236 /. H. L. VOGT 



The same also applies to slags as 



SiO^ Al^Oj CaO MgO FeO 



46-50 14-18 7-13 3-5 about 20 



which at about i3oo°-i40o° are rather viscous in spite of the high 

 percentage of FeO. 



Feldspars. — According to the investigations of the Geophysical 

 Laboratory of the Carnegie Institute, orthoclase (microcUne) and 

 albite are characterized by extremely high viscosity just above the 

 melting-point. It is so great, and at the same time the speed of 

 the melting so slow, that these minerals, as well as quartz, may be 

 superheated for a while to some degrees above the theoretical 

 melting-point without melting.^ On account of the extremely high 

 viscosity of microcHne, this mineral has for more than 100 years 

 been used as a glaze for porcelain. Melted microcUne is extremely 

 viscous, even when heated to several hundred degrees above the 

 melting-point. 



Melted anorthite (melting-point 1550°), on the other hand, 

 just above its melting-point is medium thin, both according to 

 the experiences of the Washington Institute and of myself. The 

 melts lying between Ab and An show intermediate viscosity. 



Melted SiOz is extremely viscid, and silicate melts with more 

 than 60 per cent SiOz and the remainder mixtures of various bases 

 (CaO, MgO, FeO, AI2O3, etc.) show increasing viscosity with 

 increasing percentages of SiOa. 



In short, numerous observations indicate that the viscosity 

 which at a certain temperature characterizes a complex silicate 

 melt containing several components, as SiOa, KAlSijOg, NaAlSisOg, 

 CaAljSizOs, CaMgSijOe, etc., may be derived from the viscosity 

 which is characteristic of each of the components. But the viscosity 

 is hardly a hnear function of the viscosity of the components. 



As to the alkali-silicates, we refer to the investigations of Greiner. 

 Here it may be pointed out, however, that KjSiOa (alone or with an 

 admixture of SiOa) is considerably more viscous than the corre- 

 sponding soda-siHcate, and potassium-glass is, as is known, con- 

 siderably more viscous than soda-glass. According to Greiner, 



' See Day and Allen's well-known Feldspar studies (1905). 



