GEOPHYSICAL LABORATORY. 163 



fication. Specific examples of the devitrification of optical glasses are given, 

 together with identification of the crystalline phases separating. 



(16) The hydrated ferric oxides. Eugen Posnjak and H. E. Merwin. Am. J. Sci., 47, 

 311-348 (1919). 



The work described in this pubHcation proves rather conclusively that no 

 series of hydrates of ferric oxide exists among the natural minerals. The only 

 existing hydrate is ferric oxide monohydrate. This substance occurs in 

 nature in two polymorphic forms — goethite and lepidocrocite — and in an 

 "amorphous" condition — Hmonite. The two crystalUzed forms are con- 

 trasted as follows : 



Goethite. — Orthorhombic, a:h:c — 0.91: 1:0.602; density (grams per c.c.) 

 4.28 ±0.01; a = 2.26, /3 = 2.394, y = 2.400; streak, dull orange-yellow; 

 pleochroism faint. When crystalHzed in dense aggregates of thin blades and 

 fibers inclosing much adsorbed and capillary water, it has commonly been 

 called Hmonite; however, sufficient proof is now given to show that such 

 crystallized material is really goethite. 



Lepidocrocite. — Orthorhombic, a: 6: c = 0.43: 1: 0.64; density (grams per 

 c.c.) 4.09 ± 0.04; a = 1.94, /3 = 2.20, y = 2.51; streak, dull orange; pleo- 

 chroism very strong. 



The name hmonite is retained for material which appears to be essentially 

 isotropic ferric oxide monohydrate with adsorbed and capillary water. How- 

 ever, this substance should not be considered a distinct form of ferric oxide 

 monohydrate, as the real nature of such "amorphous" substances is still 

 uncertain. 



The fibrous mineral turgite is variable in composition, and considerable 

 evidence is given that it probably represents solid solutions of goethite with 

 hematite, together with inclosed and adsorbed water. 



The genetic conditions of the hydrated ferric oxides and the stabiHty re- 

 lation of the two monohydrates are unknown. 



No definitely crystallized synthetic hydrated ferric oxide has up to the 

 present been prepared. However, it seems certain that only two distinct 

 types of "amorphous" hydrated ferric oxide exist, one yellow and the other 

 reddish-brown. The yellow is apparently essentially ferric-oxide mono- 

 hydrate, while the reddish-brown substance may hold its water in either a 

 dissolved or an adsorbed condition (or both). Thus the synthetic and the 

 natural hydrated ferric oxides exhibit, chemically, great similarity. 



(17) Strains due to temperature gradients, with special reference to optical glass. E. D. 



Williamson. J. Wash. Acad. Sci., 9, 209-217 (1919). (Papers on Optical 

 Glass, No. 10.) 



General equations are derived for the elastic stresses produced by tem- 

 perature differences in spheres, cylinders, and slabs when the temperature 

 distribution is symmetrical about the center, axis, or central plane, respectively. 

 More specific equations are given for the case of the temperature distribution 

 due to uniform surface-heating, which is the most important case in practice. 



(18) A furnace temperature regulator. Walter P. White and Leason H. Adams. Phys. 



Rev., 14, 44-48 (1919). 



By making the heating coil of an electric furnace one arm of a wheatstone 

 bridge, and combining this with a galvanometer regulator, thus keeping 

 constant the resistance of the coil, we can, regardless of variations in the cur- 

 rent-supply, and with no attention, maintain constant the temperature of 

 furnaces not too directly influenced by the temperature of the room, or where 

 the surrounding air is kept constant. The power available in this regulator 



