52 Self-sustaining Voltaic Battery. 
Could we view such a surface, or rather I should say want of 
surface, we doubtless should find it many homens fold more 
rugged, uneven, and porous, than the common spong 
The varions metals let go the hydrogen in the shes: circuit 
with very different degrees of readiness. From my cheers 
I conclude that the attraction of the various metals for the gas is 
directly as their specific gravities. All the less dense metals de- 
compose water, (evolve hydrogen). Sodium and_ potassium 
evolve the gas in torrents. The base metals proper have less 
action on water, and a stronger attraction for the gas. The 
noble metals hold the gas very firmly, and are without action on 
water. 
The order in respect to evolution is the reverse in respect to 
gravity ; and the order in respect to gravity is consequently the 
reverse of the order of fitness for a conducting plate in ee 
to the evolution of hydrogen. Platinum, gold, and lead, hol 
the gas very hard. When polished plates of these metals are 
used, the hydrogen adheres in large bubbles, | very slowly 
creep up the plate. Mercury I do pot compare, because its me- 
chanical form is the best possible for adhesion ; but could we 
but polish the solid noble metals as perfectly as the atomic polish 
of the mercnry, I have no doubt but that the mercury, according 
to its density, would follow after gold. Silver answers better 
than the other noble metals. Experiment has not enabled me to 
decide that copper is better than silver, but I am much inclined 
to consider the copper as best. Iron is decidedly better than an 
metal above it in density, and requires no special preparation to 
make it evolve freely. Zine isso prone to evolution that it is — 
with difficulty that the hydrogen can be made to adhere. The — 
metals of the alkalies cannot be invested with brdeeaie like the 
denser. A mere particle of zinc will coat a surface of copper or 
iron with hydrogen, and protect it from oxydation forever; but 
soon as potassium or sodium is deposited, it is instantly re- 
combined with oxygen, because it cannot be coated with hydro- 
gen. Here I may remark that the newly reported aluminum 
which is said to have the nobility of silver, with the density of 
only 2:5, ought, by the above views, to make a most admirable 
conducting plate. 
y the above view, the adhesion of hydrogen is very nearly 
the reverse of the affinity for oxygen. Here we find silver with 
a medium adhesion and a low affinity. This at once indicates 
that it is the metal which will be generally used for making bat- 
teries. Iron, which is the most oxydable metal that can be em- 
oyed for seas plates, has a very low adhesion, and for- 
anels a mechanical advantage from its ever-fibrous or granu- 
lar form, which. eae increases its oo for evolution. Could 
main as iron in the battery, it ei = geo 
