142 



REPORT — 1894. 



issued from the Physical Technical Institute of Berlin, 'Zeitschrift fiir 

 Instrumentenkunde,' 1893, Heft 5. 



The Zinc Sulphate Solution. — The object to be attained is the pre- 

 paration of a neutral solution of pure zinc sulphate saturated with 

 ZnSO^THaO. 



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 tem- 

 perature. 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 re- 

 moved by the method given in the directions already quoted, ' Zeitschrift 

 fiir 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. Tlie 

 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 difiiculty is 

 avoided by adding as described about 1 2 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 tlie addition 

 of a very considerable quantity of crystals of 

 zinc sulphate from the stock bottle, in order 

 to insure saturation, when tlie washed mer- 

 curous 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 



Fig. 2. 



