CHEMISTRY OF SOIL. 45 



some one as unity. In these pages, hydrogen is considered 

 as 1, or unity. As the atoms may be thus expressed by 

 numbers, it is customary, in referring to chemical compounds, 

 to speak only of the number of atoms, in which each ele- 

 ment enters into their composition. The modern system of 

 chemical notation, substitutes for the name of the elements 

 its first, or two first letters, and writes after it the number 

 of atoms, existing in any compound, as the powers of roots 

 are expressed arithmetically by exponents. Where only 

 single atoms combine, their exponents are omitted. Thus, 

 H is hydrogen, O is oxygen ; then water is H O, that is one 

 atom each of hydrogen and oxygen. C is carbon, O 

 oxygen ; then C O'^ is carbonic acid, that is, 1 carbon and 

 two of oxygen. The combining number of carbon is 6, and 

 of oxygen 8, then 1 carbon = 6, and 2 oxygen (8 x 2) = 16. 

 Then the atomic number of carbonic acid is 22, (6 -f 16 = 22. 

 One little conversant with chemistry is apt to confound the 

 combining number with the number of atoms, especially 

 when the first is called " atomic number." A distinction is 

 to be here remembered, the atomic number is one thing, the 

 number of atoms another. When it is said, that water is 

 composed of 8 parts of oxygen to 1 part of hydrogen, by 

 weight, an ultimate fact only is expressed. When it is said 

 that water is composed of an atom of oxygen united to an 

 atom of hydrogen, we express a theoretical opinion. The 

 difficulty lies, in understanding how water can be both a 

 combination of 1 to 1, and of 1 to 8 ; that 9 of water can 

 yet be only composed of 1 to 1. This discrepancy vanishes, 

 where the distinction is remembered, between the combining 

 or atomic number, and the number of atoms. Water is an 

 example, where single atoms are united. But cases contin- 

 ually occur where the combining number of one body unites 

 to more than one combining proportion of another. In this 



