872 The*American Naturalist. [October, 
the assumption that hydroxyl and fluorine are isomorphous. Their 
results show that whereas the ratio SiO, : A1,O,: F varies from 1:1: 
1.50 to 1: 1: 1.84, the ratio SiO,: ALO, : (F. OH) is constant and 1: 
1:2,s0 the formula of topaz becomes (Al'[F. OH] ), SiO, or (Al 
[F. OH], Al SiO, Their study of the physical properties of the 
mineral establishes a definite relation between them and the per cents 
of fluorine and water present, clearly indicating the isomorphous 
character of the fluorine and hydroxyl The hydro-topaz has the 
smaller optical angle and the smaller specific gravity. The same fact 
is brought out by the determined values for a, f, and y, and by exact 
. measurements of interfacial angles. The optical anomalies of some 
Brazilian erystals are explained by zonal growth of topazes of different 
composition. 
Composition of Chondrodite, Humite, and Clinohumite.— 
Penfield and Howe' have undertaken the study of the composition of 
the members of the humite group with the result not only of bringing 
order out of chaos, but also of establishing the fact that chondrodite, 
humite, and clinohumite constitute an homologous series both in a 
chemical and in a crystallographical sense. Sjögren has assumed that 
fluorine and hydroxyl are isomorphous, and derived new formulas for 
the members of this series, but as the authors point out the older 
analyses which Sjógren utilized are low as regards water, and Sjogren 
neglected to take into account the replacement of magnesia by ferrous 
iron and the consequent lowering of the silica percentage. The for- 
mulas derived by the authors, reckoning ferrous iron as magnesia, are 
as follows: 
Chondrodite Mg, (Mg [F. OH), (SiO), 
Humite Mg, (Mg [F. OH], (SiO), 
Clinohumite Mg, (Mg [F. OH], (SiO)), 
The common difference of this homologous series is a molecule of 
chrysolite, Mg, SiO, As shown by Sacchi and vom Rath, if the `c, 
axis of ‘crystals of chondrodite be divided by 5, that of humite by 7, and 
that of clinohumite by 9, the axial ratios of the three minerals become 
practically identical. Now these divisors, 5, 7, and 9, are the same as the 
number of magnesia atoms in the formulas of the corresponding min- 
erals. A most interesting relation is thus brought out connecting the 
erystal forms and chemical compositions of the members of this group. 
The authors think it probable that other members of this series will be 
discovered, such as a mineral of the composition Mg (Mg [F. OH]. 
"Am. Jour. Sci., (3) xlvii, pp. 188-206. 
