CHEMISTRY OF SOIL. 47 



1st. Bodies combine only in definite proportion. 

 2d. " " " multiple proportion. 



3d. *' " " equivalent proportion. 



These are the laws of chemical combination. The atomic 

 theory attempts to, and does account for them. Once admit 

 the principle that bodies combine only by indivisible atoms, 

 these laws follow as consequences. If bodies only unite by 

 atoms, atom to atom, their composition must be definite. If 

 a body unites an atom to two or more of another, then as 

 atoms are indivisible, the second, or other added portion, 

 must be a multiple of the first, by a whole number. When 

 bodies unite in proportions which imply half atoms, it is 

 because union has oc<;urred between two atoms of one, and 

 three atoms of another, as iron may unite with oxygen so as 

 to be seemingly a compound of 1 iron to 1 J oxygen. Truly 

 this is a compound of 2 iron to 3 oxygen. Alumina always 

 oxidates itself in this proportion, but it will simplify our 

 views to consider it as uniting atom to atom. Again, if 

 bodies unite only by atoms, the atom of one may be 

 replaced by that of another; or, which is the same thing, 

 the combining proportion of one may replace the combining 

 proportion of another, for they are equivalent to each other. 

 One body may be thus successively united to others, in 

 doses which represent their atomic weights. 



56. Calculating on this fixed principle, that the combining 

 weight of any substance is the quantity necessary to unite 

 with 8 of oxygen, it is found, that the proportions in which 

 the bases of silicates combine, are 



8 oxygen, 8 silicon =16 silica, 



8 " 10 aluminum == 18 alumina, 

 8 " 20 calcium = 28 lime, 



8 " 12 magnesium = 20 magnesia, 

 8 " 40 potassium = 48 potash, 



