PRIMARY AND SECONDARY RESULTS OF ELECTROLYTIC ACTION. 93 



effect IS to be measured, the effect is to be obtained, and then the indication read off. 

 In exact experiments it is necessary to correct the volume of gas for changes in tem- 

 perature and pressure, and especially for moisture*. For the latter object the volta- 

 electrometer (fig. 11.) is most accurate, as its gas can be measured over water, whilst 

 the others retain it over acid or saline solutions. 



738. I have not hesitated to apply the term degree, in analogy with the use made of 

 it with respect to another most important imponderable agent, namely, heat ; and as 

 the definite expansion of air, water, mercury, &c., is there made use of to measure heat, 

 so the equally definite evolution of gases is here turned to a similar use for electricity. 



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 inter- 

 missions 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. 843.). 



741. By the aid of this instrument I have been able to establish the definite cha- 

 racter 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. 



^ 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 distinc- 

 tion, already recognised among scientific men, relative to the products of that action, 

 namely, their primitive 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 respecting 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 sub- 



* For a simple table of correction for moisture, I may take the liberty of referring to my Chemical Manipu- 

 lation, edition of 1830, p. 376. 



