Geology and Natural History. 231 



tube is at first very thin, and through its walls tin- ascending acid 

 continues to act upon the surrounding silicate, the walls of the 

 tul.c in consequence constantly growing in thickness by the depo- 

 sition of additional silica on its outer surface. As long as the flow 



|»>-it of successive layers, and tin 



result is a hollow gy 



" The carbonic acid evolved from the carbonate in the solution is 

 essential to the successful commencement of the tube, but when 



this is once formed, the sulphuric or other acid can be itself forced 



thii'l in the pipette, the action can be kept up for a long time, and 

 stalactites ,,f ; inch in diameter can be formed with little difficulty. 



air highly charged with an acid.' through the alkaline silicate 



