46 THE MECHANISM OF LIFE 



isolated system. So we might imagine an apparatus, or experi- 

 mental plant, containing its own source of energy and absorbing 

 itself all the energy given off, or wasted, or dissipated. That would 

 be an isolated, energetic system. It is true that such an isolated 

 physical system is a fiction, for there must always be some 

 interchange of energy between any apparatus that we can devise 

 and work and its surroundings; the only really isolated physical 

 system is the entire universe. But let it be possible to take 

 account of the energy lost by the system to without, or received 

 by it from without; then we approximate to our concept of 

 physical isolation. 



Making all allowances, then, for experimental error, friction, 

 loss of heat by radiation, convection, and conduction, imperfect 

 rigidity and elasticity of materials, etc., we can, very approxi- 

 mately at least, trace the quantity of available energy that 

 becomes unavailable, or is dissipated. Thus we arrive at our 

 fundamental principle of energy: 



In all the transformations undergone by an isolated system 

 the total quantity of energy contained is neither increased 

 nor diminished . . . . . . . . . . (6) 



This is the law of conservation of energy, and it means that 

 although the capacity for doing work that is, available energy 

 diminishes, energy in the abstract does not diminish. When 

 available energy diminishes, unavailable energy always appears in 

 its place. Very often we can prove this. If we cannot do so we 

 assume the law, and our assumption is always justified by other 

 experimental results. But we are compelled, anyhow, to make 

 the assumption, because the law of conservation is an a priori 

 mode of our thought. To this matter we return later. 



We may now state the principal result of our work in the 

 form of a quotation: " In all the transformations of a material 

 system considered in this book, there is a certain entity which 

 (1) remains constant in quantity, and (2) is capable, under 

 certain conditions, of assuming the forms of kinetic and potential 

 energy, which are dealt with under the study of Rational Dyna- 

 mics. This Entity is Energy."* 



Several points arising from this definition must now be dis- 

 cussed: first of all the nature of " Rational Dynamics." In this 



* G. H. Bryan, Thermodynamics, Teubner's Lehrbuchen der Mathe- 

 matischen Wissenschaften, xxi., Leipzig, 1907. 



