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Method of making a Measurement.

The platinum bowl is washed with nitric acid and distilled water, dried by heat, and then left to cool in a desiccator. When thoroughly dry it is weighed carefully.

It is nearly filled with the solution, and connected to the rest of the circuit by being placed on a clean copper support to which a binding screw is attached. This copper support must be insulated.

The anode is then immersed in the solution so as to be well covered by it and supported in that position; the connections to the rest of the circuit are made.

Contact is made at the key, noting the time of contact. The current is allowed to pass for not less than half an hour, and the time at which contact is broken is observed. Care must be taken that the clock used is keeping correct time during this interval.

The solution is now removed from the bowl and the deposit is washed with distilled water and left to soak for at least six hours. It is then rinsed successively with distilled water and absolute alcohol and dried in a hot-air bath at a temperature of about 160° C. After cooling in a desiccator it is weighed again. The gain in weight gives the silver deposited.

To find the current in amperes, this weight, expressed in grammes, must be divided by the number of seconds during which the current has been passed, and by 0.001118.

The result will be the time-average of the current, if during the

interval the current has varied.

In determining by this method the constant of an instrument the current should be kept as nearly constant as possible, and the readings of the instrument observed at frequent intervals of time. These observations give a curve from which the reading corresponding to the mean current (time-average of the current) can be found. The current, as calculated by the voltameter, corresponds to this reading.

SPECIFICATION B.

ON THE PREPARATION OF THE CLARK CELL.
Definition of the Cell.

The cell consists of zinc or an amalgam of zinc with mercury and of mercury in a neutral saturated solution of zinc sulphate and mercurous sulphate in water, prepared with mercurous sulphate in excess.

Preparation of the Materials.

1. The Mercury.-To secure purity it should be first treated with acid in the usual manner, and subsequently distilled in vacuo.

2. The Zinc.-Take a portion of a rod of pure redistilled zinc, solder to one end a piece of copper wire, clean the whole with glass paper or a steel burnisher, carefully removing any loose pieces of the zinc. Just before making up the cell dip the zinc into dilute sulphuric acid, wash with distilled water, and dry with a clean cloth or filter paper.

3. The Mercurous Sulphate.-Take mercurous sulphate, purchased as pure, mix with it a small quantity of pure mercury, and wash the whole

thoroughly with cold distilled water by agitation in a bottle; drain off the water, and repeat the process at least twice. After the last washing drain off as much of the water as possible.

4. The Zinc Sulphate Solution.-Prepare a neutral saturated solution of pure (pure recrystallised') sinc sulphate by mixing in a flask distilled water with nearly twice its weight of crystals of pure zinc sulphate, and adding zinc oxide in the proportion of about 2 per cent. by weight of the zinc sulphate crystals to neutralise any free acid. The crystals should be dissolved with the aid of gentle heat, but the temperature to which the solution is raised should not exceed 30° C. Mercurous sulphate treated as described in 3 should be added in the proportion of about 12 per cent. by weight of the zinc sulphate crystals to neutralise any free zinc oxide remaining, and the solution filtered, while still warm, into a stock bottle. Crystals should form as it cools.

5. The Mercurous Sulphate and Zinc Sulphate Paste.--Mix the washed mercurous sulphate with the zinc sulphate solution, adding sufficient crystals of zinc sulphate from the stock bottle to ensure saturation, and a small quantity of pure mercury. Shake these up well together to form a paste of the consistence of cream. Heat the paste, but not above a temperature of 30° C. Keep the paste for an hour at this temperature, agitating it from time to time, then allow it to cool; continue to shake it occasionally while it is cooling. Crystals of zinc sulphate should then be distinctly visible, and should be distributed throughout the mass. If this is not the case add more crystals from the stock bottle, and repeat the whole process.

This method ensures the formation of a saturated solution of zinc and mercurous sulphates in water.

To set up the Cell.

The cell may conveniently be set up in a small test-tube of about 2 centimetres diameter and 4 or 5 centimetres deep. Place the mercury in the bottom of this tube, filling it to a depth of, say, 5 centimetre. Cut a cork about 5 centimetre 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. Wash the cork thoroughly with warm water, and leave it to soak in water for some hours before use. Pass the zine rod about 1 centimetre through the cork.

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 ends of the tube; one end forms the terminal, the other end and a portion of the glass tube dip into the mercury.

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 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 1 centimetre.

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 twentyfour 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 containing the platinum wire should project some way above the top of the marine glue.

The cell may be sealed in a more permanent manner by coating the marine glue, when it is set, with a solution of sodium silicate, and leaving it to harden.

It

The cell thus set up may be mounted in any desirable manner. 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.

In using the cell sudden variations of temperature should as far as possible be avoided.

The form of the vessel containing the cell may be varied. In the H form the zinc is replaced by an amalgam of ten parts by weight of zinc to ninety of 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.

NOTES TO THE SPECIFICATION ON THE PREPARATION OF THE

CLARK CELL.

The Mercurous Sulphate. The treatment of the mercurous sulphate has for its object the removal of any mercuric sulphate which is often present as an impurity.

Mercuric sulphate decomposes in the presence of water into an acid and a basic sulphate. The latter is a yellow substance-turpeth 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 electro-motive 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 ZnSO47H2O.

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 time 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 became 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 electro-motive 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

FIG. 2.

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 insure 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 doos 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 ninety parts of pure mercury mixed with ten 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 vertical tubes. These, as shown in the figure, 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 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 Application of Photography to the Elucidation of Meteorological Phenomena. Fourth Report of the Committee, consisting of Mr. G. J. SYMONS (Chairman), Professor R. MELDOLA, Mr. J. HOPKINSON, and Mr. A. W. CLAYDEN (Secretary). (Drawn up by the Secretary.)

IN presenting their report on the work of the last year your Committee have but little to say on the subject of the representation of clouds and lightning by photography. They consider that their collection is nearly complete so far as the different varieties of cloud form are concerned, and it is only likely to be increased slowly and at long intervals by photographs of scarce forms of clouds or by particularly interesting series. During the year the Secretary has secured many new negatives; but since the collection already includes satisfactory examples of the same types, it has not been thought desirable to add more duplicates, and the offers of co-operation from other photographers have not been fulfilled. With regard to photographs of lightning also the collection has not been increased, for your Committee have not been made aware of any such photographs which show any features not already familiar, and no opportunity has occurred for the Secretary to make any observations for the further elucidation of the known phenomena.

Your Committee propose to invite the Royal Meteorological Society to take charge of such photographs from their collection as are not likely to be required for further investigation.

The attention of the Committee has been drawn to another application of photography which seems to open up a possibility of very valuable work; this is in the measurement of cloud altitudes. This is a question which has become more important since the acceptance by the Munich

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