Part 23 (1/2)

764. From all the experiments, combined with these considerations, I conclude that muriatic acid is decomposed by the direct influence of the electric current, and that the quant.i.ties evolved are, and therefore the chemical action is, _definite for a definite quant.i.ty of electricity_. For though I have not collected and measured the chlorine, in its separate state, at the _anode_, there can exist no doubt as to its being proportional to the hydrogen at the _cathode_; and the results are therefore sufficient to establish the general law of _constant electro-chemical action_ in the case of muriatic acid.

765. In the dilute acid (761.), I conclude that a part of the water is electro-chemically decomposed, giving origin to the oxygen, which appears mingled with the chlorine at the _anode_. The oxygen _may_ be viewed as a secondary result; but I incline to believe that it is not so; for, if it were, it might be expected in largest proportion from the stronger acid, whereas the reverse is the fact. This consideration, with others, also leads me to conclude that muriatic acid is more easily decomposed by the electric current than water; since, even when diluted with eight or nine times its quant.i.ty of the latter fluid, it alone gives way, the water remaining unaffected.

766. _Chlorides._--On using solutions of chlorides in water,--for instance, the chlorides of sodium or calcium,--there was evolution of chlorine only at the positive electrode, and of hydrogen, with the oxide of the base, as soda or lime, at the negative electrode. The process of decomposition may be viewed as proceeding in two or three ways, all terminating in the same results. Perhaps the simplest is to consider the chloride as the substance electrolyzed, its chlorine being determined to and evolved at the _anode_, and its metal pa.s.sing to the _cathode_, where, finding no more chlorine, it acts upon the water, producing hydrogen and an oxide as secondary results.

As the discussion would detain me from more important matter, and is not of immediate consequence, I shall defer it for the present. It is, however, of _great consequence_ to state, that, on using the volta-electrometer, the hydrogen in both cases was definite; and if the results do not prove the definite decomposition of chlorides, (which shall be proved elsewhere,--789. 794. 814.,) they are not in the slightest degree opposed to such a conclusion, and do support the _general law_.

767. _Hydriodic acid._--A solution of hydriodic acid was affected exactly in the same manner as muriatic acid. When strong, hydrogen was evolved at the negative electrode, in definite proportion to the quant.i.ty of electricity which had pa.s.sed, i.e. in the same proportion as was evolved by the same current from water; and iodine without any oxygen was evolved at the positive electrode. But when diluted, small quant.i.ties of oxygen appeared with the iodine at the _anode_, the proportion of hydrogen at the _cathode_ remaining undisturbed.

768. I believe the decomposition of the hydriodic acid in this case to be direct, for the reasons already given respecting muriatic acid (763. 764.).

769. _Iodides._--A solution of iodide of pota.s.sium being subjected to the voltaic current, iodine appeared at the positive electrode (without any oxygen), and hydrogen with free alkali at the negative electrode. The same observations as to the mode of decomposition are applicable here as were made in relation to the chlorides when in solution (766.).

770. _Hydro-fluoric acid and fluorides._--Solution of hydrofluoric acid did not appear to be decomposed under the influence of the electric current: it was the water which gave way apparently. The fused fluorides were electrolysed (417.); but having during these actions obtained _fluorine_ in the separate state, I think it better to refer to a future series of these Researches, in which I purpose giving a fuller account of the results than would be consistent with propriety here[A].

[A] I have not obtained fluorine: my expectations, amounting to conviction, pa.s.sed away one by one when subjected to rigorous examination; some very singular results were obtained; and to one of these I refer at 1340.--_Dec. 1838._

771. _Hydro-cyanic acid_ in solution conducts very badly. The definite proportion of hydrogen (equal to that from water) was set free at the _cathode_, whilst at the _anode_ a small quant.i.ty of oxygen was evolved and apparently a solution of cyanogen formed. The action altogether corresponded with that on a dilute muriatic or hydriodic acid. When the hydrocyanic acid was made a better conductor by sulphuric acid, the same results occurred.

_Cyanides._--With a solution of the cyanide of pota.s.sium, the result was precisely the same as with a chloride or iodide. No oxygen was evolved at the positive electrode, but a brown solution formed there. For the reasons given when speaking of the chlorides (766.), and because a fused cyanide of pota.s.sium evolves cyanogen at the positive electrode[A], I incline to believe that the cyanide in solution is _directly_ decomposed.

[A] It is a very remarkable thing to see carbon and nitrogen in this case determined powerfully towards the positive surface of the voltaic battery; but it is perfectly in harmony with the theory of electro-chemical decomposition which I have advanced.

772. _Ferro-cyanic acid_ and the _ferro-cyanides_, as also _sulpho-cyanic acid_ and the _sulpho-cyanides_, presented results corresponding with those just described (771.).

