RECENT ADVANCES IN SCIENCE 37 



473, 1 91 6) have examined the stability relations and the con- 

 ditions of formation of the several forms of calcium carbonate. 

 Under ordinary conditions, calcite is the only stable form, but 

 the two unstable modifications, aragonite and fi-CaCOs, may be 

 deposited under particular circumstances. The numerous other 

 forms, vaterite, conchite and so forth, which have been described, 

 are shown to belong to one or other of the above modifications. 

 The two first-named authors (Jour. Geol. 24, 729, 191 6) discuss the 

 role of inorganic agencies in the deposition of calcium carbonate 

 and conclude that the two chief factors are the variation of 

 temperature and the concentration of free carbon dioxide in 

 the ocean. These affect the relative degree of saturation to a 

 considerable extent, and are obviously of importance in con- 

 nection with the formation both of chemically precipitated and 

 organic limestones. 



A question of some economic importance is considered by 

 Zies, Allen, and Merwin in a paper on the reactions involved 

 in secondary copper sulphide enrichment (Econ. Geol. 11, 

 407, 191 6). On allowing copper sulphate solution to react 

 with natural sulphides, a precipitation of sulphide, accom- 

 panied by an oxidation to sulphate, ensues, the reactivity 

 varying with the nature of the sulphide. Starting with pyrites, 

 the first product of the reaction is covellite and the final chal- 

 cocite, while pyrrhotite is altered to chalcopyrite. The process 

 is not reversible, except possibly in the case of bornite, where 

 the direct reaction does not occur. The rate of reaction is 

 greater at high temperatures, but the products, except for 

 secondary reactions, are essentially the same. The stability 

 of the various minerals towards copper sulphate is also deter- 

 mined. Allen and Lombard (Amer. Jour. Set. 43, 175, 191 7) 

 have devised a method of determining the dissociation pressure 

 of sulphides, and have applied it to the minerals pyrites and 

 covellite through a pressure range extending from 1 mm. to 

 500 mm. of mercury. The method is likely to be useful in the 

 synthesis of those sulphides which can be prepared in the dry 

 way. 



Boeke (Zeit. Anorg. Chem. 98, 203, 19 16) discusses the 

 utility of the tetrahedron in the graphical representation of the 

 equilibrium of four-component systems. In a paper on the 

 composition of tourmaline (Neues Jahrb. f. Min. 191 6, ii. 

 109) the principles of n-dimensional geometry are applied to 



