METHODS OF INVESTIGATION. 235 
Income: 
Food 
Outgo: 
Feces 
Urine 
Perspiration ° 
Combustible gases 
Storage of tissue 
Work 
Heat 
+ Potential energy. 
{ Kinetic energy. 
Determination of Potential Energy. 
The Energy of the Food.—The potential energy of the food is 
conveniently measured by converting it into the kinetic form of heat; 
that is, by determining its heat of combustion. This determina- 
tion is effected by means of an instrument known as a calorimeter, 
in which the heat produced by the complete combustion of a known 
weight of the substance under examination is absorbed by some 
calorimetric substance and its amount measured by the change of 
temperature or of physical state of the latter. The calorimetric 
substance ordinarily employed is water, the intrease in tempera- 
ture of a known weight of this substance giving directly the amount 
of heat in calories. It is, of course, essential either that all the heat 
produced shall be transferred to the calorimetric substance or that 
it shall be possible to correct the observed results for any heat that 
may escape absorption. 
Another essential is that the oxidation shall be complete, a 
condition whose fulfillment it is by no means easy to secure. Two 
general methods have been employed for this purpose. The first 
was that of Thompson,* as used by Frankland and subsequently 
modified by Stohmann,{ in which the oxidation is effected by 
means of pure potassium chlorate, corrections being made for the 
heat evolved in the decomposition of the latter substance. The 
second method, which has almost entirely replaced the first, con- 
* Described by Frankland, Proc. Roy. Inst. of Great Britain, June 8, 
1866, and Phil. Mag. (4), 32, 182. 
+ Jour. pr. Chem., 127, 115; Landw. Jahrb., 18, 513. 
