106

QUANTITATIVE AGRICULTURAL AXALYMH

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in the observation of fuel value, the magnitude of the error
depending upon the extent to which nitric acid is formed. As a
rule the error is small and it may be ignored for ordinary fuel test-
ing but if a correction is to be made the nitric acid is titrated
by standard base, at the end of the experiment.
The heat of formation and solution of nitric acid from elemen-
tary nitrogen is 230 calories per gram. It is convenient to use a
standard solution of base, 1 cc of which is equivalent to 5 calories.
The normality of such a solution is
230 X 0-06302 " ™*° N'
The number of cubic centimeters of base required to titrate the
nitric acid in the bomb after the combustion is multiplied by 5,
the product being subtracted from the observed calories.
Radiation.—Radiation or absorption of heat by the* calorim-
eter may be avoided by making the calorimeter "aeliabatic."
This may be done in a number of ways, three of which will be
mentioned.
1. The water in the surrounding jacket may be heated by
electrical means, so as to keep pace with the rise in temperature
of the calorimeter water. This is the most satisfactory method,
although somewhat complicated and expensive* apparatus in
required.
2. The water in the jacket may be wanned by chemical
action. By Richards' method a basic solution is used to fill the
jacket and an acid is run in from a burette at a rale which
depends upon the rate of change in temperatures of the* calorim-
eter water and upon the concentration of the acid. The acid
solution may be standardized in terms of the* number of calorics
liberated by the action of each cubic centimeter upon the* bane, in
which case the proper rate of addition is more easily determined.
3. The jacket of the calorimeter may be evacuated, on the
principle of the Dcwar flask, the transfer of heat outwardly then
being limited to that which occurs through conductivity of the
glass of the jacket. This would appear to he* the leant trouble-
some method but it has not worked well in practice.
Radiation Corrections.—If adiabatie conelitionH cannot be
maintained several methods for making radiation corrections
are available.