"Mercuric sulphate decomposes in the presence of water into an acid and a basic sulphate. The latter is a yellow substanceturpeth mineral-practically insoluble in water; its presence, at any rate, in moderate quantities has no effect on the cell. If, however, it is formed, the acid sulphate is formed also. This is soluble in water, and the acid produced affects the electromotive force. The object of the washings is to dissolve and remove this acid sulphate, and for this purpose the three washings described in the Specification will in nearly all cases suffice. If, however, a great deal of the turpeth mineral is formed, it shows that there is a great deal of the acid sulphate present, and it will then be wiser to obtain a fresh sample of mercurous sulphate rather than to try by repeated washings to get rid of all the acid,

"The free mercury helps in the process of removing the acid, for the acid mercuric sulphate, attacks it, forming mercurous sulphate and acid which is washed away.

"Pure mercurous sulphate, when quite free from acid, shows, on repeated washing, a faint primrose tinge which is due to the formation of a basic mercurous salt, and is distinct from the turpeth mineral or basic mercuric sulphate. The appearance of this primrose tint may be taken as an indication of the fact that all the acid has been removed, and the washing may, with advantage, be continued until this primrose tint appears. Should large quantities of this basic mercurous salt be formed, the sulphate should be treated as described in the instructions for setting up Clark's cells issued from the Physical Technical Institute of Berlin, Zeitschrift für Instrumentenkunde, 1893, Heft 5.

"The Zinc Sulphate Solution.-The object to be attained is the preparation of a neutral solution of pure zinc sulphate saturated with ZnSO4.7H2O.

"At temperatures above 30°C. the zinc sulphate may crystallise out in another form; to avoid this, 30°C. should be the upper limit of temperature. At this temperature water will dissolve about 1:9 times its weight of the crystals. If any of the crystals put in remain undissolved they will be removed by the filtration.

"The zinc sulphate should be free from iron and should be tested before use with sulphocyanide of potassium to ascertain that this condition is satisfied. If an appreciable amount of iron is present, it should be removed by the method given in the directions already quoted, Zeitschrift für Instrumentenkunde, 1893, Heft 5.

"The amount of zinc oxide required depends on the acidity of the solution, but 2 per cent. will, in all cases which will arise in practice with reasonably good zinc sulphate, be ample. Another rule would be to add the zinc oxide gradually until the solution become slightly milky. The solution, when put into the cell, should not contain any free zinc oxide; if it does, then, when mixed with the mercurous sulphate, zinc sulphate and mercurous

oxide are formed; the latter may be deposited on the zinc and affect the electromotive force of the cell. The difficulty is avoided by adding as described about 12 per cent. of mercurous sulphate before filtration; this is more than sufficient to combine with the whole of the zinc oxide originally put in, if it all remains free; the mercurous oxide formed, together with any undissolved mercurous sulphate, is removed by the filtration.

"The Mercurous Sulphate and Zinc Sulphate Paste.-Although, after the last washing of the mercurous sulphate, as much water as possible may have been drained off, sufficient water generally remains to necessitate the addition of a very considerable quantity of crystals of zinc sulphate from the stock bottle, in order to ensure saturation when the washed mercurous sulphate is added to the zinc sulphate solution as described in No. 4 of Specification B appended to the Order in Council.

"If the sides of the test-tube above the cork be soiled by the introduction of the paste, the marine glue does not adhere to the glass; the liquid in the cell rises by capillary action between the glue and the glass, and may damage the cell.

"The form of the vessel containing the cell may be varied. In the H form devised by Lord Rayleigh and modified by Dr. Kahle the zinc is replaced by an amalgam of zinc and mercury. The other materials should be prepared as already described. Contact is made with the amalgam in one leg of the cell, and with the mercury in the other, by means of platinum wires sealed through the glass.

