78 CHEMICAL PEOPEHTTES 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 meant 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 potassium, arsenic replacing phosplwvviA 

 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 1 of Fe (iron) to 1 of O (oxygen) and Fe.,(_) :; , 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 evi ent that the 

 iron must be in two different states, (1) a protjx'ule state, and 

 (2) a sesquioxide state. One part of iron in this sesquioxide 

 state (= | Fe) often replaces in compounds one part of iron in 

 the protoxide state (or lFe), with no greater change of quali- 

 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 statemjnt becomes 1 of Fe 3 

 replaces 1 of Fe. Aluminium occurs only in the sesqui- 

 oxide state, and the ordinary symbol of the oxide is A1,0 3 , 

 or A10 3 . £ut 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 3 , or 1 of Mg 3 , and so on. Similarly, 

 writing R for any metal, 1 of R replaces 1 of R 3 . Again, in 

 potash (K 3 0), soda (NaX>), lithia (LLO), water (H,0), one of 

 oxygen (O) is combined severally with 2 of K (potassium), of 

 Na (sodium), of Li (lithium), of hydrogen; and hence 2K, 

 3Na, 2Li. that is, K,, Xa_, Li., may each replace in compounds 

 ICa, or IMg, etc. 



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, 

 txesium, thallium, silver, and also fluorine, chlorine, bromine, 

 iodine. Still other elements combining by two parts in their 

 oxygen or sulphur compounds, etc., are nitrogen, phosphorus, 



