134 KEPOET— 1893. 



when the cork is pushed into the tube. "Wash the cork thoroughly with 

 warm water, and leave it to soak in water for some tours before use. 

 Pass the zinc rod about 1 centimetre through the cork. 



Clean the glass tube and platinum wire carefully, then heat the ex- 

 posed 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 2 centimetres. 



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 ex- 

 pelled, 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 desirable 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. 



In using the cell sadden variations of temperature should as far as 

 possible be avoided. 



Notes. 



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

 paration of a neutral solution of pure zinc sulphate saturated with 

 ZnS04,7H20. 



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 tempera- 

 ture. 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 amount of zinc oxide required depends on the acidity of the solu- 

 tion, 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 sufiBcient 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 Sidphate. — 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 — 