"The amalgam consists of about 90 parts of pure mercury mixed with 10 parts of pure redistilled zinc. These are heated in a porcelain crucible to about 100°C., and gently stirred until the zinc is completely dissolved in the mercury. The amalgam is liquid while warm, and must be poured into the cell before it becomes solid on cooling.

"The vessel containing the element consists of two vertica tubes. These, as shown in the figure [212], are closed below, and open above into a common neck, which can be closed by a ground stopper of glass. The two tubes should be 2 cm. in diameter and 3 cm. in length. The neck should be at least 1.5 cm. in diameter and 2 cm. long. A short length of platinum wire is sealed through the bottom of each tube.

"The end of the wire in one tube is covered by a small quantity of pure mercury; that in the other tube, by the zinc-mercury amalgam.

"Above the mercury a layer about 1 cm. thick of the mercurous sulphate paste is placed; above this, and also above the amalgam, a layer, also about 1 cm. in thickness, of zinc sulphate crystals, and the vessel is filled up with the saturated zinc sulphate solution.

"The zinc sulphate crystals are obtained by evaporating at a

temperature of less than 30°C. some of the zinc sulphate solution, prepared as in 4 of the Specification.

"The stopper is then inserted, leaving a small air-bubble above

Fig. 212.-Kahle's Modification of the Rayleigh H form of Clark Cell (full size). ZS.8, zinc sulphate solution; ZS.c, zinc sulphate crystals; MZ.S, mercurous sulphate and zinc sulphate paste: M, mercury; A, amalgam of zinc and mercury.

the liquid, and sealed on the outside with shellac dissolved in alcohol.

"The ends of the platinum wires outside the cell form the two poles, and should be connected to suitable terminals."

The Americans and the Germans prefer the type of Clark cell shown in Fig. 212, partly because this form avoids a possible slight variation in the E.M.F., arising from the zinc rod in the sample-tube form (Fig. 211) being in contact with liquid at different depths, and,

Fig. 212a.-Cooper's Modification of the Kalle H form of Clark cell (full size).

therefore, of different densities. Whereas, on the other hand, the greater ease with which the latter type can be immersed in a water-bath for ascertaining its temperature led the Board of Trade to give greater prominence in the Order in Council to this sample-tube form illustrated in Fig. 211.

By the addition, however, of two small glass tubes,

t, t' (Fig. 212a), sealed on to the main tubes to contain and insulate the platinum leading-in-wires, in accordance with the plan proposed by Mr. W. R. Cooper, the H type form of Clark's cell can be adapted for insertion in a water-bath. Considerable care, however, must be taken in annealing the joints between the small tubes and the main ones, otherwise a crack is very likely to occur at these places.

146. Temperature Variation of the E.M.F. of the Clark Cell. Although the E.M.F. of a Clark cell made in accordance with the Board of Trade Specification will not, as a rule, differ by more than 0·001 volt from 1·434 volt if the cell be maintained for some time at a constant temperature of 15°C., this value of the E.M.F. will be considerably changed by raising or lowering the temperature of the cell through several degrees. By observing cells at different temperatures, which were maintained constant for two or three hours at each temperature before the observations were taken, Lord Rayleigh found that the E.M.F. was diminished by about 0·077 per cent. per 1°C. rise of temperature, and this result has been generally confirmed by other observers. In reality, the relation is not a simple linear one, as it involves a term depending on the square of the temperature. But, for all practical purposes, we may accept the above percentage variation of the E.M.F., which, however, is more conveniently expressed by saying that the E.M.F. of a Clark cell falls by 0·0011 volt for each degree rise of temperature above 15 ̊C.

There are, however, one or two modifying causes that must be taken into account. For example, after a cell has been set up the mercurous sulphate settles down, leaving a layer of clear solution above it; now, if the zinc rod dips into this solution only, and not into the paste, it is found that the temperature coefficient is only about one-half the value given above. Again, if the zinc sulphate is not saturated at the temperature at which the cell is used, the temperature coefficient is also