CLARKE: CHEMICAL STABILITY 



339 



point. It was found that in the case of certain bricks made of 

 heterogeneous material of relatively low melting point, the melting 

 points were slightly higher after six hours heating to 1550°, appar- 

 ently as the result of the gradual running together of dissimilar 

 particles to form a mixture having a higher melting point than the 

 most fusible of the original materials. 



The results are summarized in the following table: 



Melting Points of Fire Bricks 



MATERIAL 



Fire clay brick . 



Bauxite brick. . 



Silica brick 



Chromite brick 

 Magnesia brick 



Kaolin 



Bauxite 



Bauxite clay. . . 



Chromite 



Pure alumina. . 

 Pure silica 



NUMBER OF 

 SAMPLES 



41 



8 

 3 

 1 

 1 

 3 

 1 

 1 

 1 



MELTING POINT 

 CENTIGRADE 



1555 to 1725 



mean 1649 



1565 to 1785 



1700 to 1705 



2050 



2165 

 1735 to 1740 



1820 



1 795 



2180 



2010 



1750 



The value 1750° given for silica is not the true melting point, 

 but represents approximately the temperature at which the silica 

 flows distinctly. It was found that silicon carbide does not melt 

 below 2700°; it becomes unstable at much lower temperatures. 



CHEMISTRY. — Notes on chemical stability. F. W. Clarke, 

 Geological Survey. 



The term chemical stability is one of those qualitative expres- 

 sions which convey a definite idea to the mind, even tho they 

 cannot be formulated quantitatively. One substance is easily 

 formed and difficultly decomposable; it is therefore said to be 

 stable. Another is made with much trouble and decomposes 

 readily, and is described as unstable. The distinction is clear, 

 but the conditions of formation and decomposition are due to 

 various agencies which may reinforce or oppose one another. 



