PRINCIPLES OF SCIENCE, W 



because geometry defines many of the assump- 

 tions and suppositions of the science of optics, 

 as well as of other sciences, that analogy be- 

 tween them may often be admitted. 



Analogy may be admitted throughout the 

 various species of common matter which exist, 

 whose nature and properties are the same, how- 

 ever different in appearance they may be ; be- 

 tween the capacity of a lump of clay, and of a 

 piece of flint; between the flexibility of lead 

 and of iron ; between the elasticity of steel and 

 of whalebone ; and between the expansibility 

 of gases in general, however different in other 

 attributes. The reason why analogy between 

 these different genera is admissible is, that 

 however different from each other their parti- 

 cular, or secondary properties may be, they, 

 nevertheless, always continue to retain the 

 same generic character; that, although the 

 chemical characters of particular gases are 

 proved to be totally different from others, all 

 however are expansible. The same may be 

 said of elastic, and of flexible bodies. 



To bodies such as these, Sir ISAAC NEWTON'S 

 second rule of philosophising may apply, that 

 " of natural effects of the same kind, the same 

 causes are to be assigned." It must, however, 

 be very obvious, that this rule can never ap- 

 ply to bodies whose nature and properties are 

 essentially different from each other, and be- 



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