to bring it into calories, as is done by the authorities in Italy, makes
more than 100 heat units--a serious difference when 5d. per ton fine is
attached to every 100 calories lower than the number guaranteed.

Taking the latent heat of steam as 537 deg. C., and multiplying this number
by 14.74, the evaporative power of the coal used in these experiments,
its equivalent in calories is 7,915. From the analysis of this coal,
disregarding the nitrogen and deducting an equivalent of hydrogen
for the oxygen present, the _total heat units_ given by Favre and
Silbermann's figures for carbon (8,080) and hydrogen (34,462) will
be 8,746. It will be seen, therefore, that the calorific power, as
determined by Thompson's apparatus, gives a much lower result when
multiplied by 537 than the heat units calculated from the chemical
composition of the coal. When I used Thompson's apparatus in the
chemical laboratory at Turin to determine the evaporative power of
various cargoes of South Wales coal, it was agreed by mutual consent
that the temperature of the water at starting should be 39 deg. F. (the
temperature at which the _heat unit_ was determined). The temperature
of the room was about 60 deg., but this varied, as the weather was somewhat
severe and changeable. Under these conditions, with the water at 39 deg. and
room 60 deg., the coal which gives 14.74 lb. of water per lb. of coal,
will give as high as 15.88 lb. of water per lb. of coal. This result
multiplied by 537=8,496 calories, approaching much more nearly to the
theoretic value. This method of working is still practiced abroad, but
experience has shown that very widely differing results follow when
working in this manner, especially if the temperature of the room is
changeable, as it naturally is where ash determinations and other
chemical work is proceeding simultaneously. The time the experiment
lasts, taking the reading on a quickly rising thermometer and other
considerations, render the experiments anything but trustworthy when