LAW OF HESS 5 



Corollaries of the First Law. 



The followino- two (Icductions arc of biological interest : 



1. The total energy of a system in a given state is for the system 

 in question a definite characteristic of iJuit state, and it is totally 

 independent of how the system reached that state. 



The energy that would be liberated by the fall of a kiloerani, e.g., of lead, 

 from a height of 3UU metres would be the same no matter how the kilogram 

 was first elevated to the point from which it was dropped. For instance : 



(a) It might be lifted bodily and vertically. 



[h) It might be lifted bodily and up an inclined plane. 



[c) It might be lifted bodily and rapidly. 



[d) It might be lifted bodily and with infinite slowness. 



[e) Or it might be lifted in small pieces, say 1 milligram, at a time. 



(/) It might he lifted as a series of chemical compounds weighing more than 1 

 kilogram as salts of lead and reduced to )netallic lead before dropping. 



(g) Or it might be dug out from the top of a hill, 300 metres high, and 

 transported horizontally by aeroplane. 



The essential conditions are that it weighs 1 kilo, and that it falls 300 metres, 

 cosmic influences being constant. 



Similarly, glucose has the same energy-content, no matter how 

 it has been prepared, provided the measurement is carried out 

 under similar conditions, e.g. glucose may be synthesised from 

 simple substances ; it may be prepared by the natural or artificial 

 hydrolysis of more complex carbohydrates, or it may be derived 

 from such substances as proteins, fats, etc. 



2. When a system changes from one state to another, the 

 alteration of total energy which accompanies that change is 

 altogether independent of the process by which the change is 

 brought about. 



Law of Hess. — This second corollary w^as enunciated by Hess in 

 1840 as the result of experiments. He worded it, " The amount of 

 heat generated by a chemical reaction is the same whether it takes 

 place all at once or in steps." 



The examples given above, if reversed, will act as examples 

 of this corollary. The rate and angle of fall are not determining 

 factors in the liberation of energy, nor does the way in which 

 the energy of glucose is liberated have any effect on the total 

 amount set free. 



These two corollaries, as we shall see later, make it possible to 

 construct a balance sheet of the energy intake and output of the 

 organism. 



Degraded Energy (Second Law of Thermodynamics), 



When a substance or group of substances is changed into a 

 substance or group of substances with a smaller energy-content, 



