214 T. S. Hunt on the Constitution 
tical for the pyroxenes and hornblendes, as well as for the tour- 
malines and many other groups of isomorphous species; the vol- 
umes thus obtained he has designated by C, the first being the 
A volumes,* If instead of taking the A volume, we divide the 
gross equivalent weights by the number of atoms, we obtain the 
mean equivalent weight of these atoms, and it is clear that C rep- 
resents only the volume corresponding to the atoms in these 
isomorphous species, where the condensation is alike. The 
equivalent weight of silica SiOs is 46°22; this divided by four 
the number of atoms according to Mr. Dana’s notation, gives 
11:55; in the same way Al2O3=614+5=103; MgO=20°36 
—+2=1018; CaO=28-—2=14 etc.; so that in formulas made 
up of varying proportions of these elements, the mean equivalent 
weight will be nearly the same, and in isomorphous species where 
the density varies as the gross equivalent, the C volume will only 
vary as the mean equivalent. But if elements of a higher equiv- 
* This Jourual, [2] vol. ix, pp. 221 and 407, and xii, p. 204. 
+See this Journal, vol. xv. p. 230. 
} Poggendorf’s Annal., vol. Lxviii, p. 319, and lxxxiv, p, 321, also this Journal, vol. 
¥. p. 381, and xiv, p. 37. : ss 
