98 
erally experimented with three different kinds, which shall 
be referred to as 
I. Recent quite transparent. 
II. Fossil opalescent. 
III. Fossil opaque. 
To find the general differences displayed by the three 
kinds, several experiments with various chemicals were 
first performed, placing all the three kinds in the same 
vessel under the same circumstances. On placing the three 
kinds in hydrofluoric acid it was seen that the recent rod 
was dissolved in about 80 minutes, the opalescent in about 
two hours, and the opaque rod only partially dissolved after 
four hours immersion in the acid. Sometimes the opaque 
rod dissolved irregularly, leaving protruberances, and some- 
times leaving a spongy substance covering the still undis- 
solved interior. On heating the three kinds in a four per 
cent, solution of caustic soda in a sealed tube to a tempera- 
ture of 110° C., so that they were under pressure, the recent 
rods dissolved in about 10 minutes, No. II. was nearly all 
dissolved in 80 minutes, leaving only a thin core of anhy- 
drous silica, while after two hours No. III. was only 
irregularly eroded, and in some places seemed very little 
affected. On heating the three specimens to 100° with 
water for two days very little change was observed, in fact, 
water at the boiling point, was found to be without practical 
effect, but when heated to 150° C. in a sealed tube, so that it 
was under a pressure of several atmospheres, the water had 
the effect of makingNo. II. much more transparent. It seemed 
then that at this high pressure that the substance took up 
water, and so was rendered more similar to the recent sponge 
rods. I found, however, that if a substance like ether, 
which boils at a much lower temperature, were introduced 
into the tube above the water, so that the pressure would 
increase much more rapidly than with water, the rods might 
be hydrated at a lower temperature. On heating the rods 
