The equili-brium of the system Cobalt-Silicon is thoroughly investigated by means of thermal, microscopic, X-ray, dilatometric and magnetic analyses, and a new equilibrium diagram is proposed. In this system there exist four compounds, i.e., Co
2Si(ε), CoSi(η), CoSi
2 and Co
3Si(γ); the first three melt without decomposition at 1332°, 1460° and 1336° respectively, while the last one is formed by the following peritectic reaction ;
melt+δ_??_Co
3Si (γ).
The compound Co
2Si changes to δ phase at 1320°: the change is probably an allotropic one. The compound CoSi
2 has a slight thermal change at 1258°, but it is not an'allotropic change. The compounds Co
3Si
2 and CoSi
3, Proposed by K. Lewkonya, do not exist. The α_??_β allotropic transformation point of Cobalt rises with the increasing content of Silicon, and decomposes to P and S by the peritectoid reaction at 755° and 6.5per cent silicon.
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