pure recrystallised' zinc sulphate as purchased. To neutralise the solution, if slightly acid, a little zinc oxide may be added; the amount necessary depends on the acidity to be neutralised, and as zinc oxide must not be left in solution care is required. A method of avoiding the error to which the presence of the zinc oxide may lead is given below. The crystals of zinc sulphate should be dissolved by the aid of gentle heat, but the temperature should not be raised above 30°, and the solution should be filtered while still warm into a stock-bottle, any crystals which remain undis solved being thus removed. This solution is afterwards to be mixed with the mercurous sulphate, treated as described below. If the solution contains zinc oxide, when the mercurous sulphate is introduced mercurous oxide and zinc sulphate are formed; if this takes place in the cell, the mercurous oxide may be a source of error. To avoid it it is well to mix with the zinc sulphate solution, before it is filtered and while still warm, some of the mercurous sulphate which has been washed and treated as described below. mercurous oxide is formed, it is removed from the solution, together with the undissolved mercurous sulphate, by the filtration, and the liquid in the stock-bottle is pure zinc sulphate containing mercurous sulphate in solution. If The Mercurous Sulphate.-Pure mercurous sulphate is a white powder. The salt as purchased is frequently greyish. This is due to the presence of metallic mercury in a finely divided state, and this is rather an advantage than otherwise, for it preserves the basicity of the salt. But it often contains large quantities of mercuric sulphate also, and this may be a source of considerable error. The mercuric sulphate is a dead white in colour, so that the colour of the mercurous sulphate is not a guide to its purity. The object of the following treatment is to get rid of this impurity. Mercuric sulphate when washed with water decomposes into an acid and a basic mercuric sulphate. The latter is a yellow substance (turpeth mineral) insoluble in water, and its presence (at any rate, to a moderate amount) is not seriously harmful. The acid sulphate is soluble in water, and if present the E.M.F. of the cell will not be normal. The rationale of the following process for preparing the mercurous sulphate will now be intelligible. Wash the mercurous sulphate with cold distilled water by agitation in a bottle; drain off the water. If the sulphate has turned yellow, the presence of acid mercuric sulphate is shewn. If a large quantity of the yellow turpeth mineral is formed, it means that there is a great deal of acid sulphate present, and it may be desirable to obtain a fresh supply of the mercurous sulphate. If the quantity of yellow salt formed is small, drain off the water, and repeat the process of wash ing twice at least. After the last washing drain off as much of the water as possible. This washing removes the soluble acid mercuric sulphate, and leaves only mercurous sulphate and the insoluble turpeth mineral. Mix the washed mercurous sulphate with the zinc sulphate solution, adding sufficient crystals of zinc sulphate from the stockbottle to ensure saturation, and a small quantity of pure mercury; shake these up well together to form a paste of the consistency of cream. Heat the paste, but not above a temperature of 30°. Keep the paste for an hour at this temperature (this is best done by putting the bottle containing the paste into a water-bath kept at about 30°), and shake it well from time to time; then allow the paste to cool, but continue to shake the bottle occasionally, thus securing that the materials are all uniformly mixed. Crystals of zinc sulphate form as the paste cools; these should be distinctly visible and should be distributed throughout the mass; if this be not the case, add more crystals from the stock-bottle, and repeat the process of heating and mixing. Contact is made with the mercury by means of a platinum wire about No. 22 gauge. This is protected from contact with the other materials of the cell by being sealed into a glass tube; the ends of the wire project from the tube; one end forms the terminal, the other end and a portion of the tube dip into the mercury. The cell may conveniently be set up in a small testtube of about 2 cm. diameter and 6 or 7 cm. deep.. Place the mercury in the bottom of this tube, filling it to a depth of, say, 15 cm. Cut a cork about o'5 cm. thick to fit the tube; at one side of the cork bore a hole, through which the zinc rod can pass tightly; at the other side bore another hole for the glass tube which covers the platinum