1904.] Physical Characters of the Sodium Borates, etc. 



295 



crystallises, hence some point between these mixtures will give the 

 maximum solubility of this borate in boric anhydride. On further 

 increasing the amount of this borate present, the glass becomes a 

 supersaturated and superfused liquid, the amount of crystallisation 

 measuring the super saturation. 



Somewhere between Na 2 O.6B 2 3 and Na 2 0.4B 2 3 no free boric 

 anhydride remains in the solution, and the whole mass will crystallise 

 for the first time. 



On further addition of sodium, another borate of composition near 

 5Na 2 0.4B 2 3 begins to be formed, and we have shown that in any 

 mixture between these two compounds, both crystals and glass have 

 identical compositions. We are here probably dealing with a case of 

 solid solution, and the glass is merely its superfused form. 



The character of the curve would indicate that there are two types of 

 crystals, one rich in B 2 O3, the other rich in Na 2 0, and the lowest point 

 of the curve is the eutectic point formed by mixtures of these two. 



That the crest of the curve does not occur with the composition 

 Na 2 O.B 2 3 can be most conveniently explained by the fact that solid 

 solutions cannot be treated as pure substances, and so a slight shifting 

 of the maximum point is to be expected. The further depression of 

 the melting point curve probably indicates the eutectic point between 

 these crystals, rich in Na 2 0, and sodium carbonate. 



In conclusion we wish to express our thanks to Professor H. B. Dixon 

 and Mr. D. L. Chapman, for the kindly interest they have taken in 

 our work. 



