0.2 of a degree makes a difference of more than 100 calories. In the
instructions supplied with Thompson's calorimeter nothing is said as to
the temperature of the room in which the experiment is performed, but
simply that the water shall be at 60 deg. F. If, with the water at 60 deg., a
room were at 50 deg., as it often is in winter, a good coal would give 14
lb. of water per lb. of coal as the evaporative power; but if in summer,
the room were at 75 deg. and the water at 60 deg., the same coal would give 15
lb. of water per lb. of coal. If further evidence were needed of the
effect of temperature consideration of the experiments already referred
to will show how necessary it is that some general rule shall be
adopted. Considerable stress is laid (in the instructions) upon the
quantity of oxygen mixture used being determined by rough experiments.
This I have found leads to erroneous conclusions unless a number of
experiments are tried in the calorimeter, as it often happens that the
quantity which appears to be best adapted is not that which yields a
trustworthy result. There are many samples of South Wales coal, 30
grains of which will require 10 parts of oxygen mixture in order to burn
completely, but since a little oxygen is lost in drying and grinding,
and few samples of chlorate are free from chloride, it is not safe to
use less than 11 parts of oxygen mixture, but this amount is sufficient
in _all_ cases, and never need be exceeded. I have made numerous
experiments with various coals (anthracite, steam, semi-bituminous, and
bituminous, including a specimen of the ten yard coal of Derbyshire),
and find that with 11 parts of chlorate and nitrate of potash, they are
all perfectly manageable and yield the best results. It is quite clear
that the excess of chlorate is decomposed in all instances, and the
latent heat of the oxygen evolved, but those coals which are best to
experiment with did not yield results that differed when the quantity of
oxygen mixture was reduced to nearly the limit required for combustion
of the coal. Under these circumstances, therefore, the constant use