358 On the non-evistence of polarizing Silica, etc. 
As these statements are contained in the last editions of each 
of the above-mentioned works, it is evident that no contradiction 
of the error involved in them has been pointed out; yet, notwith- 
standing the high anthority on which they rest, the statements 
so far as the polarizing action of the silica is concerned are 
wholly erroneous. If the cuticle of the above-mentioned plants 
is completely deprived of its carbonaceous tissues it will be found 
wholly devoid of action on polarized light, and any preparation 
of the cuticle which is found to affect polarized light will also 
be found to blacken when heated in concentrated sulphuric acid, 
and if then decarbonised by throwing into the hot acid solution 
a little chlorate of potassa, the residual silica shows no signs 0 
action under the polariscope, either alone or with the —_ plate, 
although it still retains the forms of the cells, stomata, 
It is clear then that the error in the above statements a been 
caused by the imperfect removal of the dense carbonaceous tis- 
sues which are deposited beneath the silica. I have examined 
several species of Equisetum and a large number of plants of the 
grass tribe which are most remarkable ‘for their siliceous cuticles, 
but have found no trace of any action upon polarized light, when 
the carbonaceous matter was removed. But it is unnecessary to 
resort to artificial preparations to prove the correctness of my 
statements, Nature has made her own preparations, and depos- 
ited them by myriads beneath every peat bog, where may be 
found not only the siliceous shells of the Diatoms, and the spic- 
ules of the fresh-water sponges, but also a large number of the 
siliceous parts of the grasses, sedges, &c. Ehrenberg has shown, 
(Berlin Monthly Reports, May, 1848) and I can confirm his state- 
ments, that the silica in these Phytolitharia, as well as in the 
Diatomacez, Polycistineze and Spongiolites is not doubly refrac- 
tive. He makes an exception in the case of the shell of Arach- 
noidiscus, but my own experiments prove that when properly 
cleaned this shell "fans no exception. As I have shown above 
that the silica in the cuticle of the Equisetum and grasses, agrees 
with that in the lower tribes in characters, I think the ‘conclt- 
sion is warranted, that doubly refractive silica has no existence 
in the organic wor 
