116 
MR. DANIELL ON VOLTAIC COMBINATIONS. 
rather more rapid than before. Even iron plates when substituted for the exhausted 
platinum renewed the action with great effect. 
The decline thus seemed to depend upon some state taken up by the conducting 
plates, and this I endeavoured to remove in various ways. I polished them with 
rotten stone ; I heated them red hot ; and I boiled them in a strong solution of po- 
tassa, but without any decided effect. Boiling them, however, in nitric or muriatic 
acids completely restored them. Thus, frequently renewing the plates but retaining 
the old charge of acidulated water, the first action was always very brisk, but lasted 
for shorter and shorter periods, till the decline at last became almost immediate. 
Upon examining the plates after a long-protracted action of this kind, I found a 
roughness upon their edges and faces which conveyed the idea of a corrosion of the 
platinum. A more attentive investigation proved this to arise from a deposition upon 
them of metallic zinc ; and upon leaving the battery in connexion for eighteen hours 
the coating increased to such a degree that it could be detached in large flakes. The 
precipitated metal was deposited chiefly upon the surface opposed to the zinc plate, 
but in some had extended considerably on the opposite side. It had mostly a beau- 
tiful mammillated appearance ; but in one or two instances a crystalline structure was 
visible with a lens. I ascertained the weight of zinc thus deposited upon a plate by 
removing it with an acid, and found it 27‘86 grains. The metal was pure, and when 
detached did not dissolve in the dilute acid till touched with a piece of platinum, when 
abundance of hydrogen gas was immediately given off from the latter. A similar 
deposition, in equal abundance, was formed upon iron when the conducting-plates 
were made of that metal. It originated doubtless from the oxide of zinc formed by 
the action of the battery at the generating-plate, and reduced by the nascent hydrogen 
on the conducting-plate ; and its varying quantity and accumulation was amply suffi- 
cient to account for the variations and ultimate annihilation of the circulating force. 
In some instances, indeed, the casing of the platinum was so entire as to have the 
complete effect of one zinc surface opposed to another. This local deoxidating power 
of the hydrogen is exerted upon every oxide within its reach, and is more or less 
injurious to the action of the battery according as the resulting deposition upon the 
plates is more or less capable of generating a counter current by its reoxidation. The 
most injurious of all precipitations must, of course, be that of zinc upon the conduct- 
ing-plate ; and it must take place with a rapidity proportioned to the quantity of 
oxide held in solution by the electrolyte. In the common construction of the voltaic 
battery the solution of the metal is very rapid, on account of the local as well as the 
current action, and the deoxidating action must very soon be established; and the 
reason of the incrustation of zinc never having been remarked, is doubtless the ex- 
treme facility with which it is redissolved upon breaking the circuit. By its close 
contact with the copper it forms simple voltaic circles, and is under circumstances 
the most favourable for its removal. The momentary recovery of the strength of the 
