232 PRINCIPLES OF ANIMAL NUTRITION. 
For the measurement of heat various units are in use, but the ones 
most commonly employed in physiology are the small and the large 
calorie. The small calorie (cal.) is defined as the amount of heat 
required to raise the temperature of 1 gram of water through 1° C. 
Since, however, the specific heat of water varies somewhat with the 
temperature, it is necessary to specify the average temperature 
of the water. The temperature of 18° C. has been quite commonly 
used for this purpose, the resulting unit being indicated by the 
abbreviation cal,,. Atwater & Rosa,* however, in their work 
with the respiration-calorimeter, have employed the temperature 
of 20° C., designating their unit by cal,,. The difference between 
the two is very slight, 1 cal,, equaling 1.0002 cal,,. The large 
calorie (Cal.) is the amount of heat required to raise the tempera- 
ture of one kilogram of water through 1° C., or is equal to 1000 small 
calories. The temperature at which the large calorie is measured 
may be indicated as in case of the small calorie. 
The calorie, however, while commonly used, and while in some 
respects a convenient unit, is in a sense not a rational one. Since 
heat is one form of energy, and since, in accordance with the law of 
the conservation of energy, there is a fixed relation between it and 
other forms of energy, a rational unit would be one bearing a simple 
numerical relation to the units employed to measure other forms 
of energy, or in other words, the erg or some simple multiple of it. 
As already noted, the Kilojoule (J) is a convenient unit for this 
purpose. It has two advantages over the Calorie: first, it permits 
of a direct comparison of heat with other forms of energy (expressed, 
of course, in units of the same system); and second, it is an “abso- 
lute” unit, that is, it is based on the fundamental units of space, 
mass, and time, and has a perfectly definite magnitude, while the 
Calorie has not unless the temperature at which it is measured is 
stated. To this may be added that in discussing physiological 
relations it avoids the sometimes confusing implication that the 
quantities of energy dealt with actually exist in all cases as heat. 
The relation between the Calorie and the Kilojoule is as follows: 
1Cal,s=4.183 J | =41,830,000,000 ergs; 
1J | =0.2391 Cal,,=10,000,000,000 ergs. 
* U.S. Dept. Agr., office of Expt. Stats., Bull. 63, p. 55. 
Digitized by Microsoft® 