wire ; at the edge of the cork cut a nick through which the air can pass when the cork is pushed into the tube. Pass the zinc rod about 1 cm. through the cork. Clean the glass tube and platinum wire carefully, then heat the exposed end of the platinum red hot, and insert it in the mercury in the test-tube, taking care that the whole of the exposed platinum is covered. Shake up the mercurous sulphate paste and introduce it without contact with the upper part of the walls of the test-tube, filling the tube above the mercury to a depth of rather more than 2 cm. Then insert the cork and zinc rod, passing the glass tube through the hole prepared for it. Push the cork gently down until its lower surface is nearly in contact with the liquid. The air will thus be nearly all expelled, and the cell should be left in this condition for at least twenty-four hours before sealing, which should be done as follows :— Melt some marine glue until it is fluid enough to pour by its own weight, and pour it into the test-tube above the cork, using sufficient to cover completely the zinc and soldering. The glass tube should project above the top of the marine glue. The cell thus set up may be mounted in any desired manner. It is convenient to arrange the mounting so that the cell may be immersed in a water-bath up to the level of, say, the upper surface of the cork. Its temperature can then be determined more accurately than is possible when the cell is in air. Fig. xliii gives a drawing of the cell thus set up. The cell thus prepared should have, at a temperature of 15° C., an E.M.F. of 1'434 volts. The E.M.F. decreases as the temperature rises by 00077 of its value per 1° C., so that FIG. xliii. Pt Zn Cork Zn.SO, with at to the E.M.F. is 1434 (100077(t-15)}. If the cell is carefully set up in accordance with these instructions, its E.M.F. at the end of a Marine glue week or so should be within 2 or 3 in 10,000 of this value. Some cells shew considerable changes of E.M.F. when first made. This is usually due to one of two causes: either the solution is acid (in this case the free acid attacks the zinc, and the evil cures itself), or there is zinc oxide in the solution. In this case mercurous oxide is formed, and may be deposited as a grey powder on the zinc. The E.M.F. falls greatly, and remains too low. Neither of these defects should be taken to use neutral zinc sulphate, Paste of Mercury present if care has been free from zinc oxide. The cell in the form just described' is not suitable for use as a source of current. E. M.F., though in many It is intended as a standard of cases it may conveniently be employed to measure a current in the manner described in (3) below. For standard The cell may be set up in various other forms. purposes the H form devised by Lord Rayleigh is probably best. To secure portability, Professor Carhart introduces between the mercury and the paste a thin disc of cork, which fits the tube tightly; the cork must be well washed with warm water and left to soak in zinc sulphate solution before being used. Thus the mercury cannot become impure through contact with the zinc. Professor Carhart also coats the top of the marine glue with a thin layer of silicate of soda, which forms a hard and lasting glaze. The Berlin Reichsanstalt now use the H form with some modifications. (2) To use the Clark Cell as a Standard of E.M.F Poggendorff's method of comparing electromotive forces has been described in § 80. If one of the two cells employed be a Clark's standard, any other E.M.F. can be compared with this. For many purposes it is desirable to use higher resistances than can be conveniently employed on a stretched wire bridge, and then the following method may be adopted : Connect up in series two resistance boxes A B, C D (fig. xliv) with a suitable battery. Let us suppose that we can take 10,000 ohms out of each box. The E.M.F. of the battery will depend on the value of the E.M.F. to be measured. If this be comparable with the E.M.F. of a Clark's cell, two Leclanché cells will be convenient. The boxes are to be used in such a way that the total resistance in circuit remains constant, and equal to say 10,000 ohms. Thus, if 4,500 be out in one box, 5,500 will be out in the other; and if an additional plug, say 5 ohms, is inserted in the first, the plug of the same value is taken out of the second. The connexions are made as in fig. 76, the resistance from A to P in that figure being represented by one box, AB, that from P to B by the second box, CD. It is desirable to put a high resistance in with the galvanometer when commencing the experiment. If increased sensitiveness is PP |