PHYSICAL CHEMISTRY 361 



two benzene nuclei causes the disappearance of the liquid 

 crystal as the enantiotropic form. These changes destroy the 

 axis of symmetry of the molecule. When the NH group is 

 similarly introduced, the range of stability of the liquid crystals 

 is reduced to zero, but these crystals can exist in a metastable 

 form below the true melting point of the compounds. This is 

 an interesting case of monotropy. In the homologous series 

 obtained by the successive introduction of CHj between the 

 two benzene rings, a periodic disappearance and reappearance 

 of the property is observed. 



GEOLOGY. By G. W. Tyrrell, F.G.S., A.R.C.S., University, Glasgow. 



Igneous Rocks. — N. L. Bowen has summarised his conception 

 of the mutual relations of minerals in igneous magmas in a 

 principle which is likely to become fundamental in petrological 

 science (" The Reaction Principle in Petrogenesis," Journ. 

 GeoL, 30, 1922, pp. 177-98). The gist of the paper can be best 

 given in his own words. He believes that the eutectic relation 

 is unimportant, and that another relation between crystal and 

 liquid phases — the reaction relation — is of fundamental signi- 

 ficance. " The ordinary solid solution series such as the 

 plagioclases may be regarded as a continuous reaction series, 

 because during crystallisation each member is produced from 

 an earlier member by reaction with the liquid, the variation in 

 composition being continuous. There are also discontinuous 

 reaction series exhibiting related characters but with discon- 

 tinuous changes of composition. The series olivine-pyroxene- 

 amphibole-mica is a prominent example among the rock- 

 forming minerals. On the basis of these considerations the 

 minerals making up the rocks of an igneous sequence can be 

 arranged as reaction series, and it is the existence of such 

 series that controls the crystallisation and differentiation of 

 the rocks of the sequence." 



In a monumental memoir by Dr. P. Niggli (" Die Leicht- 

 fliichtigen Bestandtheile im Magma," Preissch. . . . von der 

 Fiirstlich Jablonowskischen Gesell. zu Leipzig, 45, 1920, pp. 272) 

 magmas are treated from the point of view of their composition 

 as mixtures in various proportions of easily volatilised and 

 difficultly volatilised constituents. It is impossible to sum- 

 marise adequately a work of this kind. There is a thorough 

 review of systems composed of the above-mentioned con- 

 stituents. Then natural magmatic processes are treated in 

 relation to their dependence upon the volatile constituents. 

 Finally, there is a review from this standpoint of the petro- 

 graphicai literature dealing with plutonic, laccolithic, and 

 volcanic rocks, contact metamorphism, pneumatolytic ore 

 deposits, and pegmatites. 



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