A few minutes afterwards the odour of sulphuretted hydro- 
gen was distinctly observed. To prove the absence of sul- 
phuretted hydrogen in laboratory air, a similar paper was 
placed over a jar containing the infusion of ultramarine in 
distilled water, which remained unchanged during the 
experiments. 
These experiments, though qualitative, show that carbon 
dioxide in aqueous solution, viz. carbonic acid, can attack 
ultramarine, and therefore the latter is not a safe indicator 
of free hydrochloric acid in presence of carbonic if the for- 
mer exist in very small quantity. 
“On Siliceous Fossilization,” by J. B. Hannay, F.R.S.E., 
Assistant Lecturer on Chemistry in the Owens College, 
communicated by Professor Roscoe, F.R.S., &c. 
My friend Professor Young having mentioned to me that 
certain questions connected with fossilization were in a 
very undecided state, and having expressed his opinion that 
a chemical examination was realty what was required to 
throw light on the question, I obtained from him some 
siliceous fossils in which he was interested, and of which he 
and Mr. J. Young, F.G.S., have published an account,* 
and proceeded to make an examination of them by 
chemical means. The rods were samples of Hyalonema 
Smitliii, and, as they were in the fossil state siliceous, 
and probably had been so in the original state, I took 
for comparison in my examination of them, some rods 
of a recent siliceous sponge. On examination of the fossil 
rods it was seen that they might roughly be divided 
into two classes : I. rods partly transparent or opalescent, 
and containing some chemically combined water, and 
II. rods opaque, and apparently consisting of anhy- 
drous crystalline silica, both classes, however, containing 
many samples stained brown by ferric oxide. Thus I gen- 
# Annals of Nat. Hist., Yol. 20, No. 119, P. 425. 
