318 LETTERS ON NATURAL MAGIC. 



lighted at the flame, and the cock d is shut. 

 Professor Gumming, of Cambridge, found it 

 necessary to cover up the plantinum with a cap 

 after every experiment. This ingenious chemist 

 likewise found, that, with platinum foil the 

 9,000th part of an inch thick kept in a close 

 tube, the hydrogen was inflamed ; but when the 

 foil was only the 6,000th of an inch thick, it was 

 necessary to raise it previously to a red heat. 



Spontaneous combustion is a phenomenon 

 which occurs very frequently and often to a great 

 extent within the bowels of the earth. The heat 

 by which it is occasioned is produced by the de- 

 composition of mineral bodies and other causes. 

 This heat increases in intensity till it is capable 

 of melting the solid materials which are exposed 

 to it. Gases and aqueous vapours of powerful 

 elasticity are generated, new fluids of expansive 

 energy imprisoned in cavities under great pressure 

 are set free, and these tremendous agents, acting 

 under the repressing forces of the superincumbent 

 strata, exhibit their power in desolating earth- 

 quakes ; or, forcing their way through the super- 

 ficial crust of the globe, they waste their fury in 

 volcanic eruptions. 



When the phenomena of spontaneous combus- 

 tion take place near the surface of the earth, its 

 effects are of a less dangerous character, though 

 they frequently give birth to permanent confla- 

 grations, which no power can extinguish. An 

 example of this milder species of spontaneous 

 combustion has been recently exhibited in the 

 burning cliff at Weymouth ; and a still more 

 interesting one exists at this moment near the 

 village of Bradley, in Staffordshire. The earth 



