40 Revieivs — Recent Geophysical Reprints. 



Recent Geophysical Reprints. 



1. An Apparatus for growing Crystals under Controlled 

 Conditions. J. C. Hostetter. Journ. Wash. Acad. Sci 

 vol. ix, 1919, p. 85. 



2. The Hydrated Ferric Oxides. E. Posnjak and H. E. 

 Merwin. Am. Journ. Sci., vol. xlvii, 1919, p. 311, 



A S is well known the Geophysical Laboratory of the Carnegie 

 -'^ Institute put aside its usual scientific pursuits to help win the 

 War, and on this account only two of the latest issue of reprints 

 from the Laboratory are sufficiently geological to call for notice 

 in these pages. 



1. The apparatus for growing crystals described by Hostetter 

 has its origin in a German patent of Kriigen & Finke. Its principle 

 is the circulation of solution from a saturator to a crystallizer and 

 back again. The temperature difference varied with the salt used 

 and was generally kept well under 0-5° C. The description is clear, 

 and is illustrated by an admirable diagram. 



2. The hydrated ferric oxides, though a difficult subject, have 

 attracted considerable attention in the past. Many of the individual 

 results attained by Posnjak & Merwin are of the nature of con- 

 firmation rather than of original discovery, but this does not detract 

 from their importance. Our knowledge in its toute ensemhle was 

 hopelessly confused, and the present contribution is most welcome. 

 Its main features may be summarized as follows : — ■ 



Ferric hydrates, both natural and artificial, fall into two great 

 classes : Class I, orange to orange-yellow ; and class II, red to 

 reddish-brown. Class I has the empirical formula Fefio.H.fi, 

 with variable adsorbed and capillary water. Class II is of a different 

 constitution. 



Only the natural products have been met with as definite crystals. 

 These are of three kinds : Class I, Goethite and Lepidocrocite •, 

 Class II, Turgite. 



Limonite falls to Class I. Much of what is commonly called 

 limonite is shown to be fibrous goethite, but other samples are 

 indecipherably " amorphous ", and for these the name limonite 

 is retained. Limonite in this restricted sense may or may not be 

 goethite. Its density and refractive index, when corrected for the 

 effect of adsorbed and capillary water, point in the direction of the 

 goethite correlation, but as yet the evidence is not considered 

 conclusive. Might not the absorption spectrum of the streak help 

 to fill in this gap in our knowledge ? 



Turgite is interpreted as a solid solution of haematite and 

 goethite, but in this direction the authors have not developed 

 their case very clearly. They base upon the density of the three 

 minerals, and then remark " all other evidence at hand is in favour 

 of this assumption ". But in relation to the dehydration behaviour 

 of turgite, we had previously been told " the shape of the curve 



