GEOPHYSICAL LABORATORY. 139 



pressure also. The critical point of water is only a secondary factor in 

 determining the nature of the product, its influence being effected princi- 

 pally through the change in concentration of the solvent (liquid or fluid) in 

 the neighborhood of the critical point. 



The thorough investigation of hydrothermal syntheses is beset with many 

 difficulties, apart from the technical problems inherent in operating on 

 heterogeneous systems within closed bombs at high temperatures. Never- 

 theless our knowledge of the real relationships of these minerals can be 

 advanced materially if care is taken to control the factors involved, the 

 most important of which are the initial composition of the system (including 

 therein the relation between the amount of water and the volume of the 

 bomb) and the temperature. 



(20) Die hydrothermale Silikatbildung. Paul Niggli and George W. Morey. Z. anorg. 



Chem., 83, 369-416. 1913. 



A German translation of "The hydrothermal formation of silicates" 

 (Jour. Am. Chem. Soc, 35, 1086-1130, 1913). Reviewed under No. 19 above. 



(21) Geophysical Research. Arthur L. Day. Smithsonian Annual Report for 1912, 



pp. 359-369. 



Presidential address delivered at the seven hundredth meeting of the 

 Philosophical Society of Washington, November 25, 1911, and printed in 

 the Journal of the Washington Academy of Sciences (1, 247-260, 1911), 

 for December 4, 191 1 . This address has now been reprinted, by permission of 

 the Journal of the Washington Academy of Sciences, in the Annual Report 

 of the Smithsonian Institution for 1912. 



(22) The stability relations of the silica minerals. C. N. Fenner. Am. Jour. Sci. (4), 



36, 331-384. 1913. 



The relations between the mineral species quartz, tridymite, and cristo- 

 balite have been found to be enantiotropic. The inversion-temperatures 

 under atmospheric pressure are 



870° ± 10° quartz zzl tridymite 

 1470° ±10° tridymite ^Hl cristobalite 



The velocity of transformation of one form of silica into another has been 

 found to be very slow and in many cases the process follows Ostwald's rule — • 

 that is, an unstable form does not pass directly into the most stable form, 

 but the action progresses through successive steps, and intermediate phases 

 appear which eventually reach the stage of greatest stability. The appear- 

 ance of unstable phases in this manner has suggested an explanation of 

 natural occurrences of tridymite and cristobalite under conditions which 

 preclude the idea of their deposition within their range of stability, and 

 an inquiry has been directed toward the circumstances attending their 

 formation. 



The preparations of artificial tridymite and cristobalite which have been 

 made have yielded the minerals in very pure form and a redetermination 

 has been made of a number of their optical and other physical constants. 

 The relations of chalcedony have been investigated and evidence has been 

 obtained tending to show that it is a distinct mineral species. 



