MAX SCHULTZE, ON THE DIATOM-VALVE. 123 
with other siliceous bodies of clearly amorphous nature, such 
as sponge-spicules, Polycystina, &c., as to render their sepa- 
ration impossible. 
But how is it that, according to Ehrenberg and others, the 
diatom-valves do not possess double refraction, like crystallized 
silex? In reply to this, it may be said that Hugo von 
Mohl has lately asserted,* in opposition to previous state- 
ments, that, with the very essential improvement in the 
polarizing apparatus introduced by him, P. angulatum, with 
its hexagonal spots, appears so distinctly doubly refractive, that 
even the hexagons on the surface may be perceived with the 
Nicol’s prism, and consequently upon a dark field. 
It may be added that the artificial siliceous pellicles are 
very distinctly doubly refractive. Thus, on this account, 
there is nothing opposed to the explanation of the structure 
of certain diatomaceous valves, as being due to crystallization 
of the silex. And this supposition is materially supported 
by the results of the experiments above related, with respect 
to the specific gravity of the pellicles. These results are, 
at any rate, compatible with the notion of a mixture of 
erystallized and amorphous silex in those bodies. 
But further investigation rendered the correctness of this 
assumption in the highest degree doubtful, and it soon be- 
came quite certain that neither in the artificial siliceous 
pellicles nor in the diatom valves are the peculiar forms due 
to a crystalline structure. 
In the first place, it clearly appeared that the pellicles 
in question are not, as it was at first supposed, pure silex. 
It was manifest further that—(1). These pellicles contain a 
constant quantity of fluorin or of fluoride of silicium. When 
the latter was expelled by a red heat, the substance was found 
at once to possess the low sp. gr. of amorphous silex. (2). 
The phenomena of double refraction afforded by them are 
not the same as those exhibited by rock crystal; that is to say, 
not like those presented in a body with a positive axis of 
double refraction, but resembling what is seen in substances 
with negative double refraction. (3). The appearances de- 
seribed by H. von Mohl, and confirmed by Valentin, as being 
due to double refraction, exhibited in certain diatoms, are 
not phenomena due to double refraction at all, but are to be 
referred to depolarization by refraction. They are no longer 
visible when the valves are immersed in Canada balsam or 
other medium possessing a similar refractive power to silex. 
The author then proceeds to detail the methods by which 
* Poggendorff’s ‘ Annalen,’ 1859, Bd. eviii, pp. 179—185; ‘ Botanische 
Zeitung,’ 1858, p. 10. 
