Part 21 (1/2)

687. _Chloride of sulphur_ does not conduct, nor is it decomposed. It consists of single proportionals of its elements, but is not on that account an exception to the rule (679.), which does not affirm that _all_ compounds of single proportionals of elements are decomposable, but that such as are decomposable are so const.i.tuted.

688. _Protochloride of phosphorus_ does not conduct nor become decomposed.

689. _Protochloride of carbon_ does not conduct nor suffer decomposition.

In a.s.sociation with this substance, I submitted the _hydro-chloride of carbon_ from olefiant gas and chlorine to the action of the electric current; but it also refused to conduct or yield up its elements.

600. With regard to the exceptions (679.), upon closer examination some of them disappear. Chloride of antimony (a compound of one proportional of antimony and one and a half of chlorine) of recent preparation was put into a tube (fig. 68.) (789.), and submitted when fused to the action of the current, the positive electrode being of plumbago. No electricity pa.s.sed, and no appearance of decomposition was visible at first; but when the positive and negative electrodes were brought very near each other in the chloride, then a feeble action occurred and a feeble current pa.s.sed. The effect altogether was so small (although quite amenable to the law before given (394.)), and so unlike the decomposition and conduction occurring in all the other cases, that I attribute it to the presence of a minute quant.i.ty of water, (for which this and many other chlorides have strong attractions, producing hydrated chlorides,) or perhaps of a true protochloride consisting of single proportionals (695, 796.).

691. _Periodide of mercury_ being examined in the same manner, was found most distinctly to insulate whilst solid, but conduct when fluid, according to the law of _liquido-conduction_ (402.); but there was no appearance of decomposition. No iodine appeared at the _anode_, nor mercury or other substance at the _cathode_. The case is, therefore, no exception to the rule, that only compounds of single proportionals are decomposable; but it is an exception, and I think the only one, to the statement, that all bodies subject to the law of liquido-conduction are decomposable. I incline, however, to believe, that a portion of protiodide of mercury is retained dissolved in the periodide, and that to its slow decomposition the feeble conducting power is due. Periodide would be formed, as a secondary result, at the _anode_; and the mercury at the _cathode_ would also form, as a secondary result, protiodide. Both these bodies would mingle with the fluid ma.s.s, and thus no final separation appear, notwithstanding the continued decomposition.

692. When _perchloride of mercury_ was subjected to the voltaic current, it did not conduct in the solid state, but it did conduct when fluid. I think, also, that in the latter case it was decomposed; but there are many interfering circ.u.mstances which require examination before a positive conclusion can be drawn[A].

[A] With regard to perchloride and periodide of mercury, see now 1340, 1341.--_Dec. 1838._

693. When the ordinary protoxide of antimony is subjected to the voltaic current in a fused state, it also is decomposed, although the effect from other causes soon ceases (402, 801.). This oxide consists of one proportional of antimony and one and a half of oxygen, and is therefore an exception to the general law a.s.sumed. But in working with this oxide and the chloride, I observed facts which lead me to doubt whether the compounds usually called the protoxide and the protochloride do not often contain other compounds, consisting of single proportions, which are the true proto compounds, and which, in the case of the oxide, might give rise to the decomposition above described.

694. The ordinary sulphuret of antimony its considered as being the compound with the smallest quant.i.ty of sulphur, and a.n.a.logous in its proportions to the ordinary protoxide. But I find that if it be fused with metallic antimony, a new sulphuret is formed, containing much more of the metal than the former, and separating distinctly, when fused, both from the pure metal on the one hand, and the ordinary gray sulphuret on the other.

In some rough experiments, the metal thus taken up by the ordinary sulphuret of antimony was equal to half the proportion of that previously in the sulphuret, in which case the new sulphuret would consist of _single_ proportionals.

695. When this new sulphuret was dissolved in muriatic acid, although a little antimony separated, yet it appeared to me that a true protochloride, consisting of _single_ proportionals, was formed, and from that by alkalies, &c., a true protoxide, consisting also of _single_ proportionals, was obtainable. But I could not stop to ascertain this matter strictly by a.n.a.lysis.

696. I believe, however, that there is such an oxide; that it is often present in variable proportions in what is commonly called protoxide, throwing uncertainty upon the results of its a.n.a.lysis, and causing the electrolytic decomposition above described[A].

[A] In relation to this and the three preceding paragraphs, and also 801, see Berzelius's correction of the nature of the supposed now sulphuret and oxide, Phil. Mag. 1836, vol. viii. 476: and for the probable explanation of the effects obtained with the protoxide, refer to 1340, 1341.--_Dec. 1838._

697. Upon the whole, it appears probable that all those binary compounds of elementary bodies which are capable of being electrolyzed when fluid, but not whilst solid, according to the law of liquido-conduction (394.), consist of single proportionals of their elementary principles; and it may be because of their departure from this simplicity of composition, that boracic acid, ammonia, perchlorides, periodides, and many other direct compounds of elements, are indecomposable.

