202 HAROLD L. ALLING. 
solutions we find a more satisfactory answer to our query. Some 
fine work on this subject has been done by the Italians, especially 
by Ciamician and his co-workers. 
Ciamician* and Bruni? have shown that single-ring organic 
compounds can form solid solutions only with other single-ring com: 
pounds. Thus, benzene (not benzine), Figure 1-(1), can form solid 
solutions only with such compounds as: thiophene, Figure 1-(2), 
pyrrol, Figure 1-(3), and pyridine, Figure 1-(4). In the same way a 
double-ring compound can only form a solid solution with another 
double-ring compound. Thus naphthalene, Figure 1-(s5), only with 
quinoline, Figure 1-(6), etc. Under that same rule applied to triple- 
ring compounds anthracene, Figure 1-(7), forms solid solutions with 
its isomer, phenanthrene; with carbozol, Figure 1-(8), or with 
other triple-ring structures. 
The complete solubility of one solid in another is more likely 
to occur if the chemical structure of each is similar. The similarity 
of the two, however, does not necessarily have to be so close as the 
term “isomorphism”? would imply. For example some feldspars 
contain nephelite in solid solution, but this mineral cannot be 
regarded as isomorphous with the normal feldspar components. It 
does not seem unreasonable to assume that the mineralographic 
term “solid solution” is more comprehensive than the crystallo- 
graphic term ‘‘isomorphism.”’ 
“It is convenient to regard . . . . [an isomorphous series] as 
formed by the replacement of one element or radical by another 
isomorphous with it, rather than as a mixture of different individual 
molecules.’ It certainly is true that the term “‘replacement” is very 
commonly used, and no criticism to such use can be made provided 
that the conception of the solubilities of the components is not lost 
sight of, but there is danger of confusion in such cases. To the eco- 
nomic geologist the word “‘replacement”’ immediately suggests such 
phrases as “‘pyrite replaced by chalcocite” or ‘“‘limestone replaced 
™ Ciamician, Zeitschr. fiir Physt. Chem., XIII (1894), 1; XVIII (1894), 51; XLIV 
(1903), 505. 
2G. Bruni, Rendiconti dell’ Accademia dei Lincet., Vol. VIII (1899), p. 570. 
3A. J. Moses and C. L. Parsons, Mineralogy, Crystallography, and Blowpipe 
Analysis, 5th ed., 1916, p. 234. 
[alin 2 
