MAX SCHULTZE, ON THE DIATOM-VALVE. 121 
thus produced was first poimted out to the author by Pro- 
fessor Heintz, of Halle, some years ago. The silex is depo- 
sited in the form of thin-walled vesicles, of various dimen- 
sions and forms, as spherical, pyriform, or subcylindrical, and 
usually filled with air, so that they float on the surface of 
water. If some of the deposit be crushed between two 
pieces of glass, and examined with a power of about 3800 
diameters, a marking will be perceived on the outer or convex 
surface of many of the fragments of these vesicles similar to 
that of many Diatomacez, such as Pleurosigma, Coscino- 
discus, &c. Rounded elevations, more or less hexagonal at 
the base, and more or less regularly arranged, cover the sur- 
face of the siliceous pellicle, and not unfrequently this kind 
of marking is so regular as to give the fragments exactly the 
appearance of portions of diatomaceous valves. 
This remarkable circumstance struck the author very 
strongly, but the investigation of the subject was not pur- 
sued until his attention was again awakened to it by the 
appearance of a paper by H. Rose, “On the Different Condi- 
tions of Silicic Acid,’ in Poggendorff’s ‘ Annalen’ for 1860, 
p. 147, in which that chemist pointed out more fully than 
had previously been done the difference between amorphous 
and crystallized silex, with respect to their physical and 
chemical properties. 
Tn this paper especial stress was laid upon the difference 
in the specific gravity of the two forms as a diagnostic 
character between them, although the existence of such a 
difference had been, to some extent, previously known. 
In crystalline silex the sp. gr. is pretty constantly 2°6, 
whilst in the amorphous form it never exceeds 2°3, and is 
usually much under that, and even may not exceed 1°8. 
Under the circumstances, therefore, it became a matter of 
considerable interest to determine the specific gravity of the 
silex deposited in the way above mentioned. ‘The results of 
three experiments made by the author at different times, to 
determine this point, gave as the sp. gr. of this form of silex 
—2°437, 2°61, 2°58, or a mean of 2°54. 
The appearances presented under the microscope by the 
siliceous pellicles were such as to suggest that they were due 
possibly to crystallization. The minute elevations on the sur- 
face, when viewed on the side, often appear sharply acuminate, so 
as readily to convey the impression that they are formed by 
minute crystals of silex ; and this impression is strengthened 
at first sight by their sharply defined, hexagonal bases, when 
viewed vertically. The circumstance, again, that these eleva- 
