40 PRINCIPLES OF CHEMISTRY. 



consists of molecules, and as these molecules 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 compound substance is composed of yet smaller particles of ele- 

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

 combination are called atoms, while molecules are the smallest particles 

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



Having now established the difference between atoms and mole- 

 cules, we may give a better definition of elements and compounds by 

 saying that an elementary substance is one in which the atoms com- 

 posing its molecules are alike, while in a compgiuid__substance the 

 molecules contain atoms of different kmds. 



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

 acting between atoms and causing them to unite and form 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 ajjinity. Also, we may say that chemistry 

 is that branch of science which treats of the composition of substances, 

 the changes in their composition, and the laws governing such changes. 



The scheme below may help to illustrate the relation of chemistry 

 to some other branches of physical science : 



GENERAL FORCE OF ATTRACTION, 

 acting between 



Heavenly bodies Surfaces. Molecules. Atoms, 



or masses. 



is termed : 



Gravitation. Surface-action. Cohesion, Chemical affinity. 



Adhesion. 

 Capillary attrac- 

 tion, etc, 



The phenomena caused by these respective actions are considered 



by: 



Astronomy or Physics. Physics. 



Mechanics. Crystallography. 



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

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

 atoms also and of the molecules of which matter consists. It is, of 

 course, impossible to weigh a single atom or a single molecule, yet 



