comparative ease and proceeded quite rapidly enough, but in every
instance a portion of the carbon was unconsumed, and consequently
instead of about 13 deg. of rise in temperature only 10 deg. were recorded.

Since the calorific power of a coal is determined by the number of
degrees Fahrenheit which a given quantity of water is raised in
temperature by a known weight of fuel, it follows that every care should
be taken that the experiment be performed under similar atmospheric
conditions. The oscillation of barometric pressure does not appear to
affect the working, but the temperature of the room in which the
work was done, and especially that of the water, are most important
considerations. It has been observed by some who have used this
apparatus--and I have frequently noticed it myself--that the lower the
temperature of the water is under which the fuel is burnt the higher is
the result found. This has been explained on the assumption that the
colder the water used, the greater is the difference between the
temperature of the room and that of the water; hence it would be
expedient that in all cases when such experiments are made the same
difference of temperature between the air in the room and the water
employed should always exist. For example, if the temperature of the
room were 70 deg., and the water at 60 deg., then the same coal would give a
like result with the water at 40 deg. and the room at 50 deg.. This has been
regarded as the more evident, because the gases passing through
the water escape under favorable conditions of working at the same
temperature as the water, and are perfectly deprived of any heat in
excess of that possessed by the water. Under these circumstances it
would seem only reasonable that this assumption should be correct. It
was, however, found after a large number of experiments upon the same
sample of coal that this was not the case. 30 grammes of coal which
raises the temperature of the water 13.4 deg., when the water at starting