CLARK CELL AS A STANDARD OF ELECTROMOTIVE FORCE. 
58 7 
“ To prepare paste we may rub up together in a mortar 150 gTms. mercurous sulphate, 
as purchased, 5 grms. zinc carbonate, and as much of the saturated solution as is 
required to make a thick paste. Carbonic anhydride is liberated, and must be allowed 
a sufficient time to separate. I have found it convenient to leave the paste in a mortar 
for two or three days, rubbing it up at intervals with additions of the zinc sulphate 
solution, until the gas has escaped. By the addition of a small crystal, and by 
evaporation, we have security that the paste is saturated, and will remain so, not¬ 
withstanding such moderate elevation of temperature as the cells are expected to bear. 
The paste may then be transferred to a tightly corked bottle, and, so far as my 
experience extends, will remain available for many months at least. Before pouring 
the bottle of paste should be well shaken up.” 
Fig. 3. 
Sealing wax' 
The cell takes the form of a small tube with the platinum wire sealed in at 
its lower end. (See fig. 3.) “In charging the cells the first step is to pour in 
sufficient pure mercury to cover the platinum effectively. The paste is introduced 
with the aid of a small funnel, care being taken not to soil the sides above the 
proper level. The rods, cut from rods of pure zinc, as supplied by Hopkin and 
Williams and not re-cast, are soldered to copper wires and cleaned in the lathe. 
Just before use they are dipped in dilute sulphuric acid, washed in distilled water, 
and dried with a clean cloth or filter paper. Each zinc is mounted in a short 
piece of cork fitting the tube (but not too tightly) and nicked in order to allow 
of the passage of air. The cork is pushed gradually down until its lower face 
is almost in contact with the paste. The object is to leave but little air, and at the 
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