180) ALFALFA. 
contain the substarce to be burned and the oxygen for com- 
bustion, a cover to close the cylinder, and a collar to hold 
the cover tightly upon the cylinder. The material to be 
burned is compressed into a pellet in a press and is then 
placed in a small platinum capsule which is suspended by 
platinum wires from the cover of the bomb. A coil of fine 
iron wire, for igniting the substance electrically, is stretched 
between the two platinum wires which support the capsule 
and is made to rest upon the pellet within the capsule. The 
cover is then screwed tightly upon the cylinder of the bomb, 
and the oxygen is introduced through a valve in the cover 
until the pressure in the bomb is 20 atmospheres. Thevalve 
is then tightly closed and the bomb is placed in a Brittania 
metal receptacle containing a definite quantity of water the 
temperature of which is known. A metal stirrer, operated 
in this laboratory by a water motor, keeps the water in the 
receptacle in motion during the whole operation, and thus 
equalizes the temperature. When the bomb is immersed, a 
current of electricity is passed through the coil of wire rest- 
ing upon the substance in the capsule, causing the wire to 
become incandescent and ignite the substance. Combustion 
takes place immediately, and the heat passes through the 
metal of the bomb and is absorbed by the water surround- 
ing it. A calibrated Fuess thermometer is immersed in the 
water, and measures the rise in temperature of this known 
amount of water, from which the heat of combustion of the 
material is calculated, due allowance being made for the 
hydrothermal equivalent of the bomb and apparatus con- 
taining it (7. ¢., for the amount of heat absorbed by these), 
for the heat introduced by the current of electricity in ignit- 
ing the substance, the heat developed by oxidation of the 
metal fuse and small amount of nitrogen, etc. The hydro- 
thermal equivalent of the bomb used in these investigations 
was determined by the combustion of samples of sucrose and 
of other substances of known purity.”’ 
The digestibility of a food is measured by the difference 
between the total nutrients of the food and of the feces. The 
materials in the feces do not consist entirely of undigested 
parts of the food but contain also products of metabolism, 
or the materials resulting from the breaking down of the 
