220 NUTRITION OF FARM ANIMALS 



307. Heat energy unique. In one respect heat energy 

 occupies a peculiar position. Other forms of energy are in 

 general readily and completely transformed into heat but there is 

 no known method by which heat can be completely transformed 

 into other forms, such as kinetic energy. Whatever portion of 

 the heat is thus transformed obeys the law of the conservation 

 of energy but part of it always remains in the form of heat. 1 



308. Units of energy. Quantities of energy are measured 

 by converting them into the same form and comparing them 

 with some quantity of the same form of energy arbitrarily as- 

 sumed as a unit. 



Since quantities of kinetic energy can be expressed in terms 

 of mass, space and time, a unit based on these concepts is taken 

 as the fundamental unit of energy. The so-called C. G. S. 

 (centimeter-gram-second) unit is the erg. An erg is a quantity 

 of energy equal to twice the kinetic energy possessed by a mass 

 of one gram moving with a velocity of one centimeter per second. 

 Since this is a very small quantity, a unit called the joule, equal 

 to ten million ergs, is often employed in practical measurements 

 of energy, that is, i joule = io 7 ergs. For purposes where a still 

 larger unit is desired the kilo-joule equal to one thousand joules 

 is also employed. 



In practice, however, heat is the form of energy which gen- 

 erally lends itself most readily to exact determination and, 

 since other forms of energy are easily converted into heat, units 

 of heat are extensively employed in the study of energy. The 

 most common unit for this purpose is the calorie, which is the 

 quantity of heat required to raise the temperature of one gram 

 of water one degree centigrade. 2 



The foregoing is known as the small, or gram calorie (cal.). 

 Where larger quantities of heat are to be measured the large, 



1 This is, of course, one aspect of the second law of energetics. Theoretically, 

 a perfect heat engine with a lower temperature limit of absolute zero would convert 

 heat completely into kinetic energy. Since, however, we can neither obtain the 

 temperature of absolute zero nor construct a perfect heat engine, this theoretical 

 conception is simply a limit which may be more or less remotely approached in 

 practice but never attained. 



2 Since the specific heat of water varies at different temperatures, an exact defini- 

 tion of the calorie must specify the temperature at which it is measured. Practice 

 differs in this respect but the preferable unit is the mean calorie, which is one one- 

 hundredth of the amount of heat required to raise the temperature of one gram of 

 water from o to 100 C. 



