739.  The instrument offers the only actual measurer of voltaic electricity which we at present possess.  For without being at all affected by variations in time or intensity, or alterations in the current itself, of any kind, or from any cause, or even of intermissions of action, it takes note with accuracy of the quantity of electricity which has passed through it, and reveals that quantity by inspection; I have therefore named it a VOLTA-ELECTROMETER.

740.  Another mode of measuring volta-electricity may be adopted with advantage in many cases, dependent on the quantities of metals or other substances evolved either as primary or as secondary results; but I refrain from enlarging on this use of the products, until the principles on which their constancy depends have been fully established (791. 848.);

741.  By the aid of this instrument I have been able to establish the definite character of electro-chemical action in its most general sense; and I am persuaded it will become of the utmost use in the extensions of the science which these views afford.  I do not pretend to have made its detail perfect, but to have demonstrated the truth of the principle, and the utility of the application[A].

[A] As early as the year 1811, Messrs. Gay-Lussac and Thenard employed chemical decomposition as a measure of the electricity of the voltaic pile.  See Recherches Physico-chymiques, p. 12.  The principles and precautions by which it becomes an exact measure were of course not then known.—­Dec. 1838.

P vi. On the primary or secondary character of the bodies evolved at the Electrodes.

742.  Before the volta-electrometer could be employed in determining, as a general law, the constancy of electro-decomposition, it became necessary to examine a distinction, already recognised among scientific men, relative to the products of that action, namely, their primary or secondary character; and, if possible, by some general rule or principle, to decide when they were of the one or the other kind.  It will appear hereafter that great mistakes inspecting electro-chemical action and its consequences have arisen from confounding these two classes of results together.

743.  When a substance under decomposition yields at the electrodes those bodies uncombined and unaltered which the electric current has separated, then they may be considered as primary results, even though themselves compounds.  Thus the oxygen and hydrogen from water are primary results; and so also are the acid and alkali (themselves compound bodies) evolved from sulphate of soda.  But when the substances separated by the current are changed at the electrodes before their appearance, then they give rise to secondary results, although in many cases the bodies evolved are elementary.