20 PHYSIOLOaY AND NATIONAL NEEDS 



This twofold aspect of tlie usefulness of foods was first 

 grasped many years ago by Liebig, and lie divided foods 

 into two groups : (1) the " heat-forming," or more accu- 

 rately the " energy-forming," foods, which are the fats and 

 the carbohydrates (starch and sugar) ; and (2) the " flesh- 

 forming," or better the " tissue-forming," foods, which are 

 par excellence those complex nitrogenous substances termed 

 the proteins or albuminous foods. The proteins, however, 

 in addition are partially burnt in the body, and so serve 

 also as a source of energy. A nutritious diet, then, is one 

 which is able to repair tissue waste and provide the re- 

 quisite amount of energy. 



Energy. — When the fuel in the engine's furnace is 

 burnt, there is no real destruction of matter, for the pro- 

 ducts of combustion (carbonic acid and the like) are equal 

 in weight to the original fuel, plus the oxygen of the air 

 which has entered into combination with it. During this 

 combustion or oxidation, energy is liberated, and energy, 

 like matter, is also indestructible though it exhibits trans- 

 formations. In the unburnt fuel the energy is latent or 

 potential, but as the coal burns three forms of actual 

 energy or force are liberated : one of these is light, another 

 is heat, and the third is mechanical work which makes the 

 wheels go round. There is a fixed relationship between 

 these forms of energy ; heat, for example, can be trans- 

 formed into mechanical work, but always in a definitely 

 fixed proportion. Consequently energy can be measured 

 by selecting one kind as the standard, and then the value 

 of other forms of energy can be calculated. The standard 

 which has been selected is the heat unit or calorie. 



The calorie is the amount of heat required to raise 

 the temperature of one kilogramme (2 lb. 3 oz.) of water 

 from freezing-point to 1° C, and the instruments which 

 measure the calorific value of substances are called calori- 

 meters ; these may be briefly alluded to as vessels con- 

 taining a known weight of water ; the material to be 



