\. 
: Atomic Volume of some Minerals. 221 
of form and volume. The relations between the aggregate 
atomic volume of isomorphous compounds is in many cases nec- 
essarily complex ; for we find that this relation is expressed most 
nearly by the proportional number of molecules of elements in 
thase compounds. Between ryacolite and loxoclase, for example, 
ing the atomic volume will thus be clearly shown: and while 
not underrating the ratios ascertained by the first, we think that 
an additional value will be found in the relations developed by 
the third method ; and also that some importance may attach to 
the second. In some cases the (B) relation is singularly close and 
of interest. The atomic volumes of alumina and arsenic are al- 
most identical (161-7 and 163); while if we divided by 5, the 
number of atoms in alumina, it gives, instead of a ratio of equality, 
the ratio of 1: 5. These modes of viewing the subject of atomic 
Volume appear to open the way for important conclusions bearing 
Upon some of the most recondite points in chemical science. — 
Hermann has written at considerable length upon isomorphism 
among compounds of unlike atomic constitution.| He has con- 
fined himself almost exclusively to pointing out such well known 
cases as the chemically unlike varieties of epidote, scapolite, etc., 
(which the erystallographer has often united and the chemist as 
often pulled asunder)—besides some acknowledged instances of 
‘isomorphism. He has introduced for such cases the new term 
heteromerism, a term of indefinite signification, since all com- 
* Poggendorff’s Annalen, 1849, No. 1, lxxvi, p. 80. 
+ Erdmann und cers Journal fir Praktischen Chemie, xliii, 35, 
