648 SCIENCE PROGRESS 



The second thing to notice is that the equations show that 

 chemical reactions are directed changes. In all exothermic 

 reactions, that is, in the vast majority, heat is evolved. If the 

 reaction takes place of itself, that is, apart from intelligent 

 ordering of the conditions under which it occurs, this heat is 

 dissipated : it is conducted away, or radiated, and it raises the 

 temperature of its physical surroundings to an infinitesimal 

 degree, so that it can no longer be recovered or made use of to 

 produce further transformations. In all physical or chemical 

 changes of this category the heat so produced becomes un- 

 available; and the products of the changes, if they are chemical 

 changes, possess less intrinsic energy than did the original 

 substances. Thinking about chemical changes in general we 

 see, then, that each one that takes place exothermically reduces 

 the probability of the occurrence of further exothermic changes, 

 since in it some part of the available energy of the universe has 

 become unavailable. Every such chemical reaction tends 

 towards stability, since the products are less likely to react 

 with each other, or with other substances, than were the 

 original substances. 



The opposite tendency is exhibited by endothermic changes, 

 for in such the products of the reaction possess a greater 

 quantity of intrinsic energy than did the original substances. 

 The reaction tends towards instability, for the substance which 

 is formed endothermically is more likely to react (it is often 

 explosive) than were the substances from which it was formed. 

 In the preparation of an endothermic substance some heat is 

 certainly dissipated, since we cannot avoid the loss of heat by 

 conduction and radiation, and we cannot, as a rule, control 

 perfectly the progress of the reaction. But the general result 

 of the change is that energy remains in the available form, and 

 can be made use of in the production of other energy-trans- 

 formations. 



We may now generalise these statements. All physico- 

 chemical changes whatever, organic or inorganic, exothermic or 

 endothermic, are said to conform to the two laws of thermo- 

 dynamics. The first law states that the energy of an isolated 

 system is constant, that is, by no conceivable change occurring 

 within the system can the sum of potential and kinetic energy 

 contained in it be diminished or augmented. If we couple the 

 law of conservation of energy with the law of conservation of 



