IDENTICAL CHANGES 



Neither is the amount of heat required to raise 

 a substance from the freezing point of water to 

 the point where the substance is turned to vapor, 

 uniform for every degree of heat. The lighter a 

 gas the more heat is required to raise a given vol- 

 ume, under equal pressure, through one degree. 



Therefore, it is apparent that the Mass of the 

 substance is not all that enters into, and is con- 

 cerned in, the phenomena of heat. And the 

 changes produced in substances by heat cannot be 

 a fundamental change in the Atoms alone, although 

 based upon them, but the changes must involve the 

 organizations of the second degree, and farther on 

 in the scale of organization. Plainly we can per- 

 ceive that Molecules are dismembered by heat, and 

 our ball of butter or of lead will be changed in 

 visible shape even by heat. 



The particles of solids are interlocked and each 

 solid substance is a law to itself when it comes to 

 expansion by heat. 



And liquids possess some degree of cohesion and 

 each identical liquid has some individuality under 

 expansion by heat. 



But all gases and vapors expand at the same 

 uniform rate by heat, and therefore offer the clear- 

 est view of the problem of the nature of heat. 



Let Fig. A represent a metal Cylinder B, with a 

 tightly fitting piston C, whose rod projects through 

 a hole of the cylinder, and with a thermometer 

 connected with the inside of the cylinder. Let 

 the Cylinder be pumped full of any gas or vapor, 

 153 



