PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 57 
Omitting the description of further experiments, the final mode of 
preparing the mercurous sulphate for standard cells is here given. 
Fuming sulphuric acid saturated with SO, (32 per cent. of SO; is a con- 
venient specification) is added to sufficient pure distilled mercury to ensure 
the latter being always in excess. The mercury should be contained in a 
clean glass vessel and violently agitated by a glass stirrer, so that the 
product may be in a fine state of division. After seven or eight hours the 
reaction will be sufficiently advanced for the sulphate to be separated from 
the acid, but if convenient the action may go on for some days. Care- 
fully pour off as much of the strong acid as possible into a large volume 
of water or into an empty vessel, and afterwards add the pasty product 
left to thirty or forty times its bulk of distilled water. Mercury is 
precipitated and a considerable quantity of heat is evolved owing to the 
dilution of the acid. A few minutes suftice for the sulphate to settle, when 
the acid liquid may be decanted and the salt well washed by agitation 
with acidulated water (1 part of conc. H,SO, to 10,000 parts of distilled 
water). Filtering follows, a filter pump being employed to effect exhaus- 
tion. It is advisable next to pound the damp sulphate thoroughly in an 
agate mortar to ensure the absence of small caked masses, after which 
acidulated water is again added, filtering effected, and the salt washed on 
the filter-paper with two or three lots of neutral saturated cadmium 
sulphate solution (or zinc sulphate solution for Clark cells). The salt is 
now removed to a small flask, saturated cadmium sulphate solution added, 
and the whole well shaken and then allowed to stand for twenty-four 
hours. Filtering follows, then three more washings with cadmium sulphate 
solution, removal to a flask once more with CdSO, solution, and at the 
end of twenty-four hours the solution should still be neutral to Congo red. 
If so, the sulphate may be filtered and is ready for the manufacture cf 
the paste. The whole of the operations should be conducted in a room 
screened from sunlight. As thus prepared the mixture of mercurous 
sulphate and mercury is of a dark grey colour. Cells set up with paste 
prepared from it require no ageing, and the constancy obtained with 
pastes made from materials obtained from different sources is an indica- 
tion of the purity of the salt. 
Table III. gives the results of comparisons between cadmium cells set 
up with pastes prepared in this way and cadmium cell W17. The latter 
in every case has the greater E.M.F. Differences are expressed ir 
hundredths of a millivolt. 
Taste III. 
ee Cell No. 66 Cell No, 67 Cell No. 68 
a b c a b c a b c 
May 12,1904) -—24 -—26 —24 se ES 
” 12, ” SOT tee 20. ee =— ee 
ee = — 20502 -_ a 
” 16, ” ae —27 —'25 —28 —_ 
» 16, — aD ee ail a 
mE 16, SO P10 P10) Hot =21" =20 = 
June 13, ,, — = E99 29) 27 
” 13, ” v == — 23 —22 — 23 
» 13, 4 oie 20a —o1ee |) o 20s 2009 = l= 20) ¢— 9 
oly 6; v5, ,) —21 —21. —20 —19, —20 —19, | -—20 -—21 —20 
STEELS) 035 —20, —20, —20 —20 -21 —20 = 20. —20) —1 Gr 
oe eee ne) 
