Masterpieces of Science 



in the volcanic pipe, and may eventually run 

 over the lip of the crater, or force its way through 

 cracks in the hill, forming red-hot streams which 

 generally present a consistence something like 

 that of treacle. These lava-torrents are often of 

 great magnitude; thus, it was estimated that in 

 the famous eruption of Skaptar Jokul, in Iceland, 

 in 1783, the mass of lava brought up from sub- 

 terranean regions was equal to the bulk of Mont 

 Blanc. The lava rapidly cools on the surface, 

 though long retaining its heat beneath the pro- 

 tecting crust; and, ultimately, the entire mass 

 solidifies, forming a hard rock, more or less like a 

 slag from an iron furnace. In different speci- 

 mens, however, the lava exhibits great variations; 

 some being dark-coloured and comparatively 

 heavy, while others are lighter in colour and 

 much less dense; in some cases the rock is com- 

 pact, while in others it is spongy or cindery, 

 when it is said to be scoriaceous. The little 

 cavities, or vesicles, in this scoria or cellular lava, 

 are formed by the disengagement of bubbles of 

 gas or vapour, when the matter is in a pasty con- 

 dition; just as the porous texture of a piece of 

 bread is due to the presence of bubbles of gas 

 evolved by the fermentation of the yeast. The 

 stone largely used for scouring paints under 

 the name of pumice is a lava of very porous 

 texture; its name recalling its origin as the froth 

 or scum of lava. Sometimes, the masses of lava, 

 which are tossed into the air, are rotated during 

 their flight, and fall as more or less rounded 

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