A STANDARD OF ELECTRO-MOTIVE FORCE. 
787 
or neutral. In the former case the cell probably neutralises itself by dissolution of 
zinc, a certain amount of gas being liberated. 
It is convenient to keep a stock bottle of saturated solution of zinc sulphate. This 
may be prepared in a flask by mixing distilled water with about twice its weight of 
crystals. A little carbonate of zinc is added to neutralise free acid, and solution is 
effected with the aid of gentle heat. If time can be afforded, it is a good plan to let 
the solution stand, as a good deal of iron is usually deposited, even when “ pure ” zinc 
sulphate is used. The solution may then be filtered in a warm place into the stock 
bottle. When it is intended to charge H-cells, or prepare paste, the bottle should be 
exposed to a gentle warmth for a few hours, and the solution should be drawn with a 
pipette from near the crystals at the bottom of the bottle. Otherwise there is no 
security that the liquid used will be saturated. 
To prepare paste we may rub up together in a mortar 150 grins, mercurous 
sulphate (as purchased), 5 grms. zinc carbonate, and as much of the saturated zinc 
sulphate 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 con¬ 
venient to leave the paste in the mortar for two or three days, rubbing it up at 
intervals with additions of 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, notwithstanding 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. 
The performance of the newer cells has been satisfactory (one irregularity will be 
mentioned later), and the substitution of marine glue for paraffin wax promises a 
longer life. A large number of observations have been recorded, but it does not 
appear necessary to give them here in detail. 
§ 46. I have been anxious to compare with my cells some prepared by others, 
and have to thank many physicists for the opportunity of doing so. Dr. Alder 
Wright was good enough to send me several of his cells, with which comparisons 
of especial value have been made. I shall have occasion to remark more at length 
upon the results obtained with these cells, and for the moment will only say that the 
difference between them and mine is under TF 5 ~oth part. Cells prepared by 
Mr. Threlfall (not those previously mentioned), by Dr. Fleming, and by Prof. G. 
Forbes also agreed well. 
Dr. Fleming’s cells, of‘ which six remained in my hands, were at first irregular, 
and even now show somewhat larger variations than the best cells are liable to. The 
cause is, I believe, to be found in insufficient purity of mercury, as is suggested by 
* It is usually found that on neutralisation the mercurous sulphate turns yellow, so that the paste 
presents ultimately somewhat the appearance of mustard. I do not know whether the change of colour 
is normal, or is to he attributed to impurity. 
