IDEAS OF PHYSICAL CHEMISTRY 19 



the two reacting substances will combine more frequently, 

 the greater their mutual affinities. This relation of chemical 

 change to time is one of the most important aspects of modern 

 physical chemistry. 1 



In recent years it has become possible to produce an in- 

 tense cold, and it has been observed in the laboratories of Pro- 

 fessor Dewar and others, that as the temperature decreases, 

 the speed of reaction also decreases. Furthermore, it has long 

 been known that matter usually contracts, that is to say, 

 occupies less space, as it grows colder. At a temperature 

 minus 273 centigrade, all heat energy ceases, all chemical 

 reactions become exceedingly slow, and all volume approaches 

 as near as possible to nothing without the actual disappear- 

 ance of substance. It is fair to assume that at such an abso- 

 lute zero various other properties by means of which we dif- 

 ferentiate substances would disappear, and this, too, is veri- 

 fied by recent observations. Nevertheless, all substances 

 by no means become identical at this point. Because heat is 

 a form of energy, and cold is only the absence of that form 

 of energy, we have here only another aspect of the ever-re- 

 curring effect of the changing relation of matter to energy 

 in producing the phenomena of our actual world. 



In summing up the outcome of our work in this lecture, 

 it is clear that we have built a sort of platform, floating upon 

 a sea of uncertainty, but of sufficient strength and stability 

 to permit the erection of a more solid edifice of physico-chemi- 

 cal theory. Let us cast another glance at the planks. We have 

 the four fundamental ideas, matter, energy, space and time 

 - which occur in everything superposed upon one another 

 but not in a constant quantitative ratio; these abstract es- 



1 An experiment demonstrating the effect of concentration and temperature 

 upon the speed of reaction is given at the end of the Appendix to this volume. 

 [Editor.] 



