DOBEBEINER'S LAMP. 315 



flammation commences near the centre of the 

 mass, at the depth of five or six inches beneath 

 its surface, and at this spot the temperature is 

 always higher than at any other. Black charcoal, 

 strongly distilled, heats and inflames more easily 

 than the orange, or that which is little distilled, 

 or than the charcoal made in boilers. The most 

 inflammable charcoal must have a mass of at 

 least 661bs. avoirdupois, in order that it may be 

 susceptible of spontaneous inflammation. With 

 the other less inflammable varieties, the inflam- 

 mation takes place only in larger masses. 



The inflammation of powdered charcoal is more 

 active in proportion to the shortness of the inter- 

 val 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 inter- 

 esting 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 pru- 

 dent to leave these mixtures in too large masses 

 after trituration.* 



A species of spontaneous combustion, perfectly 

 analogous to that now described, but produced 

 almost instantaneously, was discovered by Pro- 

 fessor 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 



* See Edinburgh Journal of Science, New Series, No. 

 viii. p. 274. 



