HEAT OF COMBUSTION

109

stance containing a large amount of water, 100 gm or more is first weighed
(to centigrams only). It is then evaporated to dryncss over the steam
bath and reweighed. The loss of water gives the necessary data for calcu-
lating the fuel value of the dry material to a basis of the original sample.
Thus, if M represents the per cent of water, c the calorific value of the dry
residue and C that of the original sample,

(100 -

"loo"

C

As suitable materials for exercises in calorimetry of foods, such substances
as dried egg albumen, starch, sugar and butter fat may be used. Coal,
coke, crude oil, kerosene or gasoline are fuels whose calorific power may be
determined. Volatile liquids, such as the last named two, can be weighed
and burned in a gelatine capsule, such as are used for medical preparations.
Blank determinations must then be run on other similar capsules, so that
corrections may be subtracted. All of the capsules are weighed.
Fill a weighing bottle with the prepared sample and weigh accurately to
0.1 mg. Pour from this into the pan in the bomb, until the pan is approxi-
mately half full. Weigh the bottle again, the difference between the above
weighings giving the weight of the fuel in the bomb. This weight should be
greater than 0.5 gm and not more than 1.2 gm. For hard coal the charge
should be not greater than 1 gm. Hard coal should not be as finely divided
as soft coal or foods.
The upper half of the bomb is now placed in position and the nut is
screwed down as far as may be by hand, care being taken not to cross
the threads. The shoulder on the upper half of the bomb, over which
the nut makes bearing contact, should be lubricated with oil. Extreme
care should be taken that no oil or grease is deposited on the lead gasket.
The bomb is now ready to be filled with oxygen. The nipple is coupled
to the oxygen piping by means of the attached hand union. In handling
the bomb, care should be taken not to tip or jar it, as fuel may be thrown
from the pan.
The spindle valve on the bomb is opened one turn and then the valve
on the oxygen supply tank is-very cautiously opened. The pressure gauge
should be carefully watched and the tank valve so regulated that the pres-
sure in the system shall rise very gradually. When the pressure reaches 300
Ib per square inch, the tank valve is closed and the spindle valve immediately
afterward. The bomb should be immersed in water immediately to detect
any possible leaks. The bomb is now ready for the calorimeter, which is
prepared as follows:
Nineteen hundred grams of distilled water, weighed or measured in a cali-
brated flask, is placed in the calorimeter can at a temperature about 1.5°
below the jacket temperature (which should be in the proximity of the room
temperature). The bomb is then placed in the calorimeter and the stirrer
and thermometer are lowered into position as indicated by Fig. 28. The
thermometer is immersed about 3 inches in the water. The bulb of the ther-
mometer should not touch the bomb.