294 Messrs. C. H. Burgess and A. Holt, Junr. [Oct. 27, 



We also fractionally crystallised borax itself five times, when the 

 final crystalline portion was found to contain 30 '8 per cent. Na 2 0, 

 theory for Na 2 B 4 07 requiring 30 '7 per cent. Na 2 O. 



These differences are negligible, and the crystals that separate out 

 from the glass have exactly the same composition from the beginning. 



These results show at once that this crystallisation is not due to the 

 separation of any single compound as we had previously supposed, but 

 that it is more probably due to the formation of mixed crystals or a 

 solid solution from a superfused liquid. 



All the various crystalline fractions obtained from any one glass of 

 composition Na 2 0.4B 2 O 3 to Na 2 0.2B 2 3 were found to melt at 

 identical temperatures, as also did the residual glasses. Further, the 

 crystalline portions when melted, gave glasses which had a melting 

 point identical with that of the previous residual glass, and these 

 residual glasses could, in their turn, be changed to crystals with 

 similar melting points to those which had originally been separated. 



This shows most conclusively the identity of the crystals and glass 

 in chemical composition. We have also determined in the case of 

 several mixtures the temperatures at which this crystallisation takes 

 place. The curve indicating these temperatures is shown in fig. 2 by 

 the line joining AA. 



A bead of the glass was put on the platinum wire of our melting 

 point apparatus, and the temperature raised extremely slowly, whilst 

 the bead was watched with a lens. In the case of those mixtures 

 which melted before the change occurred, no weight was attached to 

 the bead. The crystallisation appeared to start at a fairly well defined 

 temperature, and, if it was maintained, would gradually spread through 

 the bead. 



It is a little hard to say exactly what is the meaning of this curve 

 AA, fig. 2, and all that seems certain is that it gives approximately 

 the temperatures at which the rate of crystallisation assumes a sufficient 

 velocity to visibly change the state of the substance. 



From a consideration of the melting point curves, together with the 

 analytical and other observations we have described, it is possible to 

 form some idea as to the nature of these glasses and the crystals into 

 which they are wholly or partially transformable. 



The glass must be regarded as a superfused, and, therefore, metastable 

 form of the crystals, behaving in several respects as if it -were a liquid 

 of enormous viscosity. 



Now, on considering the behaviour of the glasses ranging in composi- 

 tion from pure boric anhydride to Na 2 0.6B 2 O 3 , it will be noticed that 

 the supposition of the existence of a borate of about the composition 

 Na 2 0.5B 2 3 , which is dissolved in boric anhydride, will explain the 

 observed facts. A mixture of composition Na 2 0.40B 2 O 3 cannot be 

 crystallised at all, and one of composition Na 2 0.16B 2 3 only partially 



