8()0 



brated in Washington, and by determination of' the melting-points of 

 y'hSO, (884° C), Li.SiO, (1201° (J.), syntlietieai diopside (1391° C), 

 and synthetical anorthite (1552° C), with each of the three mentioned 

 thermoelements successively. The potentiometer-current was kept 

 constant at 0,002 Amperes ; two WESTOTs-cells, connected in series, 

 and a Wolff auxiliary-rheostate, served as standard-electromotive 

 force ; the WF.sTON-cells were constructed and controlled several 

 times at the Physical Laboratory of the University. The furnaces 

 used were platinnm-resistance-furnaces with the heating-coil inside ; 

 they had the usual type, and their regulation was executed by means 

 of a decade-rheostate of manganin-wire. The heating-current was 

 direct current of 110 Volts and 20 or 30 Ampères. 



§ 4. The components. 



With respect to the components themselves, the following data 

 may be given. The relation between the three modifications of the 

 silicumdioxide : 810.^, can be esteemed established in general lines 

 after the recent, most accurate research of C. N. Fknner (Amer. 

 Journal of Science, 36. 331 (1913)). 



The inversiontemperature for /^quartz ^ tridymite lies at 870° 

 ±10° C. ; in the same way thai for tridymite ^ cristobalite is deter- 

 mined at 1470° =b 10° C. The three modifications are enantiotropic 

 forms, but the transformation-velocity^ is very small, and retardation- 

 phenomena, even in an enormously high degree, are almost always 

 present. This is the reason, why i]i nature some modifications of 

 SiO.^ often occur within the stability-field of other forms. 



Then there is at 575° C. an inversiontemperature for « — ^ ,?-quartz; 

 at 117" C. one for f<- -» j?-tridymite, at 163° C. one for (3- -» y-tridy- 

 mite ; these inversions occur relatively fast. Probably the «-, and in- 

 forms of cristobalite are in less or more stable equilibrium with each 

 other at temperatures, situated between 198° and 274° C. The relations 

 of those modifications to each other are very complicated, but of no 

 direct interest for the present study. 



The meltingpoint of cristobalite is very close to 1625° C; the 

 liquid is a very viscous mass, which b}' rapid cooling changes into 

 the wellknown "glass". 



Quartz is optically easy to discriminate from both the other forms, 

 by the great differences of the refractive indices ; the discrimination 

 between tridymite and cristobalite however is rather difficult. 



The refractive indices of quartz are: ?2e = 1.553 and no =^ 1.544; 

 those for tridymite are: r2;< =1 1 .469, 7iv =^ 1.473, while the trlie 



