THE SOURCES OF ENERGY 69 



These contain a chemical substance called haemoglobin, 

 which carries the oxygen, while the carbonic acid, is carried by 

 the clear liquid part of the blood, or plasma. In all animals 

 there are other corpuscles which are colourless, and some of these 

 are called phagocytes (devouring cells) because they have the 

 power of ingesting foreign substances, such as bacteria, which 

 may enter the blood. The pus which forms when an abscess 

 gathers consists largely of phagocytes. They are a protection 

 against infection. 



We have now seen how the nutritive materials — the proteids, 

 fats, and carbohydrates — and the oxygen which is to " burn " 

 them, are obtained, and how they are carried to the tissues. 



The Sources o£ Energy. — These proteids, fats, and carbo- 

 hydrates, digested, dissolved, split up, and recombined, are the 

 immediate sources of energy (the ultimate source is, of course, 

 the solar radiation, which enables the green plants to synthesise 

 water, carbonic acid, and nitrate into proteid, fat, and carbo- 

 hydrate). They contain potential, chemical energy which is at 

 a high intensity, and is available for doing work. 



How ? In the inanimate engine there is a working substance, 

 the steam, and we regard this as something different from the 

 engine, which may exist even if there is no steam in it. But in 

 the animate engine the mechanism (muscle and nerve) are the 

 same as the working substance (the proteids, fats, and carbo- 

 hydrates, which are the muscle and nerve). It may be the case 

 that a " living " muscle contains " non-living " substances which 

 are oxidised and yield energy, but since we only define the " life " 

 of the muscle by its irritability (that is, its ability to contract 

 when it is stimulated), we cannot be sure that there are parts of 

 it which are not alive. It is probable that the substances 

 carried to the muscle by the blood-stream are actually built up 

 into the living tissue — they become alive — and are then oxidised, 

 or die. 



Further, the inanimate engine — steam, gas, or petrol — is a 

 thermodynamic machine. It contains a working substance — 

 steam at a temperature, say, 120° C, or a mixture of gases 

 resulting from the explosion of gas and air, or petrol vapour and 

 air at a temperature of, say, 700° C. to over 1,000° C. At these 

 high temperatures the molecules of the gases are moving with 

 enormous velocities, and so they exert pressure. Therefore a 

 heat engine is a mechanism which converts the kinetic energy of 



