AND OF THE SECOND LAW 



PART I 



DEFINITIONS, GENERAL PRELIMINARIES, DEVELOPMENT, CURRENT 

 AND PRECISE STATEMENTS OF THE MATTERS CONSIDERED 



(i) The " State " of a Body and its " Change of State " 



As we will make constant use of the terms contained in this 

 heading and as they here represent fundamentally important 

 conceptions, we will seek to make them clear by presenting them 

 in the various forms into which they have been cast by the different 

 investigators, even at the risk of being considered prolix. 



In the Introduction to this article we called attention to the 

 two distinct modes of attacking any physical problem. Now the 

 conception "state of a body" varies with the chosen mode of 

 attack. Of course as both modes are legitimate and lead to 

 correct results, these differences in the conception of "state" 

 can be reconciled and a broader definition reached. We can 

 illustrate these different methods of approach, as PLANCK has 

 done, by assuming two different observers of the state of the body, 

 one called the microscopic-observer and the other the macro- 

 scopic-observer. The former possesses senses so acute and 

 powers so great that he can recognize each individual atom and 

 can measure its motion. For this observer each atom will move 

 exactly according to the elementary laws prescribed for it by 

 General Dynamics. These laws, so far as we know them, also 

 at once permit of exactly the opposite course of each event.' Con- 

 sequently there can be here no question of probability, of entropy 

 or of its growth. On the other hand, the "macro-observer," (who 

 perceives the atomic host, say as a homogeneous gas, and conse- 



