32 / CHEMISTRY. 



column, in the order of their affinity for any one substance at the 

 head of the column, as in the following example ; 



Sulphuric Acid. 



Baryta, 



Strontia, 



Potash, 



Lime, 



Magnesia, 



Ammonia. 



The atomic theory. — The doctrine of atomic weights and chemical 

 equivalents is based upon the supposition that every substance is 

 divisible into ultimate particles termed atoms^ which atoms unite 

 together in certain definite proportions to form various compound 

 bodies. 



The chemical equivalent of a body is the number expressing its 

 least combining proportional ; as these merely express the ratio, 

 any one body may be selected as a standard ; either oxygen or 

 hydrogen are generally employed, — more frequently hydrogen, — 

 which being placed at unity, the equivalent number or chemical 

 equivalent of oxygen would be 8 ; since eight atoms of oxygen 

 always unite with one of hydrogen. In the same way the equivalent 

 of chlorine has been fixed at 36 ; that of nitrogen at 14; that of 

 iron at 2S ; and so on. An atom of a compound body will of 

 course be compound, as an atom of water, or of sulphuric acid ; so 

 also, the chemical equivalent of a compound body is the sum of the 

 equivalents of its constituents ; thus the equivalent of water is 9, 

 (8-j-l ;) that of sulphuric acid 40, (16+24 ;) and so on. 



SECTION I. 

 SIMPLE NON-METAI/LIC BODIES. 



OXYGEN. 



Exists abundantly in nature, constituting one-fifth of the atmo- 

 sphere, and eight parts in nine of water ; also, in all organic matter. 



Discovered in 1771 by Priestley, and by Scheele in 1775; named 

 dephhgisticated air by Priestley, empyreal air by Scheele, and vital 

 air by Condorcet ; — the name oxygen^ derived from two Greek words 

 signifying to generate an acid^ was given by Lavoisier, under the 

 idea that it was the sole acidifying principle. Prepared by heating 

 either the peroxides of mercury, lead, or manganese, or the nitrate 

 or chlorate of potassa. When either of the peroxides is exposed to 

 heat, a portion of their oxygen is driven off, and they are converted 

 into either protoxides or sesquioxides. Perhaps the best mode of 

 procuring it is to heat the chlorate of potassa, which yields a large 



