101 
heated with water in a tube to 150° for five or six hours, 
the side of the rod next the glass became dissolved, this 
action seeming to be due to the solvent action of the soda 
of the glass, as water at that temperature decomposed the 
glass, removing the alkali. Fig. 9 shows a rod so treated, 
and Fig. 13 shows a section of that rod through the dotted 
line in Fig. 9. This action clearly shows at least four 
layers in the rod. Fig. 15 shows the action of water con- 
taining 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, which, as my former ex- 
periments 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 fluo- 
ride solution, sealed and placed in beside the nodule, and 
the larger tube was then sealed off and heated in the air 
bath to 150° C. In rather less than half an hour the am- 
monium 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 hydrated and 
whilst in this state was etched by the fluorine. My reason 
for wishing the etching to go on after the hydration and 
without removing the rod or cooling it, was that when the 
rod is removed from the superheated water it begins im- 
mediately 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. 
