ula is
certainly erroneous at low temperatures. A favourite temperature to
select is 4 deg. C., the temperature of maximum density, since at this
point the specific heat at constant volume is the same as that at
constant pressure But this is really of no consequence, since the
specific heat at constant volume cannot be practically realized. The
specific heat at 4 deg. could be accurately determined at the mean
over the range 0 deg. to 8 deg. keeping the jacket at 0 deg. C. But
the change appears to be rather rapid near 0 deg., the temperature is
inconveniently low for ordinary calorimetric work, and the unit at 4
deg. would be so much larger than the specific heat at ordinary
temperatures that nearly all experiments would require reduction. The
natural point to select would be that of minimum specific heat, but if
this occurs at 40 deg. C. it would be inconveniently high for
practical realization except by the continuous electrical method. It
was proposed by a committee of the British Association to select the
temperature at which the specific heat was 4.200 joules, leaving the
exact temperature to be subsequently determined. It was supposed at
the time, from the original reduction of Rowland's experiments, that
this would be nearly at 10 deg. C., but it now appears that it may be
as low is 5 deg. C., which would be inconvenient. This is really only
an absolute unit in disguise, and evades the essential point, which is
the selection of a standard temperature for the water thermometric
unit. A similar objection applies to selecting the temperature at
which the specific heat is equal to its mean value between 0 deg. and
100 deg. The mean calorie cannot be accurately realized in practice
in any simple manner, and is therefore unsuitable as a standard of
comparison. Its relation to the calorie at any given temperature, such
as 15 deg. or 20 deg., cannot be determined with the same degree of
accuracy as the ratio of the specific heat at 15 deg. to that at 20
deg., if the scale of temperature is given. The most practical unit is
the calorie at 15 deg. or 20 deg. or some temperature in the range of
ordinary practice. The temperature most generally favoured is 15 deg.,
but 20 deg. would be more suitable for accurate work. These units
differ only by 11 parts in 10,000 according to Callendar and Barnes,
or by 13 in 10,000 according to Rowland and Griffiths, so th