of a flask or bottle shape. The zinc plate z is in the middle, and a pair of carbon plates K K, one on each side of the zinc, are joined at the top and constitute the positive pole. The zinc plate z is attached to a rod a, by which it can be lifted out of the solution when the element is not in use. This is necessary, as the solution acts on the zinc even when the circuit is broken.

Bunsen's Chromate Battery.-Bunsen's battery, belonging to this class, is shown in Figs. 359 and 360. The details of the connections of the plates are shown in Fig. 360. The catches on the two sides rest on the rectangular frame, so that when the frame is lowered by the winch handle, the zinc drops into one cylindrical cell, and the carbons into the next. first quite equal to 23 Daniells, but is not constant.

The E. M. F. is at

M

Fig. 300.-Connections of Bunsen Battery.

Hauck's Chromate Battery.-Fig. 361 shows Hauck's battery. The connection of the plates with the lid M M is seen in the right-hand figure. The

clamps K K' K" are to connect the carbon of one cell with the zinc of the next. The zinc plates z may be raised through the lid, and the whole lid with its

carbon plates K K, as well as the zinc, can be lifted out by means of the screw and hand-wheel seen at the top of the figure. Trouvé's Cell.-Trouvé's elements, a very powerful current for a short time

B

Fig. 363.-Plan.

shown in Figs. 362 and 363, give The lower diagram shows how the plates are connected with each other. There are fourteen plates connected as follows: Three carbon plates are joined to each other by means of the metal rod A, thus forming one carbon plate of large surface; the zinc plates joined to the metal rod c represent also one large zinc plate. To the metal rod B four zinc plates and four carbon plates are fastened, representing respectively a large carbon and a large zinc plate. The zinc and carbon plates of в form one element, and the plates of c and A another element. The two elements are thus joined in series. The poles of this two-celled battery are at A and c. The zinc and carbon plates are placed vertically (Fig. 362), and to prevent them from touching each other, the carbon plates have india-rubber caps at both ends. The plates are kept together by a frame, which is lifted by means of the handle A. T is a leaden pipe, by means of which air can be forced through the apparatus. These elements give a very powerful current at first, but the strength very soon decreases.

Trouvé also constructed elements for medical purposes, etc., with hermetically sealed lids, as represented in Fig. 364. This cell consists of an ebonite box, closed at both ends. The amalgamated zinc rod is connected with a knob outside (the negative knob), while the carbon cylinder is connected with the positive knob. The box is only half-filled with the acid liquid, and contact is made by shaking the box. As sulphuric acid is not convenient for domestic purposes, it is avoided as much as possible, being in this case replaced by a mixture of one-third of potassium bichromate and two-thirds of potassium bisulphate.

Zinc-Carbon Elements with Salt Solutions.-Not only are combinations of solids and liquids used for diminishing polarisation, but gases also, as for instance the oxygen of the air, may effect the same purpose. Fig. 365 represents an element frequently used in Switzerland. F is a carbon cylinder fourteen centimetres high and nine centimetres in outer diameter, acting as the negative electrode. G is an amalgamated zinc strip, acting as the positive electrode. These elements are therefore described as zinc-carbon elements with salt solution.

The Maiche Element.-Fig. 366 represents the zinc-carbon element with a salt of ammonia, by L. Maiche. It is contained in a cylindrical glass vessel having an ebonite lid; a porous vessel having holes in the side is fastened to the lid. In the porous cell is placed platinised carbon, forming the negative electrode. A platinum wire leads from it to a binding screw on the lid. Through the centre of the lid an ebonite tube passes, having at its end a porcelain basin containing mercury and pieces of zinc, which form the positive electrode. A second platinum wire connects the basin containing the mercury, through the ebonite tube, with the remaining binding screw. The vessel holds about 15 litres of the liquid, which consists either of 250 grammes of ammonia salt, or 140 to 150 grammes of sodium bisulphate, mixed with about 5 to 10 per cent. of sulphuric acid. The E. M. F. of the element is equal to 125 volts, and the resistance is half an ohm. In comparison with other elements this is a cheap arrangement, because depolarisation here is caused by the air, or rather by the oxygen contained in the air. To allow the oxygen to reach the

carbon more easily, the porous vessel dips into the fluid to a depth of about two centimetres. The current soon falls off in consequence of the increased generation of hydrogen, which consumes the oxygen stored in the carbon. By taking the zinc electrode away from the carbon electrode, the deposition of crystals on the carbon is prevented.

Two-Fluid Cells.-Volta was the first who observed that a combination of two liquids and a metal produced a galvanic current. Becquerel examined more minutely the behaviour of the different liquids. To obtain a constant current, not only is it necessary to adopt some means of depolarising the electrodes, but it is necessary also to devise a means of keeping the resistance constant. Now the resistance varies with the chemical changes of the liquid, and also with the degree of concentration of the chemically unaltered fluid. On account of these differences Cazin divides the elements with two liquids into two groups:

1. Those fluids which have their composition unaltered by the chemical action. 2. Those which undergo chemical change, and consequently offer a varying resistance to the current.

ELEMENTS OF TWO LIQUIDS WHICH ARE NOT ALTERED BY THE CHEMICAL ACTION.

Daniell's Element.-On page 111 we gave a description of a Daniell's element in its original shape, and shall, therefore, find it necessary to consider here only those modifications of the Daniell's cell which are likely to be of practical interest. In the form represented in Fig. 89, a porous earthenware vessel is substituted for the membrane from the jugular vein of an ox. If we examine the porous vessel of an element that has been in use for some time, we find that figures representing the foliage and branches of trees, or little crystals, cover its surface. These are crystals of copper. The copper separated out in this manner sometimes goes through the porous cell, and is then in direct contact with the zinc, forming minute elements, which, producing only local action, decompose copper sulphate, but do no useful work. The deposition of copper on the surface of the diaphragm is sometimes caused by the zinc residue which coats the cell. This sediment consists of iron, lead, copper, carbon, etc., which dissolve but slowly in the dilute sulphuric acid, if indeed they dissolve at all.

To prevent this the porous pot is sometimes replaced by parchment. If the element is arranged so that the zinc together with the sulphuric acid are inside the porous pot, the separation of copper may be prevented by having the zinc placed in the middle of the pot, and the bottom of the diaphragm coated with wax. The zinc residue now remains at the bottom, and the solution of copper bisulphate cannot pass through it. Kramer prevents the separation of copper in the diaphragm by having the copper electrode divided into