1355.  Another point is the facilitation of electrolytic conducting power or discharge by the addition of substances to the dielectric employed.  This effect is strikingly shown where water is the body whose qualities are improved, but, as yet, no general law governing all the phenomena has been detected.  Thus some acids, as the sulphuric, phosphoric, oxalic, and nitric, increase the power of water enormously; whilst others, as the tartaric and citric acids, give but little power; and others, again, as the acetic and boracic acids, do not produce a change sensible to the voltameter (739.).  Ammonia produces no effect, but its carbonate does.  The caustic alkalies and their carbonates produce a fair effect.  Sulphate of soda, nitre (753.), and many soluble salts produce much effect.  Percyanide of mercury and corrosive sublimate produce no effect; nor does iodine, gum, or sugar, the test being a voltameter.  In many cases the added substance is acted on either directly or indirectly, and then the phenomena are more complicated; such substances are muriatic acid (758.), the soluble protochlorides (766.), and iodides (769.), nitric acid (752.), &c.  In other cases the substance added is not, when alone, subject to or a conductor of the powers of the voltaic battery, and yet both gives and receives power when associated with water.  M. de la Rive has pointed this result out in sulphurous acid[A], iodine and bromine[B]; the chloride of arsenic produces the same effect.  A far more striking case, however, is presented by that very influential body sulphuric acid (681.):  and probably phosphoric acid also is in the same peculiar relation.

  [A] Quarterly Journal, xxvii. 407. or Bibliotheque Universelle, xl.
  205.  Kemp says sulphurous acid is a very good conductor, Quarterly
  Journal, 1831, p. 613.

  [B] Quarterly Journal, xxiv, 465. or Annales de Chimie, xxxv. 161.

1356.  It would seem in the cases of those bodies which suffer no change themselves, as sulphuric acid (and perhaps in all), that they affect water in its conducting power only as an electrolyte; for whether little or much improved, the decomposition is proportionate to the quantity of electricity passing (727. 730.), and the transfer is therefore due to electrolytic discharge.  This is in accordance with the fact already stated as regards water (984.), that the conducting power is not improved for electricity of force below the electrolytic intensity of the substance acting as the dielectric; but both facts (and some others) are against the opinion which I formerly gave, that the power of salts, &c. might depend upon their assumption of the liquid state by solution in the water employed (410.).  It occurs to me that the effect may perhaps be related to, and have its explanation in differences of specific inductive capacities.