MR. J. B. HANNAY ON SILICEOUS FOSSILIZATION. 239 



water containing a small quantity of alkali at 150° on a 

 rod the end of which has dissolved to a considerable 

 extent, leaving a hard core of anhydrous silica Avhich, as 

 my former experiments showed, is unacted on by dilute 

 caustic soda. 



It occurred to me that if a section were first prepared by 

 grinding or splitting, and this section hydrated as much as 

 possible and then acted upon by the hydrofluoric acid, it 

 might show the state of the rod, as the hydration of the silica 

 would restore the rod to its original state, and the etching 

 action of the acid would show what that state was. 



The experiment was carried on in this way. A rod was 

 split by mechanical means and placed in a thick short tube 

 sealed at one end along with some water. A thin tube 

 much smaller than the other was filled with ammonium 

 fluoride solution^ sealed and placed in beside the nodule ; 

 and the larger tube was then sealed ofi" and heated in the 

 air-bath to 150° C. In rather less than half an hour the 

 ammonium fluoride dissolved its way through the thin 

 tube, and, escaping into the larger tube, etched the split 

 rod. By these means the silica of the rod was first hy- 

 drated, and whilst in this state was etched by the fluorine. 

 My reason for wishing the etching to go on after the hy- 

 dration and without removing the rod or cooling it, was 

 that when the rod is removed from the superheated water 

 it begins immediately to lose its water ; that is, the 

 hydration is only temporary. By the above method the 

 rod was not allowed to become dehydrated, but was etched 

 just as the hydration was going on. 



Figs. 8 and 12 are specimens obtained by this method ; 

 and they show the layer formation to perfection, as many 

 as fifteen layers being shown by one specimen. 



Fig. 12 is the end of fig. 8 ; and it shows the concentri- 

 city of the layers very plainly. 



