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 unobjectionable 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 class of compound combinations does not seem to be subject to the same simple law as the former class 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 illustrated 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.

P 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 quantity 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 passed, or in other circumstances.  The conclusive proofs of the truth of these statements shall be given almost immediately (783, &c.).