MR. LATIMER CLARK ON A STANDARD VOLTAIC BATTERY. 
3 
saturated solution of zinc sulphate, the positive element consisting of pure distilled zinc 
resting on the paste. 
The best method of forming this element is to dissolve pure zinc sulphate to satura- 
tion in boiling distilled water. When cool, the solution is poured off from the crystals 
and mixed to a thick paste with pure mercurous sulphate, which is again boiled to drive 
otf any air ; this paste is then poured on to the surface of the mercury, previously heated 
in a suitable glass cell ; a piece of pure zinc is then suspended in the paste, and the 
vessel may he advantageously sealed up with melted paraffin-wax. Contact with the 
mercury may be made by means of a platinum wire passing down a glass tube, cemented 
to the inside of the cell, and dipping below the surface of the mercury, or more conve- 
niently by a small external glass tube blown on to the cell, and opening into it close to 
the bottom. The mercurous sulphate (Hg 2 S0 4 ) can be obtained commercially * ; but it 
may be prepared by dissolving pure mercury in excess in hot sulphuric acid at a tem- 
perature below the boiling-point : the salt, which is a nearly insoluble white powder, 
should be well washed in distilled water, and care should be taken to obtain it free 
from the mercuric sulphate (persulphate), the presence of which may be known by the 
mixture turning yellowish on the addition of water. The careful washing of the salt 
is a matter of essential importance, as the presence of any free acid, or of persulphate, 
produces a considerable change in the electromotive force of the cell. 
The electromotive force of the elements thus formed is remarkably uniform and 
constant, provided the elements be not connected up and allowed to become weakened by 
working. A long series of comparisons was made between various elements, some of 
which had been made many months, and it was found that the greatest variation among 
them all did not differ from the mean value more than one thousandth part of the whole 
electromotive force ; such a difference as this was, however, unusual, and might have been 
due to slight differences of temperature. 
The following Table gives some of the results obtained. Temperatures are not stated, 
as the elements were approximately at the same temperature as the standards with which 
they were compared at the time. 
No. of element 
or letter. 
Date of 
construction. 
Date of 
comparison. 
Value. 
96* 
March 23, 1871 
March 25, 1871 
1-0000 
16 . 
February 16, 1871 
March 24, 18J1 
•9997 
89. 
March 23, 1871 
March 25, 1871 
•9991 
90. 
„ 
„ 
•9993 
91. 
,, 
„ 
•9985 
92. 
,, 
„ 
*9988 
93. 
„ 
•9991 
94. 
„ 
•9988 
95. 
„ 
•9998 
97. 
„ 
„ 
•9996 
98. 
„ 
•9996 
99. 
„ 
„ 
•9995 
100. 
” 
” 
•9993 
* The author has obtained it from Messrs. Hopkins and Williams, 5 New Cavendish Street. 
B 2 