773. _Acetic acid._--Glacial acetic acid, when fused (405.), is not decomposed by, nor does it conduct, electricity. On adding a little water to it, still there were no signs of action; on adding more water, it acted slowly and about as pure water would do. Dilute sulphuric acid was added to it in order to make it a better conductor; then the definite proportion of hydrogen was evolved at the _cathode_, and a mixture of oxygen in very deficient quant.i.ty, with carbonic acid, and a little carbonic oxide, at the _anode_. Hence it appears that acetic acid is not electrolyzable, but that a portion of it is decomposed by the oxygen evolved at the _anode_, producing secondary results, varying with the strength of the acid, the intensity of the current, and other circ.u.mstances.

774. _Acetates._--One of these has been referred to already, as affording only secondary results relative to the acetic acid (749.). With many of the metallic acetates the results at both electrodes are secondary (746. 750.).

Acetate of soda fused and anhydrous is directly decomposed, being, as I believe, a true electrolyte, and evolving soda and acetic acid at the _cathode_ and _anode_. These however have no sensible duration, but are immediately resolved into other substances; charcoal, sodiuretted hydrogen, &c., being set free at the former, and, as far as I could judge under the circ.u.mstances, acetic acid mingled with carbonic oxide, carbonic acid, &c.

at the latter.

775. _Tartaric acid._--Pure solution of tartaric acid is almost as bad a conductor as pure water. On adding sulphuric acid, it conducted well, the results at the positive electrode being primary or secondary in different proportions, according to variations in the strength of the acid and the power of the electric current (752.). Alkaline tartrates gave a large proportion of secondary results at the positive electrode. The hydrogen at the negative electrode remained constant unless certain triple metallic salts were used.

776. Solutions, of salts containing other vegetable acids, as the benzoates; of sugar, gum, &c., dissolved in dilute sulphuric acid; of resin, alb.u.men, &c., dissolved in alkalies, were in turn submitted to the electrolytic power of the voltaic current. In all these cases, secondary results to a greater or smaller extent were produced at the positive electrode.

777. In concluding this division of these Researches, it cannot but occur to the mind that the final result of the action of the electric current upon substances, placed between the electrodes, instead of being simple may be very complicated. There are two modes by which these substances may be decomposed, either by the direct force of the electric current, or by the action of bodies which that current may evolve. There are also two modes by which new compounds may be formed, i.e. by combination of the evolving substances whilst in their nascent state (658.), directly with the matter of the electrode; or else their combination with those bodies, which being contained in, or a.s.sociated with, the body suffering decomposition, are necessarily present at the _anode_ and _cathode_. The complexity is rendered still greater by the circ.u.mstance that two or more of these actions may occur simultaneously, and also in variable proportions to each other. But it may in a great measure be resolved by attention to the principles already laid down (747.).

778. When _aqueous_ solutions of bodies are used, secondary results are exceedingly frequent. Even when the water is not present in large quant.i.ty, but is merely that of combination, still secondary results often ensue: for instance, it is very possible that in Sir Humphry Davy's decomposition of the hydrates of pota.s.sa and soda, a part of the pota.s.sium produced was the result of a secondary action. Hence, also, a frequent cause for the disappearance of the oxygen and hydrogen which would otherwise be evolved: and when hydrogen does _not_ appear at the _cathode_ in an _aqueous solution_, it perhaps always indicates that a secondary action has taken place there. No exception to this rule has as yet occurred to my observation.

779. Secondary actions are _not confined to aqueous solutions_, or cases where water is present. For instance, various chlorides acted upon, when fused (402.), by platina electrodes, have the chlorine determined electrically to the _anode_. In many cases, as with the chlorides of lead, pota.s.sium, barium, &c., the chlorine acts on the platina and forms a compound with it, which dissolves; but when protochloride of tin is used, the chlorine at the _anode_ does not act upon the platina, but upon the chloride already there, forming a perchloride which rises in vapour (790.

804.). These are, therefore, instances of secondary actions of both kinds, produced in bodies containing no water.

780. The production of boron from fused borax (402. 417.) is also a case of secondary action; for boracic acid is not decomposable by electricity (408.), and it was the sodium evolved at the _cathode_ which, re-acting on the boracic acid around it, took oxygen from it and set boron free in the experiments formerly described.

781. Secondary actions have already, in the hands of M. Becquerel, produced many interesting results in the formation of compounds; some of them new, others imitations of those occurring naturally[A]. It is probable they may prove equally interesting in an opposite direction, i.e. as affording cases of a.n.a.lytic decomposition. Much information regarding the composition, and perhaps even the arrangement, of the particles of such bodies as the vegetable acids and alkalies, and organic compounds generally, will probably be obtained by submitting them to the action of nascent oxygen, hydrogen, chlorine, &c. at the electrodes; and the action seems the more promising, because of the thorough command which we possess over attendant circ.u.mstances, such as the strength of the current, the size of the electrodes, the nature of the decomposing conductor, its strength, &c., all of which may be expected to have their corresponding influence upon the final result.

782. It is to me a great satisfaction that the extreme variety of secondary results has presented nothing opposed to the doctrine of a constant and definite electro-chemical action, to the particular consideration of which I shall now proceed.

-- vii. _On the definite nature and extent of Electro-chemical Decomposition._