ELECTRICITY.

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sulphate of zinc which is formed in the battery cell, so that its sale shall in part repaythe expense of other material. It is probable such a market may be found in thecotton manufacture.

It is very necessary to be careful in selecting pure acid. The sulphuric acid manu-factured from pyrites commonly contains a considerable quantity of arsenic, thepresence of which is fatal by being precipitated on the zinc plate and causing localaction. This is the more dangerous from its insidious progress not being perceivedtill the galvanic action has been some time in operation. A convenient mode ofdetecting its presence is to immerse a pair of small plates in a small quantity of acidsolution somewhat stronger than that used in the battery; complete the circuit for afew minutes, and then break it, when, if arsenic has been precipitated on the zinc,it will be immediately detected by the local action it causes.

The apparatus figured in Plate II. was erected in 1841, since which it has beenfound that the constancy of action iu the battery may be conveniently maintained,merely by supplying its waste occasionally, provided the quantity of exciting liquid besufficient; and instead of separate cells of 60 gallons for each battery, as at first used,a cell has been constructed containing 154)0 gallons, into which 4 pairs of plates areplunged, each thus having 375 gallons, and the liquid being occasionally maintainedby the addition of as much acid as the diminished weight either of the zinc plate inthe battery, or the dissolving plate in the trough, shews to have been taken up toform the sulphate, and the working of the apparatus being exhibited on a galvano-meter attached to each pair of plates. The large dimensions of this cistern afford theadvantage of placing the battery plates in an oblique or horizontal position by asimple arrangement of the mercury, facilitating and maintaining the amalgamation ofthe zinc, so that, it being no longer necessary to remove the zinc so frequently forthat purpose, it may he increased to a considerable weight, and the battery left in itscell for a much longer time.

In this construction also the plates may be made of slips of metal, which, moreespecially in the negative or silver plate, is of great advantage, from the facility andeconomy it affords in the operation of platinizing, usually one of much delicacy anddifficulty in large plates. In this arrangement the sulphate of zinc which forms inthe bottom of the cell is drawn off by a siphon, a false floor being provided to receivethe plates. It is not thought necessary to figure this newer apparatus in the presentarticle, as the former is sufficiently effective for all cases likely to occur in ordinaryservice.

A few lines may, however, he added on the great importance of agitating the solu-tion in the decomposing trough, to which, as well as to the position of the plates inthe trough, it will always be found necessary to attend very carefully, whateverarrangement may be adopted for the battery plates. A very simple experiment willshew that the change in the character of the precipitated metal, when the dissolvingand receiving plates are in different positions, arises from the changing of the densityof different parts of the solution when the battery is in active operation.

If a copper dissolving and receiving plate be placed vertically in a glass jar filledwith a solution of sulphate of copper, and attached to a small battery in good work-ing order, and the solution narrowly watched, that portion from which the metal liasbeen precipitated will be seen to rise from the upper edge of the receiving plate tothe top of the solution, while a stream of greater density will he seen to flow from thelower edge of the dissolving plate to the bottom of the jar, and in time crystals of thesalt become formed at the bottom of the jar, while the solution at the top willbecome colourless as far down as the upper edge of the dissolving plate ; so that ifthe receiving plate project above the dissolving plate in the solution, it is obvious no