78 CHEMICAL PROPERTIES OF MINERALS, 
dral, and among them there is a difference of less than two de-. 
grees in the angle of the rhombohedron. Besides a carbonate 
of calcium, a carbonate of magnesium, and a carbonate of iron, — 
there is also a carbonate of calcium and magnesium, in which 
half of the calcium of the first of these carbonates is replaced 
by half an atom of magnesium; and another species in which 
the base, instead of being all magnesium, is half magnesium and 
half iron. By half is here meaut half in the proportion of their 
combining weights. 
The replacement of one of these elements by the other, and 
similar replacements among other groups of related elements, 
run through the whole range of mineral compounds. Thus we 
have sodium replacing potasstwm, arsenic replacing phosphorus 
and antimony, and so on. 
In the combinations of oxygen and iron, as illustrated above, 
oxygen is combined with the iron in different proportions. 
FeO contains | of Fe (iron) to 1 of O (oxygen) and Fe,O,, or, as 
it is often written, FeO,, contains ¢ Fe to 1 of O. As the iron in 
each of these cases satisfies the oxygen, it is evilent that the 
iron must be in two different states, (1) a protomide state, and 
(2) a sesquiouide state. One part of iron in this sesquioxide 
state (= 2 Fe) often replaces in compounds one part of iron in 
the protoxide state (or 1Fe), with no greater change of qualli- 
ties than happens in the replacement of iron by magnesium, or 
calcium, explained above; or, avoiding fractions, 3 parts of Fe 
in the protoxide state replaces 2Fe in the sesquioxide state. 
Writing Fe for the last 2Fe, the statement becomes | of Fe, 
replaces 1 of Ee. Aluminium occurs only in the sesqui- 
oxide state, and the ordinary symbol of the oxide is A1,O;, 
or AlO,. But it is closely related to iron in the sesquioxide 
state, so that, using the same mode of expression as for iron, 
1 of Al replaces 1 of Fe,, or | of Mg, and so on. Similarly, 
writing R for any metal, 1 of R replaces 1 of R,. Again, in 
potash (K,O), soda (Na,O), lithia (Li,O), water (H,O), one of 
oxygen (O) is combined severally with 2 of K (potassium), of 
Na (sodium), of Li (lithium), of hydrogen; and hence 2K, 
2Na, 2Li, that is, K,, Na,, Li,, may each replace in compounds 
1Ca, or 1Mg, ete. 
The elements potassium, sodium, lithium, hydrogen, of which 
it takes two parts to combine with 1 of oxygen, are called 
monads. Other elements of the group of monads are rubidium, 
cesium, thallium, silver, and also fluorine, chlorine, bromine, 
todine. Still other elements combining by two parts in their 
oxygen or sulphur compounds, ete., are nitrogen, phosphorus, 
