fc) 



MEASUREMENT OE ENERGY 3 



gently opposed. We are told that the machine — the scientists' 

 imitation of life — is not merely a purely inanimate mechanism. 

 In its cunning combination of valves and regulators it has a 

 brain, part of the brain of its designer. The partial likeness is 

 that of the machine to the man, of the limited imitation to the 

 original, not the other way about, which is true enough. But 

 let us bear in mind one essential and undeniable fact. Machine 

 or man, inanimate mechanism with the mechanical imitation of 

 a brain, or brain controlling an animate mechanism, what of the 

 power ? The power to live, the power to do work is not in the 

 brain nor in the body, not in the valves nor the moving parts. 

 The power, whether of life or of mechanism, is external. This is 

 the real ground of analogy " (Soddy). We must determine the 

 source of this energy, study its laws, see how it is made available 

 for living matter, and then see how it operates in living matter. 



Energy is the underlying cause of all changes in matter. This 

 does not seem a very satisfactory definition, but, so far, it is the 

 only one possible. It is a very striking fact that the two funda- 

 mentals of our external world, matter and energy, have for us 

 no existence apart from their effect on us. We cannot prove 

 that there are such things except in so far as they manifest them- 

 selves, matter by being changed, and energy by producing changes, 

 which in turn alter our sensation-complex. Energy, then, is that 

 which produces an effect on our senses. Our sense organs, as we 

 shall see later (Chap. XIX.), are stimulated by changes of energy in 

 their environment. We see, because radiant energy of a certain 

 frequency falls on our retinae ; we hear when the hair cells on the 

 basilar membrane are stimulated as the result of air waves of 

 definite frequencies transmitted to them via tympanic membrane 

 or via bone ; our senses of taste and smell depend on changes of 

 chemical energy, while changes in kinetic energy produce the 

 sensations of heat, cold, tickle, touch, pressure, position, etc. 



Energy is measured by its power to do work. The kind of unit 

 used will therefore depend on the nature of the work done. In 

 the C.G.S. system the unit of work is the erg — that amount of 

 work which is done when unit force is overcome through unit 

 distance. The unit of force is the dyne, and is defined as the force 

 Avhich acting on a gram for a second would gi\e it the velocity of 

 one centimetre per second. An erg is, therefore, a dyne-centimetre. 

 If the power developed is electrical it would be measured in watts, 

 if mechanical, u\ foot-pounds per second, or in horsepower, and so on. 

 These various units may be converted into one another (Table I.). 

 For our piu'pose it is most convenient to measure energy in heat 

 units or calories. 



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