Properties of the Feldspars. 129 



the finer the powder and the slower the heating, the earlier 

 the first traces appear. Long continued heating, even at very 

 low temperatures (700°), yields a perfectly continuous cake 

 (except for the included bubbles) the surface area of which 

 constantly tends toward a minimum. There is no doubt that 

 the sintering of powdered glasses is due to flow in the Tinder- 

 cooled liquid arid is a phenomenon in liquid viscosity. All the 

 feldspar glasses sintered readily between 700° and 900°, de- 

 pending on the fineness of the powder and the time. 



Powdered crystalline feldspars do not sinter readily below 

 their melting temperature. Indeed we were at first inclined 

 to the view that when only pure, dry, stable crystals are pres- 

 ent, they do not sinter however finely they be powdered. We 

 observed the phenomenon in natural albite at 1000° but the 

 crystals were not wholly free from inclusions which may have 

 caused chemical reactions resulting in cementation. Crystal- 

 line fluorite also sinters 300° below its melting point, but here 

 we were able to establish a decomposition. Acid fumes were 

 evolved during the experiment and the sintered cake contained 

 1 per cent of free lime. Our final experiments with long con- 

 tinued heating for specific gravity determinations, however, 

 showed that the purest feldspars we could prepare, even after 

 they had reached their maximum density, still sinter very 

 slowly. Thus AbjAng powder, which was shown by a deter- 

 mination of its specific gravity to be noncrystalline, formed a 

 compact chalky mass in four hours at a temperature about 

 150° below its melting point ; in three days it was as hard as 

 porcelain. Other feldspars snowed the same behavior. It is 

 hardly possible that inhomogeneities sufficient to produce dif- 

 fusion between portions of different concentration, could have 

 existed in these charges. There is considerable indication that 

 some of the crystalline nuclei grow at the expense of others — 

 perhaps through exceedingly slow sublimation — which may 

 account for it. 



We made repeated attempts to locate some fixed sintering 

 point which should be characteristic of a particular material 

 by means of continuous measurements of the electrical con- 

 ductivity, but they all indicated that no such point existed. 

 The conductivity of a dry powder increases enormously after 

 sintering begins and would therefore seem to offer a most 

 sensitive test, but the phenomenon is altogether gradual even 

 witli a crystalline feldspar containing only a small percentage 

 of glass. We purpose to extend these observations to other 

 substances 



Conclusions. — It now remains for us to gather the results 

 together and to draw such conclusions as they appear to justify. 



(1) If the melting points are now plotted in a system of 



