preparing Aluminium. 237 



When the aluminiutn is fused under chloride gf potassium, its 

 surface is not perfectly smooth, but exhibits minute concavities ; 

 with chloride of aluminium and sodium, this is not the case. 



The readiest method of preparing the chloride of aluminium 

 and sodium for this purpose, is by placing the mixture of alumina 

 and carbon in a glass tube as wide as possible, and in this a 

 tube of less diameter, open at both ends, and containing chloride of 

 sodium; if the spot where the mixture is placed be very strongly 

 heated, and that where the chloride of sodium is situated more 

 moderately, while a current of chlorine is passed through the 

 tube, the vapour of the chloride of aluminium is so eagerly 

 absorbed by the chloride of sodium, that no chloride of alu- 

 minium, or at most a trace, is deposited in any other part of the 

 tube. 



If the smaller tube be weighed before the operation, the amount 

 absorbed is readily determined. It is not uniformly combined 

 with the chloride of sodium, for that part which is nearest to 

 the mixture of the charcoal and alumina will be found to have 

 absorbed the most. 



I have varied in many ways the process for the preparation 

 of aluminium, but in the end* have returned to the one described. 

 I often placed the sodium at the bottom of the crucible, the 

 powdered kryolite above it, and then the chloride of potassium 

 above all. On proceeding in this manner, it was observed that 

 much sodium was volatilized, burning with a strong yellow flame, 

 which never occurred when the sodium was cut into thin slices 

 and placed in alternate layers with the kryolite, in which case 

 the process goes on very quietly. When the crucible begins to 

 get red-hot, the temperature suddenly rises, owing to the com- 

 mencement of the decomposition of the compound ; no lowering 

 of the temperature should be allowed, but the heat should be 

 steadily maintained, not longer, however, than half an hour. By 

 prolonging the process a loss would be sustained, owing to the 

 action of the chloride of potassium on the aluminium. Nor 

 does the size of the globules increase on extending the time even 

 to two hours ; this effect can only be produced by obtaining the 

 highest possible temperature. If the process be stopped, how- 

 ever, after five or ten minutes of a very strong heat, the produce 

 is very small, as the metal has not had sufficient time to con- 

 glomerate into globules, but is in a pulverulent form and burns 

 to alumina during the cooling of the crucible. No advantage 

 is gained by mixing the kryolite with a portion of the chloride 

 before placing it between the layers of sodium. Neither did 

 1 increase the product by using the chloride of sodium and 

 aluminium to cover the mixture instead of the chloride of 

 potassium. 



