Page:The American Cyclopædia (1879) Volume VII.djvu/602

 590 GALVANISM current, and which after a length of time be- came equal to it, rendering its action null. The nascent hydrogen which is evolved on the surface of the negative plate has a remarkable power, under the circumstances, of decompo- sing the salt in the fluid. When copper and zinc plates and dilute sulphuric acid are used, for instance, after a time sulphate of zinc, ZnSOi, accumulates, and the hydrogen tends to deprive the oxysulphion, SO 4, of a part of its oxygen, thus loosening its hold upon the zinc constituent, which being attracted by the copper plate is deposited in a metallic state upon it; and this action may continue until the surface of the copper is covered, when of course there will be, in place of the original arrangement of two opposite metals immersed in an exciting liquid, two plates of the same kind, and having therefore little or no electromotive force. Electricians have there- fore devised several different forms of batteries with the intention of obviating these difficul- ties, which, because of their more continuous and equable action, have been called constant batteries. The first of these was constructed by Becquerel, and consisted of a sheet of cop- per in the form of a cylinder enveloped in a bladder containing a saturated solution of sul- phate of copper. The bladder is surrounded by a cylindrical sheet of zinc, and the whole is placed in a vessel containing 'dilute sulphuric acid or common salt. The battery of Daniell, constructed in 1836, is a modification of Bec- querel's in which a porous earthenware cup is substituted for the animal membrane. It is still regarded as the most constant of batteries, al- though it is not the most powerful. Fig. 7 represents one cell of a Daniell's battery. A glass jar contains dilute sulphuric acid, in which is placed a zinc plate cast in the form of a cyl- inder; within this is a porous earthenware cy- lindrical cup ; and within this, again, a copper plate in the form also of a cylin- der. The porous cell con- tains a saturated solution of sulphate of copper, and crystals of this salt are placed in the liquid upon shelves, or in a copper- wire basket. Sulphuric acid permeates all the cells, but the porous par- tition offers a great hin- drance to the transfusion of the saline solutions. The nascent hydrogen which is evolved at the surface of the copper plate decomposes the sulphate of copper in the same way that it does the sulphate of zinc in the ordinary zinc and copper couple. The ac- tion of a Daniell's battery may be explained as follows : In the diagram a zinc and a copper Fia. 7. Cell of Daniell's Battery. plate are represented, having a porous earthen- ware partition between them ; solution of sul- phate of copper (Cu 2 SO 4 ) being the active fluid in the presence of the copper, and sulphuric acid (H 2 S0 4 ) the active fluid in the presence of the zinc plate. The polarization of the mole- cules in both fluids which is effected by connect- ing the terminal plate with a wire is represent- ed in the following diagram : + Porous partition. Copper I I I plate I Cu S0 4 Cu SO 4 I H a SO 4 H I + I + -I + 4 S0 4 Zinc plate. An interchange of particles follows this polari- zation, the terminal negative constituent, S0 4 , going to the positive or zinc plate and com- bining with it, and the terminal positive con- stituent (its electrical state heightened by in-' duction) going to the negative or copper plate, and being deposited upon it. It will be ob- served that the action here is somewhat differ- ent from that which obtains in a simple cell containing dilute sulphuric acid as the exciting liquid. In that case hydrogen is liberated at the surface of the copper plate, while in the Daniell's cell it is substituted by metallic cop- per. The hydrogen constituent of sulphuric acid is separated from it at the porous cell, but is immediately reunited to the salt radical, oxysulphion, SO 4, the electro-negative constitu- ent of the sulphate of copper. An interchange of molecules is thus effected throughout the whole chain of particles of sulphate of copper and of sulphuric acid lying between the copper and zinc plates ; a copper molecule, charged with positive electricity, being at one end de- posited upon the copper plate, which within the fluid is charged with negative electricity, and a molecule of the salt radical, S0 4, charged with negative electricity, uniting at the other end with the zinc plate, which within the fluid is charged with positive electricity. If the mole- cules of copper after they are deposited could re- tain their electro-positive condition, a state of polarization of the plate would result similar to that which takes place from the deposition of a film of hydrogen in the ordinary sulphuric acid battery, and having the effect of a deposit of metallic zinc ; but as soon as the deposition takes place the polarity of the deposited mole- cule changes from positive to negative, a ne- cessary result of its becoming a part of the copper plate. When the solution of sulphate of copper becomes dilute, hydrogen is liberated upon the surface of the copper plate, from de- composition of water or of sulphuric acid, it has not been determined which. To preserve the constancy of the battery, it is therefore necessary to keep the solution of sulphate of copper saturated. The sulphuric acid result- ing from the decomposition of the sulphate of copper tends to replenish the loss sustained by the formation of sulphate of zinc, but the accu- mulation of the latter salt weakens its action, so that it is necessary from time to time to add