VOLTAIC EXCITEMENT AFFECTED BY DILUTION. 
109 
gradual diminution of effect from iron to copper, and from copper to silver, one would 
not expect the reverse effects, or those on the other side of zero, to appear by a return’ 
back to such metals as lead and tin ( 1979 . 1989 .), but rather look for them in platinum 
or gold, which, however, produce no results of the kind ( 1976 . 1988 .). To increase 
still further this complexity, it appears, from what has been before stated, that on 
changing the acids the order must again be changed ( 1981 .). Nay, more, that with 
the same acid, and merely by changing the proportion of dilution, such alteration 
of the order must take place ( 1986 . 1988 .). 
1992 . Thus it appears, as before remarked ( 1982 .), that to apply the theory of con- 
tact electromotive force to the facts, that theory must twist and bend about with 
every variation of chemical action : and after all, with every variety of contact, active 
and inactive, in no case presents phenomena independent of the active exertion of 
chemical force. 
1993 . As the influence of dilution and concentration was so strong in affecting the 
relation of different parts of the same metal to an acid, making one part either 
positive or negative to another, I thought it probable that, by mere variation in the 
strength of the interposed electrolyte, the order of metals when in acids or other 
solutions of uniform strength, might be changed. I therefore proceeded to experi- 
ment on that point, by combining together two metals, tin and lead, through the gal- 
vanometer ( 1915 .) ; arranging the electrolytic solution in tube No. 1 , strong on one 
side and weak on the other ; immersing the wires simultaneously, tin into the strong, 
and lead into the weak solution, and after observing the effect, re-cleaning the wires, 
re-arranging the fluid, and re-immersing the wires, the tin into the weak, and the lead 
into the strong portion. De la Rive has already stated # that inversions take place 
when dilute and strong sulphuric acid is used ; these I could not obtain when care 
was taken to avoid the effect of the investing fluid ( 1918 .) : the general statement is 
correct, however, when applied to another acid, and I think the evidence very im- 
portant to the consideration of the great question of contact or chemical action. 
1994 . Two metals in strong and weak solution of potash. — Zinc was positive to tin, 
cadmium, or lead, whether in the weak or strong solution. Tin was positive to 
cadmium, either in weak or strong alkali. Cadmium was positive to lead both ways, 
but most when in the strong alkali. Thus, though there were deferences in degree 
dependent on the strength of the solution, there was no inversion of the order of the 
metals. 
1995 . Two metals in strong and weak, sulphuric acid. — Cadmium was positive to iron 
and tin both ways : tin was also positive to iron, copper, and silver ; and iron was 
positive to copper and silver, whichever side the respective metals were in. Thus none 
of the metals tried could be made to pass the others, and so take a different order 
from that which they have in acid uniform in strength. Still there were great varia- 
tions in degree; thus iron in strong acid was only a little positive to silver in weak 
* Annales de Chimie, 1828, xxxvii. p. 2-10. 
