Chemical Affinity in terms of Electromotive Force. 29 
lary stem (made by drawing out a piece of tubing before the 
blowpipe) paraffin-wax was introduced into the sealed-up limb 
so as to cover up completely the platinum wire and lower half 
of the zinc, leaving the other half exposed, b, In a similar 
way, recently boiled mercury was run into the other limb, 
previously sealed up with a second platinum wire passing- 
through the glass, c. The stem of the inverted Y-tube was 
then carefully drawn out before the blowpipe (fig. 5, m), and 
connected by means of a short piece of india-rubber tubing, n, 
with the end of a glass tube projecting from the little flask, p, 
containing mercurous sulphate paste, and connected with the 
Sprengel pump. When a fairly good vacuum was obtained, 
the paste was made to boil by applying a very gentle heat ; 
after about half an hour's boiling (the pump being at work 
the whole time) the connexion between the pump and flask 
was suddenly severed, when the sudden access of atmospheric 
pressure drove the paste into the cell, completely filling it : 
the pump was then again connected, and the boiling carried 
out again in the cell itself, and so on as before. Finally, by 
means of a blowpipe the drawn-out stem was sealed at m. To 
prevent the paste blocking up this drawn-out part, it was found 
necessary to use levigated particles of mercurous sulphate with 
a large proportion of zinc-sulphate solution; so that ultimately 
the cell contained much more fluid than solid matter. In 
order to use the cell, copper wires were soldered to the plati- 
num wires projecting from the sealed ends of the inverted Y 
and bent over so as to dip into mercury-cups, the Y being 
either held by a clamp or imbedded in paraffin- wax, and of 
course being never allowed to be upset or shaken up so that 
the mercury could pass into the limb containing the zinc, 
which is otherwise liable to occur and spoil the cell. 
136. The following numbers may be quoted as illustrations 
of the practical absence of any effect on the E.M.F. of the cell 
caused by the presence or otherwise of dissolved air. The 
values cited are the average readings, during a period of 
several months, of a dozen ceils set up with cold-saturated zinc- 
sulphate solution well aerated, and not sealed up with melted 
paraffin-wax, but only loosely corked to avoid entrance of 
dust. Each cell during this period remained sensibly constant. 
All the values are reduced to the average reading (taken as 
100*00), during the same period, of a yet larger number of 
cells prepared hot and sealed up precisely in accordance with 
Clark's directions — this average reading being the standard 
employed in the previous portions of these experiments, and 
especially in Parts V. and YI. 
