40 4 NINETEENTH CENTUAY. FT. III. 



these numbers are called the atomic weights of hydrogen, 

 oxygen, and sodium. 



This will give you a rough idea of Dalton's theory of 

 atoms. There is always this difficulty in it that we cannot be 

 quite sure when we have arrived at the smallest quantity of 

 any substance ; for suppose that one day we were to find 

 that half as much oxygen would unite with some other sub- 

 stance as now unites with sodium, then the atom of oxygen 

 would no longer weigh 1 6, and for this and other reasons 

 atoms are used by chemists merely as convenient units. 

 But if we bear this possibility in mind, then the theory is of 

 great use in giving us the symbols which are now used in 

 chemical language. For when it was once agreed that the 

 weight of an atom of hydrogen should be reckoned as i, 

 then an atom of oxygen will weigh 16, and the letters HHO 

 express a great deal. They tell us that two atoms of hydro- 

 gen weighing 2 are joined to one atom of oxygen weighing 

 1 6, to form a molecule of water. In the same way HO,NA l 

 tells us that single atoms of each of these substances, weigh- 

 ing respectively i, 16, 23, form a molecule of soda. And 

 thus a complete chemical language has sprung up, by which 

 chemists in all parts of the world can understand at once 

 what is the composition of any substance ; and by means of 

 these simple letters the most complicated chemical problems 

 can be worked out clearly and intelligibly. 



Dalton's theory was received very quickly by chemists, 

 considering how entirely new the ideas were which it taught. 

 His friend Dr. Thomson, an eminent chemist (born 1773, 

 died 1852), gave a very clear account of it in his ' System of 

 Chemistry,' and brought it under the notice of Davy and Fara- 

 day ; and a great French chemist, Gay-Lussac (born 1778, 



1 Na stands for Natrium, the Latin name for soda, now used for 

 the metal sodium. 



