Makch 1, 1894. 



KNOWLEDGE. 



67 



Let us consider for a moment what chemical changes, 

 and heat changes, take place after the death of a plant. 

 Decay and decomposition, which are mostly processes of 

 oxidation, are accompanied by heating effects. Many 

 such cases can be called to mmd, such as the heating of 

 hay in the stack. On the other hand, the surest evidence 

 of death in animals is the loss of bodily warmth. Thus, 

 the oijposite functions of plants and animals, with regard to 

 heat, are more sharply contrasted m the phenomena of 

 death than of hfe. The contrast must not be pushed too 

 far, since when decay sets in there is a heating effect with 

 animals also, though not perhaps so marked as in the case 

 of plants. 



Warmth and power of locomotion we have chosen as two 

 marked and obvious characteristics of living animals 

 distinguishing them from plants. The power of loco- 

 motion which animals possess depends upon their warmth. 

 The ordinary movements of an animal are made against 

 some kind of resistance, and when motion is so accomplished 

 physical work is performed. Physical work is measured 

 numerically by multiplying the amount of the force by the 

 distance moved against its resistance. All bodily work 

 may be expressed in this way. The systematic and 

 continuous use of the rational and thinking faculties is 

 often called work, but for present purposes we exclude 

 brain labour from the scope of the term " work," which we 

 shall employ in the physical sense only. Plants cannot do 

 work, animals can do work. Combinations of mechanism 

 (pulley, screw, lever, &c.) cannot do work, they only vary 

 the proportion of the two factors of work, distance and 

 force ; but engines, like animals, actually do work, for an 

 engine is provided with a source of energy as well as with 

 a train of mechanism. A steam engine, as far as chemical 

 science and heat science are concerned, is closely analogous 

 to an animal. From this point of view, it is an animal 

 very much simplified. Energy is obtained in the steam 

 engine and in animals by burning carbonaceous material. 

 Both the engine and the animal can do work as long as the 

 same chemical action furnishes them with the same kind 

 of energy. Without fuel or food both grow cold, and no 

 work can be done. 



By the study of the steam engine, HLrn proved that every 

 foot-pound of work done entails the disappearance of a defi- 

 nite quantity of heat between the boiler and the condenser 

 of the engine. The relation is about one thousand three 

 hundred and seventy foot-pounds of work for each unit of 

 heat, as that quantity which will raise one pound of water 

 one degree in temperature. Roughly, Hirn's method was 

 as follows : — He determined the total amoimt of heat put 

 into the boiler from the amount of the water supplied per 

 diem and from the temperature of the steam. The amount 

 of heat returned to the condenser was calculated from 

 similar data. The amount of mechanical work done was 

 measured by a Watt's indicator. In this instrument the 

 amount of work done is indicated by the movement of a 

 pencil point upon a sheet of paper. The position of the 

 pencil point, which indicates the work done, is determined 

 jointly by the pressure or force of the steam, and the distance 

 through which the working piston has moved. The work 

 registered by the indicator is partly the work done in over- 

 coming the resistance of the parts of the engine, which we 

 will call internal work, and partly external work, such as 

 raising a weight or driving a shaft. Him found that, if he 

 increased the external work of his engine, there was a 

 greater loss of heat between the boiler and condenser of 

 the engine, one unit quantity of heat disappearing for 

 each one thousand three hundred and seventy foot-pounds 

 of additional external work. A great variety of methods 

 has given practically the same value for the mechanical 



equivalent of heat, and we may confidently apply the value 

 obtained to the solution of problems upon the relation of 

 food and work in animals. In the animal body there is 

 " internal work " to be done, just as there is in the steam 

 engine. In neither case can the whole of the heat furnished 

 by the fuel or food be converted into external mechanical 

 effect. This would require, in the case of the steam 

 engine, that no heat at all should reach the condenser, and 

 that the condenser should be at the absolute zero of 

 temperature, a condition which it is impossible practically 

 to attain. In the case of an animal, if all the heating 

 power of its food were used for external work the tempera- 

 ture of the animal's body would sink to that of its inanimate 

 surroundings, a condition obviously incompatible with life. 

 As a matter of fact, about one-fifth of the energy which the 

 food develops can be obtained in the form of external 

 mechanical effect. The rest is required for the internal 

 work of the body. This is a much larger proportion of 

 external efl'ect than in the case of a steam engine, 

 or other heat engine, where the efftciency, or proportion 

 of the mechanical equivalent of the heat supplied to 

 the external work done, is more like one-twentieth per 

 cent. 



The heat developed in the body by one pound of the 

 carbo-hydrates (starch, &c.) has a mechanical equivalent 

 of about two thousand eight hundred and sixty foot-tons. 

 One pound of oil or fat is in the same sense equivalent to six 

 thousand four hundred and fifty foot-tons. About twenty 

 per cent, of this is available for external work, so that we 

 may say that if a man is to do three hundred and twelve 

 foot tons of work he may supply himself with the requisite 

 store of energy by taking four ounces of fatty food in 

 addition to the maintenance diet necessary to meet the daily 

 waste of the body. Three hundred and twelve foot-tons 

 is about the average amount of work done in a day by an 

 English labourer, whose dietwould probably comprise about 

 four and a half ounces of fat. People often puzzle them- 

 selves why it is that working makes a man hot rather than 

 cold, seeing that heat is used up to produce mechanical 

 effect. The analogy of the steam engine will enable us 

 more readily to explain this apparent anomaly. When a 

 locomotive is going sixty miles an hour, more work is 

 being done than when it is going thirty miles an hour. 

 More heat is converted into work when the engine is 

 going fast than when it goes comparativelj' slowly ; but no 

 one would expect the heat in the engine as a whole to 

 be less when the pace is great. The draught will be 

 greater and the consumption of coal greater, and so much 

 greater that, although there will be more heat converted 

 into mechanical effect by the working of the piston, yet the 

 engine as a whole will be hotter. It is so also in thexase 

 of the animal body. When bodily work is being done, 

 the contracted {i.e., the working) muscle seizes upon a 

 greater quantity of the oxygen of the arterial blood than 

 in the case of the uncontracted muscle in its state of rest. 

 The blood of the veins may contain as much as seven 

 and a half per cent, of oxygen when the muscles are at 

 rest, but a working muscle may leave as little as one and 

 a half per cent, of oxygen in the venous blood. This means 

 that there must be a corresponding increase in the amount 

 of carbonic acid given off by the lungs, which in fact may 

 be as much as five times greater dm-ing work than during 

 repose. Respiration is deeper and more rapid, more 

 oxygen is inhaled, the combustion of the food goes on 

 more quickly, and an increased supply of carbonaceous 

 food is required to supply the fuel for combustion. Thus 

 it is that the temperature of the body is maintained during 

 work, although the work is done at the expense of food 

 which otherwise would produce animal heat. 



