364 LETTERS ON NATURAL MAGIC. 



hydrogen, the diluted acid does not touch the zinc, and 

 consequently is prevented from producing any more gas. 

 The instant, however, that any gas is let off at c, the pres- 

 sure of the fluid in the globe A B, and tube m, n o p, over- 

 comes the elasticity of the remaining gas in C D, and 

 forces the diluted acid up to the zinc o p, so as to enable 

 it to produce more gas to supply what has been used. 



The lamp being supplied with hydrogen in the manner 

 now described, it is used in the following manner : The 

 spongy platinum in P being brought near c, the cock d 

 is turned, and the gas is thrown upon the platinum. An 

 intense heat is immediately produced, the platinum be- 

 comes red hot, and the hydrogen inflames. A taper is 

 then lighted at the flame, and the cock d is shut. Pro- 

 fessor Gumming of Cambridge found it necessary to cover 

 up the platinum with a cap after every experiment. This 

 ingenious chemist likewise found that, with platinum 

 foil the 9000dth part of an inch thick kept in a close tube, 

 the hydrogen was inflamed ; but when the foil was only the 

 GOOOdth 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 decomposition 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 genrated, 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 desolat- 

 ing earthquakes ; or, forcing their way through the super- 

 ficial crust of the globe, they waste their fury in volcanic 

 eruptions. 



