362 LETTERS ON NATURAL MAGIC. 



other less inflammable varieties, tlie inflammation takes 

 place only in larger masses. 



The inflammation of powdered charcoal is more active 

 in proportion to the shortness of the interval between its 

 carbonization and trituration. The free admission of air 

 to the surface of the charcoal is also indispensable to its 

 spontaneous combustion. 



Colonel Aubert, to whom we owe these interesting 

 results, likewise found that when sulphur and saltpetre 

 are added to the charcoal, it loses its power of inflaming 

 spontaneously. But as there is still an absorption of 

 air and a generation of heat, he is of opinion that it would 

 not be prudent to leave these mixtures in too large masses 

 after trituration.* 



A species of spontaneous combustion, perfectly analo- 

 gous to that now described, but produced almost instan- 

 taneously, was discovered by Professor Dobereiner of 

 Jena in 1824. He found that when a jet of hydrogen gas 

 was thrown upon recently-prepared spongy platinum, the 

 metal became almost instantly red hot, and set fire to the 

 gas. In this case the minutely-divided platinum acted 

 upon the hydrogen gas, in the same manner as the minutely- 

 divided charcoal acted upon common air. Heat and com- 

 bustion were produced by the absorption of both gases, 

 though in the one case the effect was instantaneous, and in 

 the other was the result of a prolonged absorption. 



This beautiful property of spongy platinum was happily 

 applied to the construction of lamps for producing an 

 instantaneous light. The form given to the lamp by 

 Mr. Garden of London is shown in the annexed figure, 

 where A B is a globe of glass, fitting tightly into another 

 glass globe C D by a ground shoulder m n. The globo 

 A B terminates in a hollow tapering neck m n o p, on the 



* See Edinburgh Journal of Science, Xew Scries, No. viii. p. 274. 



