292 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



so very hard, as never to wear or break in pieces : no 

 ordinary power being able to divide what God himself made 

 One, in the first creation " (Optics, Book III ; Works, 1782, 

 IV. 260. Not included in the first edition of the Optics 

 published in 1704). 



Atoms and molecules. The vague atomistic theory of 

 the Greek philosophers was of little real value until Dalton 

 showed that it could be used to explain the laws which 

 govern chemical combination. The new ideas introduced 

 into the Atomic Theory by Dalton may be summarised as 

 follows : 



(1) The ultimate particles of a pure substance, whether 

 simple or compound, are perfectly alike in size and weight. 



(2) The " simple atoms" of an elementary . substance are 

 indivisible, and ca?i neither be created nor destroyed. 



(3) The "compound atoms" of a chemical compound are 

 formed by the union of two or more elementary atoms. 



(4) Combination between atoms takes place in the simplest 

 integral ratios, e.g. i atom of A with i, 2, or 3 atoms of B. 



In order to avoid the difficulty of describing a group of 

 particles as an indivisible atom, it has become customary to 

 reserve the name ATOM for the simple elementary atoms of 

 which elements and compounds are alike composed. 

 Groups of atoms, whether of the same kind or of different 

 kinds, are distinguished as MOLECULES, i.e. little masses. 



Dalton (1808) describes his atomic theory. Dalton's 

 views as to the nature and methods of combination of the 

 atoms are set out clearly in his " New System of Chemistry," 

 Vol. I., published in 1808 : 



" Whether the ultimate particles of a body, such as water, 

 are all alike, that is, of the same figure, weight, etc., is a 

 question of some importance. From what is known, we 

 have no reason to apprehend a diversity in these particulars : 

 if it does exist in water, it must equally exist in the elements 

 constituting water, namely, hydrogen and oxygen. Now it 



