THE PATH OF EVOLUTION 



elements. When combined with one another they 

 form molecules which, with the exception of the 

 gases, are grouped into coherent bodies having an open 

 or cellular structure, by which the molecule, though 

 retaining its position in the mass, can and does vi- 

 brate throughout its substance. This structure is 

 evident from the greater or lesser elasticity of all 

 bodies, and by the action of heat, which increases the 

 rate of vibration and the distance between the mo- 

 lecules, and expands the body. This structure is 

 called a solid body. 



When a solid is heated to a certain temperature, 

 that varies with its chemical composition, the vibra- 

 tions increase so greatly as to overcome the cohesion 

 of the molecules to each other (if their chemical com- 

 binations are not altered at that temperature), so that 

 the molecules are free to move around and about 

 each other, though the vacuities between them are not 

 decreased, but rather are increased in size and num- 

 ber; in other words, the matter melts or becomes a 

 fluid. 



If the temperature is raised still higher, the mo- 

 lecules part entirely from each other ; not only cease, 

 absolutely, to cohere to each other, but seem to be 

 mutually repellent, and fly away from each other ; in 

 other words, they boil and become vapors, or, if away 

 from contact with the engendering fluid, and not sur- 



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