CHAPTER I 



MATTER AND ITS STATES 

 I. FUNDAMENTAL THEORIES OF THE NATUEE OF MATTER 



o) The atomic theory. — The whole structure of modern 

 physical science is based upon the atomic theory. This theory 

 supposes every mass of matter to be composed of numerous 

 ultimate, indivisible particles, called atoms, which possess a 

 peculiar attraction for one another. Atoms differ in the 

 amount and nature of this attractive force, those of every 

 chemical element being in this way different from those of 

 every other ; but all atoms of the same element, when under 

 the same conditions, are exactly similar. Owing to their 

 chemical attraction, atoms seldom exist free as such, but are 

 prone to unite into groups, thus causing the neutralization 

 of their mutual attraction. The groups so formed are called 

 molecules. If the atoms composing the molecules of any 

 substance are alike (i. e., of the same element), the element 

 is said to be in the molecular condition — as opposed to the 

 atomic or nascent condition, in which the atoms are not 

 united to one another, but exist free as such. When the 

 atoms forming a molecule are of different chemical elements, 

 a compound is said to be formed. The physical and chemi- 

 cal properties of molecules are very different from those of 

 their component atoms, and they are also very different 

 from those of any molecules which can be formed in any 

 other way. But all molecules which are formed of the same 

 elements and in the same manner are exactly similar under 

 the same conditions. It thus appears that the smallest par- 

 ticle of a compound which can exist and still retain the 

 properties of that compound is the molecule ; break this up, 



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