CHEMICAL DIVISIBILITY. 39 



matter which follows the action of true chemical attraction ; we 

 might define affinity to be a force by which elements unite and 

 new substances are generated. 



Atoms. Molecules, as stated heretofore, are the smallest parti- 

 cles of matter which can exist. All matter consists of mole- 

 cules, consequently the red oxide of mercury must also consist 

 of molecules. 



By heating the oxide of mercury, oxygen and mercury are 

 obtained, each of which also must consist of molecules. As 

 the oxide of mercury consists of molecules, and as these mole- 

 cules are neither pure oxygen nor pure mercury, we must come 

 to the conclusion that a molecule of the oxide of mercury is 

 composed of a small particle of oxygen and a small particle 

 of mercury. We consequently learn that a molecule of a com- 

 pound substance is composed of yet smaller particles of ele- 

 ments, and these smallest particles of elements capable of entering into 

 combination are called atoms, whilst molecules are the smallest particles 

 of matter which are capable of existing in a free state. 



Chemistry is the science of affinity, and affinity is the attraction 

 acting between atoms and causing them to unite and torm 

 molecules. As every chemical change is due to the motion of 

 atoms, chemistry may also be defined as the science of the motion 

 of atoms taking place in consequence of chemical affinity. 



Atomic weight. All matter possesses weight ; this is true of a 

 mass as well as any part of it, and must consequently be also 

 true of the atoms and molecules of which matter consists. It 

 is, of course, impossible to weigh a single atom or a single 

 molecule, yet science has formed an opinion in regard to the 

 relative weights of these minute particles. The experiment re- 

 ferred to above may be so conducted as to ascertain the weight 

 of the products of decomposition (viz., the oxygen and the 

 mercury) of a given, previously weighed quantity of oxide of 

 mercury. In doing this, it will be invariably found that every 

 18.5 parts by weight of the oxide of mercury yield upon heating 

 12.5 parts by weight of mercury and 1 part of oxygen, that we 

 have consequently in 13.5 pounds of oxide 12.5 pounds of mer- 

 cury and 1 pound of oxygen. 



If we assume that a molecule of the oxide is composed of one 



