124 INTRODUCTION TO IMMUNOCHEMICAL SPECIFICITY 



Energy 



We shall need to discuss only the first two laws of thermody- 

 namics. The first law is well known and today needs only to be 

 stated to be believed. It is simply that energy can neither be created 

 nor be destroyed. It is understood that we are not thinking of changes 

 involving changes in atomic nuclei ; if we were, we should have to 

 formulate the law more broadly. 



From the first law of thermodynamics it follows that no perpetual 

 motion machine of the "first type," i.e., one getting all or part of its 

 energy from nowhere, can ever be constructed. The total energy of a 

 completely isolated system, therefore, remains constant (If the sys- 

 tem is not isolated its total energy may change from time to time.) 

 We designate this total energy, which may be made up of heat ( which 

 Count Rumford proved to be a form of energy ) or of mechanical 

 or chemical energy and at times of other forms, as E. The science 

 of thermodynamics grew out of a study of the process by which heat 

 may be converted by suitable machines partly into work. If we let 

 Q stand for the heat content of the system and W for the work done, 

 we may write the simple equation 



AE = ^Q - AW (2) 



which states that the increase in the total energy of the system, 

 a£, equals the heat taken up, AQ, minus the work done, AlV. This 

 is a statement of the first law of thermodynamics in symbols. 



If we consider an extremely small change in the system and ignore 

 certain questions of mathematical rigor, we may replace the finite 

 dift'erences AE, AQ, and AlV by the differentials dE, dQ, and dW, 

 and write 



dE = dQ - dW (3) 



The meaning of this equation is not as obvious as the beginner 

 might think. It looks as if the equation means that, if you measure 

 the infinitesimal increase in the total energy of a system, you can 

 show experimentally that it equals the experimentally determined in- 

 finitesimal absorption of heat minus the experimentally determined 

 amount of work done. But this is not the meaning at all, for we have 

 no "energy meter" with which we can measure the total energy of 

 a system, or even the change in total etiergy. The onl}' way we have 



