Part 12 (1/2)

397. When chloride of silver and chlorate of pota.s.sa were experimented with, in a similar manner, exactly the same results occurred.

398. Whenever the current pa.s.sed in these cases, there was decomposition of the substances; but the electro-chemical part of this subject I purpose connecting with more general views in a future paper[A].

[A] In 1801, Sir H. Davy knew that ”dry nitre, caustic potash, and soda are conductors of galvanism when rendered fluid by a high degree of heat,” (Journals of the Royal Inst.i.tution, 1802, p. 53,) but was not aware of the general law which I have been engaged in developing.

It is remarkable, that eleven years after that, he should say, ”There are no fluids known except such as contain water, which are capable of being made the medium of connexion between the metal or metals of the voltaic apparatus.”--Elements of Chemical Philosophy, p. 169.

399. Other substances, which could not be melted on gla.s.s, were fused by the lamp and blowpipe on platina connected with one pole of the battery, and then a wire, connected with the other, dipped into them. In this way chloride of sodium, sulphate of soda, protoxide of lead, mixed carbonates of potash and soda, &c. &c., exhibited exactly the same phenomena as those already described: whilst liquid, they conducted and were decomposed; whilst solid, though very hot, they insulated the battery current even when four troughs were used.

400. Occasionally the substances were contained in small bent tubes of green gla.s.s, and when fused, the platina poles introduced, one on each side. In such cases the same general results as those already described were procured; but a further advantage was obtained, namely, that whilst the substance was conducting and suffering decomposition, the final arrangement of the elements could be observed. Thus, iodides of pota.s.sium and lead gave iodine at the positive pole, and pota.s.sium or lead at the negative pole. Chlorides of lead and silver gave chlorine at the positive, and metals at the negative pole. Nitre and chlorate; of pota.s.sa gave oxygen, &c., at the positive, and alkali, or even pota.s.sium, at the negative pole.

[Ill.u.s.tration]

401. A fourth arrangement was used for substances requiring very high temperatures for their fusion. A platina wire was connected with one pole of the battery; its extremity bent into a small ring, in the manner described by Berzelius, for blowpipe experiments; a little of the salt, gla.s.s, or other substance, was melted on this ring by the ordinary blowpipe, or even in some cases by the oxy-hydrogen blowpipe, and when the drop, retained in its place by the ring, was thoroughly hot and fluid, a platina wire from the opposite pole of the battery was made to touch it, and the effects observed.

402. The following are various substances, taken from very different cla.s.ses chemically considered, which are subject to this law. The list might, no doubt, be enormously extended; but I have not had time to do more than confirm the law by a sufficient number of instances.

First, _water_.

Amongst _oxides_;--pota.s.sa, protoxide of lead, gla.s.s of antimony, protoxide of antimony, oxide of bis.m.u.th.

_Chlorides_ of pota.s.sium, sodium, barium, strontium, calcium, magnesium, manganese, zinc, copper (proto-), lead, tin (proto-), antimony, silver.

_Iodides_ of pota.s.sium, zinc and lead, protiodide of tin, periodide of mercury; _fluoride_ of pota.s.sium; _cyanide_ of pota.s.sium; _sulpho-cyanide_ of pota.s.sium.

_Salts._ Chlorate of pota.s.sa; nitrates of pota.s.sa, soda, baryta, strontia, lead, copper, and silver; sulphates of soda and lead, proto-sulphate of mercury; phosphates of pota.s.sa, soda, lead, copper, phosphoric gla.s.s or acid phosphate of lime; carbonates of pota.s.sa and soda, mingled and separate; borax, borate of lead, per-borate of tin; chromate of pota.s.sa, bi-chromate of pota.s.sa, chromate of lead; acetate of pota.s.sa.

_Sulphurets._ Sulphuret of antimony, sulphuret of pota.s.sium made by reducing sulphate of pota.s.sa by hydrogen; ordinary sulphuret of pota.s.sa.

Silicated pota.s.sa; chameleon mineral.

403. It is highly interesting in the instances of those substances which soften before they liquefy, to observe at what period the conducting power is acquired, and to what degree it is exalted by perfect fluidity. Thus, with the borate of lead, when heated by the lamp upon gla.s.s, it becomes as soft as treacle, but it did not conduct, and it was only when urged by the blowpipe and brought to a fair red heat, that it conducted. When rendered quite liquid, it conducted with extreme facility.

404. I do not mean to deny that part of the increased conducting power in these cases of softening was probably due to the elevation of temperature (432. 445.); but I have no doubt that by far the greater part was due to the influence of the general law already demonstrated, and which in these instances came gradually, instead of suddenly, into operation.

405. The following are bodies which acquired no conducting power upon a.s.suming the liquid state:--

Sulphur, phosphorus; iodide of sulphur, per-iodide of tin; orpiment, realgar; glacial acetic acid, mixed margaric and oleic acids, artificial camphor; caffeine, sugar, adipocire, stearine of cocoa-nut oil, spermaceti, camphor, naphthaline, resin, gum sandarach, sh.e.l.l lac.

406. Perchloride of tin, chloride of a.r.s.enic, and the hydrated chloride of a.r.s.enic, being liquids, had no sensible conducting power indicated by the galvanometer, nor were they decomposed.

407. Some of the above substances are sufficiently remarkable as exceptions to the general law governing the former cases. These are orpiment, realgar, acetic acid, artificial camphor, per-iodide of tin, and the chlorides of tin and a.r.s.enic. I shall have occasion to refer to these cases in the paper on Electro-chemical Decomposition.

408. Boracic acid was raised to the highest possible temperature by an oxy-hydrogen flame (401.), yet it gained no conducting powers sufficient to affect the galvanometer, and underwent no apparent voltaic decomposition.

It seemed to be quite as bad a conductor as air. Green bottle-gla.s.s, heated in the same manner, did not gain conducting power sensible to the galvanometer. Flint gla.s.s, when highly heated, did conduct a little and decompose; and as the proportion of potash or oxide of lead was increased in the gla.s.s, the effects were more powerful. Those gla.s.ses, consisting of boracic acid on the one hand, and oxide of lead or pota.s.sa on the other, show the a.s.sumption of conducting power upon fusion and the accompanying decomposition very well.

409. I was very anxious to try the general experiment with sulphuric acid, of about specific gravity 1.783, containing that proportion of water which gives it the power of crystallizing at 40 Fahr.; but I found it impossible to obtain it so that I could be sure the whole would congeal even at 0 Fahr. A ten-thousandth part of water, more or less than necessary, would, upon cooling the whole, cause a portion of uncongealable liquid to separate, and that remaining in the interstices of the solid ma.s.s, and moistening the planes of division, would prevent the correct observation of the phenomena due to entire solidification and subsequent liquefaction.

410. With regard to the substances on which conducting power is thus conferred by liquidity, the degree of power so given is generally very great. Water is that body in which this acquired power is feeblest. In the various oxides, chlorides, salts, &c. &c., it is given in a much higher degree. I have not had time to measure the conducting power in these cases, but it is apparently some hundred times that of pure water. The increased conducting power known to be given to water by the addition of salts, would seem to be in a great degree dependent upon the high conducting power of these bodies when in the liquid state, that state being given them for the time, not by heat but solution in the water[A].