140 REPORT— 1894. 



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 ') sine 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 satura- 

 tion, 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 ; con- 

 tinue to shake it occasionally while it is cooling. Crystals of zinc sul- 

 phate 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 np 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 

 zinc 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 