698. With regard to salts and combinations of compound bodies, the same simple relation does not appear to hold good. I could not decide this by bisulphates of the alkalies, for as long as the second proportion of acid remained, water was retained with it. The fused salts conducted, and were decomposed; but hydrogen always appeared at the negative electrode.

699. A biphosphate of soda was prepared by heating, and ultimately fusing, the ammonia-phosphate of soda. In this case the fused bisalt conducted, and was decomposed; but a little gas appeared at the negative electrode; and though I believe the salt itself was electrolyzed, I am not quite satisfied that water was entirely absent.

700. Then a biborate of soda was prepared; and this, I think, is an un.o.bjectionable case. The salt, when fused, conducted, and was decomposed, and gas appeared at both electrodes: even when the boracic acid was increased to three proportionals, the same effect took place.

701. Hence this cla.s.s of compound combinations does not seem to be subject to the same simple law as the former cla.s.s of binary combinations. Whether we may find reason to consider them as mere solutions of the compound of single proportionals in the excess of acid, is a matter which, with some apparent exceptions occurring amongst the sulphurets, must be left for decision by future examination.

702. In any investigation of these points, great care must be taken to exclude water; for if present, secondary effects are so frequently produced as often seemingly to indicate an electro-decomposition of substances, when no true result of the kind has occurred (742, &c.).

703. It is evident that all the cases in which decomposition _does not occur, may_ depend upon the want of conduction (677. 413.); but that does not at all lessen the interest excited by seeing the great difference of effect due to a change, not in the nature of the elements, but merely in their proportions; especially in any attempt which may be made to elucidate and expound the beautiful theory put forth by Sir Humphry Davy[A], and ill.u.s.trated by Berzelius and other eminent philosophers, that ordinary chemical affinity is a mere result of the electrical attractions of the particles of matter.

[A] Philosophical Transactions, 1807, pp. 32, 39; also 1826, pp. 387, 389.

-- v. _On a new measure of Volta-electricity._

704. I have already said, when engaged in reducing common and voltaic electricity to one standard of measurement (377.), and again when introducing my theory of electro-chemical decomposition (504. 505. 510.), that the chemical decomposing action of a current _is constant for a constant quant.i.ty of electricity_, notwithstanding the greatest variations in its sources, in its intensity, in the size of the _electrodes_ used, in the nature of the conductors (or non-conductors (307.)) through which it is pa.s.sed, or in other circ.u.mstances. The conclusive proofs of the truth of these statements shall be given almost immediately (783, &c.).

705. I endeavoured upon this law to construct an instrument which should measure out the electricity pa.s.sing through it, and which, being interposed in the course of the current used in any particular experiment, should serve at pleasure, either as a _comparative standard_ of effect, or as a _positive measurer_ of this subtile agent.

706. There is no substance better fitted, under ordinary circ.u.mstances, to be the indicating body in such an instrument than water; for it is decomposed with facility when rendered a better conductor by the addition of acids or salts; its elements may in numerous cases be obtained and collected without any embarra.s.sment from secondary action, and, being gaseous, they are in the best physical condition for separation and measurement. Water, therefore, acidulated by sulphuric acid, is the substance I shall generally refer to, although it may become expedient in peculiar cases or forms of experiment to use other bodies (843.).

707. The first precaution needful in the construction of the instrument was to avoid the recombination of the evolved gases, an effect which the positive electrode has been found so capable of producing (571.). For this purpose various forms of decomposing apparatus were used. The first consisted of straight tubes, each containing a plate and wire of platina soldered together by gold, and fixed hermetically in the gla.s.s at the closed extremity of the tube (Plate V. fig. 60.). The tubes were about eight inches long, 0.7 of an inch in diameter, and graduated. The platina plates were about an inch long, as wide as the tubes would permit, and adjusted as near to the mouths of the tubes as was consistent with the safe collection of the gases evolved. In certain cases, where it was required to evolve the elements upon as small a surface as possible, the metallic extremity, instead of being a plate, consisted of the wire bent into the form of a ring (fig. 61.). When these tubes were used as measurers, they were filled with the dilute sulphuric acid, inverted in a basin of the same liquid (fig. 62.), and placed in an inclined position, with their mouths near to each other, that as little decomposing matter should intervene as possible; and also, in such a direction that the platina plates should be in vertical planes (720).