A HISTORY OF SCIENCE 



and constantly in mifid. To understand the theory, 

 one must think of all matter as composed of minute 

 isolated particles or molecules, which are always in 

 motion vibrating, if you will. He must mentally 

 magnify and visualize these particles till he sees them 

 quivering before him, like tuning-forks held in the 

 hand. Remember, then, that, like the tuning-fork, 

 each molecule would, if left to itself, quiver less and 

 less violently, until it ran down altogether, but that the 

 motion thus lessening is not really lost. It is sent out 

 in the form of ether waves, which can set up like mo- 

 tion in any other particles which they reach, be they 

 near or remote ; or it is transmitted as a direct push 

 a kick, if you will to any other particle with which 

 the molecule comes in physical contact. 



But note now, further, that our molecule, while in- 

 cessantly giving out its energy of motion in ether 

 waves and in direct pushes, is at the same time just 

 as ceaslessly receiving motion from the ether waves 

 made by other atoms, and by the return push of 

 the molecules against which it pushes. In a word, 

 then, every molecule of matter is at once a centre for 

 the distribution of motion (sending out impulses 

 which affect, sooner or later, every other atom of mat- 

 ter in the universe), and, from the other point of view, 

 also a centre for the reception of motion from every 

 direction and from every other particle of matter in 

 the universe. Whether any given molecule will on 

 the whole gain motion or lose it depends clearly on 

 the simple mechanical principles of give and take. 



From equally familiar mechanical principles, it is 

 clear that our vibrating molecule, in virtue of its vi- 



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