508 PROFESSOR HEDDLE ON THE RHOMBOHEDRAL CARBONATES. 
soda with a certain amount of water is capable of replacing lime; other such 
cases are known. 
Moreover, complicated difficulty lies in the fact that the equivalent volume 
of compounds is far from invariably the sum of the equivalent volumes of their 
constituents. The equivalent volume of sulphur is 16:1, that of copper is 7 ; the 
equivalent volume of the protosulphide is 23, which is the sum of the above 
equivalents ; but the equivalent volume of the subsulphide is only 26:5, while 
the sum of the equivalent volumes of two of copper and one of sulphur is 30. 
Again, the equivalent volume of the bisulphide of iron is only 23°6, while the 
sum of the equivalent volumes of two of sulphur and one of iron is 39°47. 
Many other examples might be adduced of a similar condensation. 
Neither are we in possession of a sufficient amount of evidence as to how far 
the law of quantivalence dominates among mineral bodies; the evidence which 
we do possess of nature’s workings is not always consonant with deductions 
elaborated at the desk. 
In this partial state of our knowledge, all probably that we can at present do 
is, in the cases where minerals with an unequal number of atoms, as expressed 
in their rational formule, replace each other, to consider them as so doing in 
equal number of atoms; that is, that of a mineral formulated with three atoms, 
we must employ two parts to replace one formulated with six. 
It will therefore be necessary in all such cases to compare the common 
atomic volume each with each. 
The atomic volume is arrived at by dividing the equivalent volume, as before 
obtained, by the number of atoms in the combination. 
Of cavernous pseudomorphs, which have to be considered under this second 
heading, there are (in the writer’s cabinet) the following :— 
Three after Pyrite— 
Hematite. Pyrite. 
Fe,0, ; FeS, 
32°65 . 
i a =6-53 120 + 5-08 — se Se 
Limonite. Pyrite. 
2¥e,0, + 3H,O 
374 + 3-8 - vase 5-18 7-4 
iy) 
Magnetite. Pyrite. 
FeO,Fe,0, 
46 
232 = 5-05 - 7 = 6-57 7-4 

