THERMO-DYNAMICS. 



39 



these two lines which I have marked at 10 C. and 20 C.; 

 that is to say, suppose a substance has a pressure of six 

 atmospheres, or any other unit of pressure, and then that 

 this volume is 2J cubit feet, then the condition of the 

 substance at that time will be represented by the point A. 

 Then let the pressure gradually be diminished until it 

 reaches 4 J cubic feet, then it will come to this point B, and 



the line of that curve will represent the condition of the 

 substance there, and its volume will be represented by the 

 perpendicular distance of the point from the line of pressure ; 

 that is to say, about 3j cubic fe"et. If, on the other hand, 

 we employ a temperature of 10 the state of the same 

 substance will be indicated by this curve in exactly the 



