1901.] on Metals as Fuel. 501 



oxide of the metal to be reduced is intimately mixed with finely- 

 divided aluminium, and heated in magnesia-lined crucibles. The 

 heat produced by the oxidation of aluminium during the operation is 

 sufficient to fuse alumina, a specimen of which is exhibited." 



The subject is, however, in a sense your own, for, as far as I 

 know, the lecture on " The Rarer Metals and their Alloys," * which I 

 delivered here in 1895, was the first occasion on which the reducing 

 action of aluminium was demonstrated on a comparatively large scale, 

 and covered an extended series of metallic oxides. Since that time 

 great progress has been made, the most noteworthy advance being in 

 the direction of the use of aluminium for the sake of the heat afforded 

 by its combustion as a true fuel, the oxygen being derived, not from 

 the air, but from a metallic oxide. In order that I may be clear, let 

 me repeat that when coal is burnt the oxygen is derived from the air. 



Fig. 1.— The oxidation in air of an amalgamated wire of alumi- 

 nium, E F. The films of alumina, A L> and C D, are those 

 which first formed on the wire. 



When aluminium is used as a fuel the oxygen is derived from a 

 metallic oxide, the metals change places: the aluminium is oxidised, 

 and the other metal set free from its oxide. This part of the subject 

 must be carefully approached, and the question at once arises as to 

 what extent the aluminium must be heated before it will begin to 

 abstract oxygen from air or from an oxide. It is well known that the 

 metal aluminium will not oxidise sensibly in the air at the ordinary 

 temperature, but the presence of a little mercury enables it to oxidise 

 readily. Le Bon f has shown how minute the quantity of mercury 

 may be. This wire of aluminium to which a thermo-couple is attached 

 will, if a mere trace of mercury be rubbed on its surface, become 

 rapidly heated by oxidation, the temperature rising to 102° C, while 



* R. Inst. Proc. vol. xiv. p. 497. ' Nature,' vol. lii. pp. 14 and 39. 

 t Comptes rendus, vol. cxxxi. p. 707. 



