2 ATOMS AND MOLECULES OF MATTER. PARTI, 



of homogeneous simple substances; molecules, or groups 

 of heterogeneous atoms united in definite proportions, 

 constitute such as are compound. High pressure steam 

 is invisible as it issues from the boiler, yet each of its 

 molecules contains two atoms of hydrogen and one 

 of oxygen. The perfume of a flower is a compound 

 invisible substance formed of molecules. 



We know nothing of the forms either of atoms or of 

 those groups of atoms which we call molecules ; but we 

 cannot suppose them otherwise than as excessively hard, 

 since conceive them how we will, we are sure that an 

 atom, whatever be its form or nature, is ever the same. 

 It never wears, it never changes, though it may have 

 formed part of thousands of bodies and entered into 

 thousands of combinations, organic and inorganic ; 

 when set free by their dissolution, it is ready to enter 

 into a new series ; it is indestructible even by fire, the 

 same now as when created. Nor has the quantity of 

 matter in our terrestrial abode ever been increased or 

 diminished; liable to perpetual change of place and 

 combination, the amount remains the same : the bed of 

 the seas may be changed to dry land, and the ocean 

 may again cover the lofty mountains, but the absolute 

 quantity of matter changes not. 



All substances, whether solid, liquid, or aeriform, are 

 supposed to consist of hard separate atoms or particles, 

 and in conformity with that supposition to be surrounded 

 by the ethereal medium, otherwise they could not trans- 

 mit light and heat, which are merely vibrations of 

 that medium. Even the hardest and most compact sub- 

 stances are capable of compression, and have been com- 

 pressed to an enormous degree by the hydraulic press ; 

 but it probably transcends mechanical force to bring 

 their atoms into contact : in fact, no known substance 

 is impervious to l>otJi light and heat, however thin. 



By far the greater number of terrestrial substances 

 consist of heterogeneous atoms chemically combined 



