Xlll INTRODUCTION. 



currents (against considerable resistance), and also as molar movement, as in the 

 attraction or repulsion of the magnetic needle. Electrical phenomena are manifested 

 in our bodies by muscle, nerve, and glands, but these phenomena are relatively small 

 in amount when compared with the other forms of energy. It is not improbable 

 that the electrical phenomena of our bodies become almost completely transformed 

 into heat. As yet experiment has not determined with accuracy a " unit of 

 electricity " directly comparable with the " heat-unit " and the " work-unit. " 



It is quite certain that within the organism one form of energy can be trans- 

 formed into another form, and that a certain amount of one form will yield a 

 definite amount of another form ; further, that new energy never arises spontane- 

 ously, nor is energy already present ever destroyed, so that in the organism the 

 law of the conservation of energy is continually in action. 



ANIMALS AND PLANTS. The animal body contains a quantity of chemically- 

 potential energy stored up in its constituents. The total amount of the energy 

 present in the human body might be measured by burning completely an entire 

 human body in a calorimeter, and thereby determining how many heat-units are 

 produced when it is reduced to ashes (see Animal Heat). 



The chemical compounds containing the potential energy are characterised by the 

 complicated relative position of their atoms, by a comparatively imperfect saturation 

 of the affinities of their atoms, by the relatively small amount of oxygen which 

 they contain, by their great tendency to decomposition, and the facility with which 

 they undergo decomposition. 



If a man were not supplied with food he would lose 50 grammes of his body- 

 weight every hour ; the material part of his body, which contains the potential 

 energy, is used up, oxygen is absorbed, and a continual process of combustion takes 

 place ; by the process of combustion simpler substances are formed from the more 

 complex compounds, whereby potential is converted into kinetic energy. It is im- 

 material whether the combustion is rapid or slow ; the same amount of the same 

 chemical substances always produces the same amount of kinetic energy, i.e., of 

 heat. 



A person, when fasting, experiences after a certain time the disagreeable feeling 

 of exhaustion of his reserve of potential energy, hunger sets in, and he takes food. 

 All food for the animal kingdom is obtained, either directly or indirectly, from the 

 vegetable kingdom. Even carnivora, which eat the flesh of other animals, only eat 

 organised matter which has been formed from vegetable food. The existence of 

 the animal kingdom presupposes the existence of the vegetable kingdom. 



All substances, therefore, necessary for the food of animals occur in vegetables. 

 Besides water and the inorganic constituents, plants contain, amongst other 

 organic compounds, the following three chief representatives of food-stufsfate, 

 carbohydrates, and proteids. 



All these contain stores of potential energy, in virtue of their complex chemical 

 constitution. 



The fats contain^l CnH,0(OH) = fatty acids ^ 



1 +C 3 H 5 (OH) 3 = glycerin J V8 ; 

 The carbohydrates contain i C c H 10 O 5 . . ( 252). 



