APPENDIX 325 



Thus we see that food not only serves to build up the vital 

 mechanism of our body, it also sets this mechanism in motion. 

 Feeding a man or an animal not only sustains his body — which 

 is obvious; it also maintains his strength — which demands 

 explanation. We ask : what energy can be concealed in a sack 

 of oats, a loaf of bread, a piece of meat ? The answer is far 

 from being a simple one, and does not occur to every mind. 



In order to get a satisfactory answer to this question it will 

 be necessarj'' to study the properties of vegetable substance, 

 and also the conditions of its production. We say ' vegetable 

 substance,' because we know that animal food proceeds in- 

 directly from vegetable food. Meat is only grass or grain 

 transformed by an animal organism. 



But before we tackle this problem we must agree as to the 

 exact meaning of certain terms we are going to use. First, 

 what do we mean by energy ? We will try to explain it by 

 some examples. Such a method of explaining scientific truths 

 is certainly not very exact, but on the other hand it is one of 

 the easiest and quickest, and therefore the most convenient in 

 the present case. 



Man gets a conception of energy from his own experience, 

 from his own muscular sensations. Energy is defined in 

 mechanics as the 'capacity of producing work,' and 'work con- 

 sists in motion against resistance' (Rankine). Let us study 

 first of all some instances of the manifestation of energy, 

 commencing with that most familiar to us, the energy of our 

 muscles. 



Let us imagine two leaden balls, kept in the position C and 

 O^ by means of two steel springs to which they are attached. 

 By overcoming the resistance of the springs, and moving the 

 balls to C 0^', I do work, as it is generally called in mechanics. 

 I do a similar work when, by lifting a weight, I overcome its 

 tendency to fall upon the earth when, so to speak, I tear it away 

 from the earth. The raising of weights is the very simplest 

 illustration of work ; such is the work of a porter, for instance. 

 We know that in that kind of work the greater the weight, 

 and the greater the height through which it has to be raised, 

 the greater the amount of energy which has to be expended. 

 If we raise a unit of weight, i.e. a pound or a kilogramme 

 through a unit of height, i.e. a foot or a metre, we do a unit 



