4 o 4 SCIENCE PROGRESS 



is in the region of the transition conditions a definite range 

 for each substance, throughout which the velocity is appar- 

 ently zero. In a final paper {ibid. 52, pp. 91-187, 1916) the 

 author collects the results and discusses the general mechanism 

 of polymorphic change. The substances examined behave in 

 very diverse ways, and it is evident that many more data, 

 especially with respect to the crystallography of the forms, 

 are necessary before a satisfactory explanation is reached. 

 Certain analogies with the behaviour of liquids under high 

 pressure are pointed out. The author suggests the restriction 

 of the term — polymorphic — to those substances which exist 

 in several forms with reversible transformations. 



Wright {Jour. Amer. Chem. Soc. 39, pp. 1515-25, 191 7) 

 has made a complete study of the various crystalline forms of 

 menthol. The latter exists in four modifications, three of 

 which undergo monotropic transformations to the stable hexa- 

 gonal a- form which melts at 40*5°, the other forms, /9, 7, S, 

 melting at 35*5°, 33-5° and 31-5° respectively. Spherulites 

 develop readily in the crystallisation of all four forms, and these 

 change from spherical to ellipsoidal at the transition tempera- 

 tures of the monotropic forms. 



Mineralogy. — Taber {Bull. Amer. Inst. Mining Eng. 19 16, 

 PP- 1 973-9 8 ) discusses the genesis of asbestos. During the 

 formation of serpentine, the volume change induces strains in 

 the rock and the chrysotile develops in the fractures thus 

 formed. The mineral structure is due to the inhibition of 

 growth in certain directions by the external conditions so. 

 that an extremely prismatic form develops. This is criticised 

 by Brauner, Dresser, Graham and Merrell {ibid. 191 7, pp. 397- 

 405), who hold that the extrapolation of laboratory results on 

 the growth of fibrous minerals is not justified on account of 

 the fact that in nature the influence of the wall rock is im- 

 portant, and further that the cavities in which the veins form 

 are due to shrinkage and not to pressure. 



Clarke and Wheeler {U.S. Geol. Surv. Prof. Paper 102, pp. 

 1-56, 191 7) describe the mineralogical nature of the inorganic 

 parts of marine invertebrates. Magnesium salts are found to be 

 more common than is generally supposed, and are usually in 

 isomorphous mixture with calcite, not aragonite. No satis- 

 factory explanation of the distribution of calcite and aragonite 

 in shells is reached. 



