of the battery in the agate cup, and the positive wire in that of the sulphate of lime. In about four hours he found a strong solution of lime in the agate cup, and sulphuric acid in that of gypsum. By reversing the order, and carrying on the process for a similar length of time, the sulphuric acid appeared in the agate cup, and the solution of lime on the opposite side. Many trials were made with other saline substances with analogous results.

The time required for these transmissions (the quantity and intensity of the electricity, and other circumstances remaining the same) seemed to be related to the length of the intermediate column of water.

To ascertain whether the contact of the saline solution with a metallic surface was necessary for the decomposition and transference, he introduced purified water into two glass tubes; a vessel containing solution of potassium chloride was connected with each of the tubes by means of asbestos; on introducing the wires into the tubes alkaline matter soon appeared in one tube, and acid matter in the other; and in the course of a few hours moderately strong solutions of potash and of hydrochloric acid were formed.

Two tubes, one containing distilled water, the other a solution of potassium sulphate, were each connected by asbestos threads with a vessel containing a dilute solution of litmus; the saline matter was negatively electrified; and as it was natural to suppose that the sulphuric acid in passing through the water to the positive side would redden the litmus in its course, some slips of litmus paper were placed above and below the pieces of asbestos, directly in the circuit: it was found that the acid and alkali passed through the litmus solution without effecting any change in colour.

“As acid and alkaline substances during the time of their electrical transfer passed through water containing vegetable colours without affecting them, or apparently combining with them, it immediately became an object of inquiry whether they would not likewise pass through chemical menstrua having stronger attractions for them; and it seemed reasonable to suppose that the same power which destroyed elective affinity in the vicinity of the metallic points would likewise destroy it, or suspend its operation, throughout the whole of the circuit.”

To test this supposition, solution of potassium sulphate was placed in contact with the negative wire, and pure water in contact with the positive wire and a weak solution of ammonia was made the middle link of the conducting chain, so that no sulphuric acid could pass to the positive pole in the distilled water without passing through the solution of ammonia.

In less than five minutes it was found that acid was collecting round the positive pole, and in half an hour the water was sour to the taste, and gave a precipitate with barium nitrate. Hydrochloric acid from common salt, and nitric acid from nitre were transmitted through concentrated alkaline menstrua under similar circumstances. Strontia and baryta readily passed, like the other alkaline substances, through hydrochloric and nitric acids; and vice versâ these acids passed with facility through aqueous solution of baryta and strontia; but it was impossible to pass sulphuric acid through baryta or strontia, or to pass baryta and strontia through sulphuric acid, as precipitates of insoluble barium and strontium sulphate were formed.