GEOPHYSICAL LABORATORY. 165 



CO very of thermoelectricity in 1821, and sanctioned by the leading texts and 

 collections of tables in English, French, and German (with the exception of 

 Mascart and Joubert), defines that metal as positive from which the cm'rent 

 flows at the cold junction. This makes antimony, iron, copper, and platinum- 

 rhodium positive to platinum, mercury, nickel, constantan, and bismuth. 

 The opposite notation has, however, been used in some cases, with the result 

 of causing much confusion. 



(16) Nephelite crystals from Monte Ferru, Sardinia. H. S. Washington and H. E. Merwin. 



J. Wash. Acad. Sci., 5, 389-391 (1915). 



Nephehte from miarolitic cavities in a trachytic phonolite has lower refrac- 

 tion and is different crystallographically and chemically from other known 

 nephelites. The differences are shown in this paper by comparative tables. 



(17) The constituents of portland cement clinker. G. A. Rankin. J. Ind. Eng. Chem., 7, 



466-474 (1915). 



In attacking a complex problem such as the question of the constituents of 

 Portland cement clinker, one must proceed in a systematic manner. After 

 ascertaining which are the essential components, one must determine by experi- 

 ment just how these components combine under the particular conditions, and 

 the nature and mutual relations of the several compounds formed; this in- 

 volves the investigation of the composition, the number and relation of the 

 various crystalline forms of the several compounds, and their optical charac- 

 teristics, as well as the study of the state of equilibrium reached by mixtures 

 of all compositions throughout the range of temperatures. One is then able 

 to state precisely what will happen when any mixture of the components is 

 heated or cooled slowly (so that equilibrium is continuously attained) and to 

 indicate the course and final products of reaction when equilibrium is ap- 

 proached but not completely attained. 



The essential chemical components of portland cement are CaO, AI2O3, Si02. 

 From the results obtained during a systematic study of the system CaO- 

 AljOs-SiOa we have found that these three oxides alone in the proportions in 

 which they occur in commercial portland cement, when perfectly burned, will 

 produce a clinker made up only of the constituents (compounds) 2CaO.Si02, 

 3CaO.Si02, and 3CaO.Al203. This clinker possesses all the properties of a 

 desirable portland cement when it is ground and treated with water. 



White commercial portland cement has a CaO, AI2O3, SiOo content of over 

 95 per cent, while the more common gray variety of cement contains over 90 

 per cent of these three oxides. Examinations of the clinker of these two types 

 of cement have shown that the main constituents (over 90 per cent) are 

 2CaO.Si02, 3CaO.Si02, and 3CaO.Al203, and that the minor constituents 

 (less than 10 per cent) are 5Ca0.3Al203, CaO, and ferrites. 



It may, therefore, be definitely stated that portland cement clinker is made 

 up largely of the three CaO-Al203-Si02 compounds, 2CaO.Si02, 3CaO.Si02, 

 and 3Ca(3.Al203, and it seems probable that the desirable properties of port- 

 land cement are due to these three major constituents and that the minor 

 constituents have little, if any, influence upon it. 



It has been argued by some that not even an approximation to equilibrium 

 conditions obtains in the actual manufacture of portland cement, and hence 

 that conclusions derived from a study of the equilibrium diagram of this sys- 

 tem would have little or no bearing on the question of the constituents of 

 commercial clinker. It may be that equilibriuto conditions are not always 

 completely attained in the burning of commercial clinker, but the work on 

 actual clinkers already published from the laboratoiy of the Bureau of Stand- 



