THE CONQUEST OF NATURE 



Even in the aggregate the molecules make up a mere 

 intangible gas, which requires to be closely confined 

 in order that its energy may be made available. Once 

 the molecules have performed their work, they are so 

 changed in their activities that they sink back, as it 

 were, exhausted, into a relatively quiescent state, which 

 enables their latent cohesive forces to reduce them again 

 to the state of a liquid. In a word, we are concerned 

 with the manifestation of energy which depends upon 

 molecular activities in a way quite different from what 

 has been the case with any of the previously considered 

 mechanisms. The tangible manifestation of energy 

 which we term heat is not merely a condition of action 

 and a by-product, as it was in the case of the animal 

 machine; it is the essential factor upon which all the 

 efficiency of the mechanism depends. 



It should perhaps be stated that this explanation of 

 the action of the steam engine is a comparatively modern 

 scientific interpretation. The earlier experimenters 

 brought the steam engine to a high state of efficiency, 

 without having any such conception as this of the nature 

 of steam itself. For practical purposes it suffices to note 

 that water when heated takes the form of steam; that 

 this steam has the property of powerful and indefinite 

 expansion; and thirdly, that when allowed to escape 

 from a state of pressure, sudden expansion of the steam 

 cools it sufficiently to cause the recondensation of part 

 of its substance, thus creating a vacuum. 



Stated in few words, the entire action of the steam 

 depends upon these simple mechanical principles. The 

 principles are practically applied by permitting the 



[so] 



