GEOPHYSICAL LABORATORY. 165 



for instance, by burning under different conditions from that obtaining during 

 the course of these experiments. With some types of pots the method would 

 undoubtedly be successful, even with the usual bm*ning schedules. The 

 possible appHcation to the removal of iron from grog, clay, and other ceramic 

 products is indicated. 



(21) Electrometric titrations, with special reference to the determination of ferrous and 



ferric iron. J. C. Hostetter and H. S. Roberts. J. Am. Chem. Soc, 41, 1337- 

 1357 (1919). 



The advantages of the electrometric method for titrating, oxidizing, and 

 reducing reactions may be summarized as follows: 



(1) This method permits the use of potassium dichromate with its numer- 

 ous advantages. 



(2) The reduction of the solution with electrometric control eliminates 

 the removal of excess reducing agent, which must be done with the usual 

 methods of reduction. 



(3) Conditions, such as acidity, need not be controlled, except within very 

 wide limits, and hj^drochloric, sulphuric, or hydrofluoric acid, or mixtures of 

 these, may all be used. In contrast to these wide limits, compare the narrow 

 acid limits (1.5 to 2.5 per cent by volume of H2SO4) within which the reduction 

 with SO2 or H2S must be carried out and the precautions which must be taken 

 in a permanganate titration in the presence of either chlorides or fluorides. 



(4) The sensitivitj^ and accuracy of the method make possible (a) the 

 determination of a few tenths of a miUigram of tin, chromium, ferrous or 

 ferric iron, and probably many other elements, in the presence of large quanti- 

 ties of some other element, and (b) the determination of blanks involved in 

 some of the ordinary determinations by reducing or oxidizing agents. 



(5) The time within which a determination can be carried out is greatly 

 shortened. The content of ferrous and ferric iron in a silicate, for instance, 

 can be determined in from 15 to 30 minutes. 



(6) The precision attainable is comparable to the best of the ordinary 

 volumetric determinations. 



(22) Electrical apparatus for use in electrometric titration. Howard S. Roberts. J. 



Am. Chem. Soc, 41, 1358-1362 (1919). 



A description of simplified and inexpensive apparatus for use in the titration 

 of salts by the electrometric method. The potentiometer consists of a sliding 

 rheostat with attached scale. Several forms of galvanometer may be used. 



(23) The ternary system CaO-MgO-Si02. J. B. Ferguson and H. E. Merwin. Am. J. 



Sci., 48, 81-123 (1919). 



The ternary system Hme-magnesia-sihca has proved to be the most com- 

 pHcated of the four possible ternary systems which may be constructed from 

 the four oxides, Ume, magnesia, alumina, and siHca. The crystaUine phases 

 which are definite compounds, and which appear as primary phases, are as 

 follows: Lime; magnesia; sihca (tridymite and cristobaHte) ; a CaO.Si02 

 (pseudowoUastonite) ; 3Ca0.2Si02; a and /3 2CaO.Si02; MgO.SiOg (cUno- 

 enstatite) ; 2MgO.Si02 (forsterite) ; CaO.Mg0.2Si02 (diopside) ; 5Ca0.2MgO. 

 eSiOz; and 2CaO.Mg0.2Si02. The melting-point of 2CaO.Mg0.2Si02 is 

 1458° ± 5° C. and the decomposition temperature of 5Ca0.2Mg0.6Si02 is 

 1365° ± 5° C. 



In addition to these, crystals representing several solid solutions also ap- 

 pear as primary phases. The soUd solutions are: 



(1) A complete series with chno-enstatite and diopside as end-members, 

 generally known as pyroxenes. 



