THE OKIGIN OF LU^AR VOLCANOES. 103 



surface. So long as we accept the generally received belief 

 that water is a necessary agent in the evolution of volcanic 

 forces, the difficulties presented by the lunar surface are rather 

 increased than diminished by further examination and specu- 

 lation. 



We know that the lava, scoriae, dust, and other products of 

 volcanic action on this earth are mainly composed of mixed 

 silicates those of alumina and lime preponderating. When 

 we consider that the solid crust of the Earth is chiefly coin- 

 posed of silicic acid, and of basic oxides and carbonates which 

 combine with silicic acid when heated, a natural necessity for 

 such a composition of volcanic products becomes evident. 



If the Moon is composed of similar materials to those of 

 the Earth, the fusion of its crust must produce similar com- 

 pounds, as they are formed independently of any atmospheric 

 or aqueous agency. 



This being the case, the phenomena presented by the cooling 

 of fused masses of mixed silicates in the absence of water 

 become very interesting. Opportunities of studying such phe- 

 nomena are offered at our great iron-works, where fused masses 

 of iron cinder, composed mainly of mixed silicates, are contin- 

 ually to be seen in the process of cooling under a variety of 

 circumstances. 



I have watched the cooling of such masses very frequently, 

 and have seen abundant displays of miniature volcanic phenom- 

 ena, especially marked where the cooling has occurred under 

 conditions most nearly resembling those of a gradually cooling 

 planet or satellite ; that is, when the fused cinder has been 

 inclosed by a solid resisting and contracting crust. 



The most remarkable that I have seen are those presented by 

 the cooling of the " tap cinder" from puddling furnaces. 

 This, as it flows from the furnace, is received in stout iron 

 boxes ("cinder-bogies") of circular or rectangular horizontal 

 section. The following phenomena are usually observable on 

 the cooling of the fused cinder in a circular bogie. 



First a thin solid crust forms on the red-hot surface. This 

 speedily cools sufficiently to blacken. If pierced by a slight 

 thrust from an iron rod, the red-hot matter within is seen to 

 be in a state of seething activity, and a considerable quantity 

 exudes from the opening. If a bogie filled with fused cinder 

 is left undisturbed, a veritable spontaneous volcanic eruption 

 takes place through some portion, generally near the centre, of 

 the solid crust. In some cases this eruption is sufficiently 



