124 MAX SCHULTZE, ON THE DIATOM-VALVE. 
the chemical composition of the pellicles was determined by 
Professor Landolt, for which we must refer to the original, 
and goes on to discuss their doubly refractive property above 
referred to. The existence of this property in them is readily 
shown by means of the common polarizing apparatus, although 
for the closer study of the phenomena it is necessary to em- 
ploy H. von Mohl’s illuminating lens in connection with the 
lower Nicol’s prism. 
Owing to the great differences presented by these pellicles 
as regards thickness, it is obvious that the phenomena of 
double refraction will vary very much in distinctness. In the 
thinnest, most delicate, and immeasurably fine pellicles, they 
can scarcely be perceived at all; whilst, on the other hand, 
it increases in distinctness in proportion to the thickness. 
The vesicles best fitted for examination in this regard are 
those of a nearly spherical form, with thick walls, on whose 
surface minute elevations are seen arranged in regular rows, 
as shown in fig. ] (Pl. VIII, 8). A hollow sphere of this kind 
appears under a Nicol’s prism, as an illuminated ring ona 
black ground; the width of the ring differing according to. 
the thickness of the wall. The ring is subdivided into four 
quadrants by a black cross. It exhibits no colours. As re- 
gards the elevations on the surface, it is obvious that those 
situated in the centre of the sphere, and consequently whose 
points are presented to the observer, exhibit no indication of 
double refraction, whilst those at the periphery of the sphere, 
nearer the light, possess the power of double refraction the 
more distinctly the more nearly the direction of their axis 
approaches the horizontal. 
The author then proceeds at considerable length to com- 
pare the phenomena of double refraction exhibited in the 
artificial siliceous vesicles with what is observed in other sili- 
ceous bodies or minerals, such as opal, hyalite, different va- 
rieties of siliceous sinter, silicified woods, &c. The general 
result at which he arrives is that, as regards structure and 
optical property, there are two kinds of amorphous siliceous 
minerals—1l, those which are perfectly homogeneous, and 
which exhibit no indication whatever of double refraction, of 
which the precious opal is a good example; and, 2, those 
whose structure presents a fine, concentric lamination, and 
which form spherical or botryoidal masses. These exhibit 
double refraction, which property is connected with the lami- 
nated structure, and is always negative. To this class belong 
hyalite, siliceous sinter, and other allied minerals. 
The production of these phenomena, as the consequence of 
the unequal tension of the different layers, is then very clearly 
