160 
DE. H. T. BAENES ON THE CAPACITY FOE HEAT OF WATEE 
published in full (‘ Proc. Roy. Soc.,’ vol. 67, p. 117 (1897)), and in consequence may 
be passed over here with but a brief mention. A thorough study of the old form of 
cell recommended by the Board of Trade formed one of our chief objects, including 
measurements of the diffusion lag on a sudden and definite change of temperature. 
It was shown that for a change of 15° C. the time required for a B.O.T. cell to assume 
its true value was of the order of 14 days when left undisturbed, but only 2 days 
when shaken three times at different intervals. In cells where the saturated solution 
of zinc sulphate was replaced by moist zinc sulphate crystals, no such effect could be 
noticed, but the cells assumed their normal value on a sudden change in temperature 
in 10 or 15 minutes, or, in other words, in only such time as was required for the cell 
to assume the temperature of the surroundings. 
These modified B.O.T. cells were studied in every detail, but more particularly as 
regards the formula governing the variation of the E.M.F. with temperature and 
their reproducibility. The remarkable constancy of these cells, their agreement 
amongst themselves, and the closeness with which they followed the temperature 
expression deduced, was a matter of much satisfaction. It was found that the 
tenrperature change of the E.M.F. depended on two conditions, a change of 
temperature and a change of strength of solution. These two changes were about 
equal and formed one-half of the total change. By keeping the strength of the 
solution constant and varying the temperature, the change was practically the same 
for all strengths, and equal to that found by Professor Carhart for the Carhart- 
Clark cell. If the temperature was kept constant and the strength of solution 
varied, then the E.M.F. followed the concentration in the linear relation in millivolts 
and grammes per cub. centim., 
c/E = 42-0 - 88'0u>. 
The variation with temperature was followed for higher temperatures, and the 
transition point for the inversion of the heptahydrate (normal) crystals at 38'78° C. 
fixed. Various types of cells were devised which have been designated as the B.O.T. 
“ crystal” cell, which is the modified Board of Trade form ; the “ sealed ” cell, which 
is a form hermetically closed by glass fusion; and the “inverted’ cell, which is 
a B.O.T. crystal cell reversed so as to place the negative electrode (zinc amalgam) at 
the bottom. These all have an identical temperature formula, which may be expressed 
in millivolts, 
E, = E 15 - P200 (t - 15°) - -0062 (t - 15°) 3 
between 0° and 30° for a mean temperature of 15°, or 
E, = E 39 — 1-6 3 5 (t — 39°) — -0140 (t — 39°)° 
for a mean temperature of 39°. 
