90 Mr. J. Davies's Attempt to assig?i the Cause 



from the wood employed by Mr. Hadfield, we have in his 

 manufacture the same operation and the same materials as 

 in M. Brunner's experiment, and may therefore expect the 

 same results. The only difference would be, that as M. Brun- 

 ner used much potash, he procured a large proportion of the 

 metallic base; while in the other case the potassium must be 

 in small quantity, because all the potash present would be 

 only that supplied by the wood subjected to distillation: and 

 yet, upon the whole, the quantity extracted under the latter 

 circumstances is not inconsiderable; for it is by the combus- 

 tion of such wood in America, where it is of comparatively 

 little value, that the potash is principally formed which is con- 

 sumed in the arts and manufactures in every part of Europe. 



It is manifest, therefore, that in the formation of Mr. Had- 

 field's charcoal, potassium must in small quantities be libe- 

 rated. 



Supposing the presence of potassium in fresh charcoal to 

 have been established, we have now to explain its operation ; 

 and this appears to be effected upon the assumption that the 

 metal lingers in the pores of the charcoal, incased, as it were, 

 in the substance, until it be at length exposed to the action of 

 atmospheric air and aqueous vapour. This view of the sub- 

 ject derives plausibility from the facts, that the combustion 

 does not commence at any considerable depth below the sur- 

 face ; and that when a thermometer is introduced into the 

 mass, the ignition generally originates in that place; that is 

 to say, the combustion occurs exactly where it might be ex- 

 pected, since it takes place at the part which, favourably situ- 

 ated, is most exposed to the action of the supporter of com- 

 bustion. 



This general view of the spontaneous combustion is directly 

 countenanced by the excellent paper of Colonel Aubert, in- 

 serted in the Bulletin des Sciences Militaires for January, 

 which Mr. Hadfield has afforded me the opportunity of con- 

 sulting. This ingenious foreigner shows by a number of de- 

 cisive experiments, that the absorption of air and moisture are 

 indispensable in the production of the phenomenon*. He 

 proves also (what Mr. Hadfield has in a different way very 

 clearly confirmed), that no carbonic acid is formed before the 

 incandescence occurs ; — a fact strictly in accordance with the 

 hypothesis which I have offered ; since, upon this hypothesis, 

 the oxygen of the air, instead of forming an acid with the 

 carbon, produces, by its superior affinity, an alkali with the 

 potassium. The next position which he establishes is, that 

 the carbon increased in weight in proportion to the quantity 



* See Phil. Mag. and Annals, N.S. vol. ix. p. 148.— Edit. 



