32. Prof. H. Rose on the different States of Silicic Acid, 
where p is the density, / the mean length of path of a particle, 
and v the mean velocity v= al mia /- ae 
Vo T 
ee (Ss ae 
ao 3K (25) 
Now Professor Stokes finds by experiments on air, 
VAs 
p 
If we suppose “k=930 feet per second for air at 60°, and 
therefore the mean velocity v=1505 feet per second, then the 
value of /, the mean distance travelled over by a particle between 
consecutive collisions, = zzy7'55 9th of an inch, and each particle 
makes 8,077,200,000 collisions per second. 
A remarkable result here presented to us in equation (24), is 
that if this explanation of gaseous friction be true, the coefficient 
of friction is independent of the density. Such a consequence of 
a mathematical theory is very startling, and the only experiment 
Ihave met with on the subject does not seem to confirm it. We 
must next compare our theory with what is known of the diffusion 
of gases, and the conduction of heat through a gas. 
[To be continued. | 
VI. On the different States of Silicic Acid. By M. H. Rosg*. 
AO aed determinations of the density of silicic acid, 
and especially those of Count Schaffgotsch, prove that 
there exist two distinct modifications of this acid, one of which 
has a density of 2-6, whilst in the other the density rises to 2-2, 
or 2°3. The first is always crystallized, or more or less crystal- 
line, the second always amorphous. 
Crystallized silica is found not only in rock-crystal, quartz, 
amethyst, sandstone, and quartzose sand, but also in a great 
number of the varieties of silica, in appearance compact, but 
really formed of an aggregation of crystalline particles, as their 
property of polarizing light proves—such are chalcedony, chryso- 
prase, jasper, flint, and certain siliceous woods. Some of these 
varieties may contain traces of water or foreign matter, which 
make their density vary a little, without, however, causing the 
same to fall below 2°6. 
The chemical and physical properties of all these substances 
are exactly the same. If crystallized quartz seems to resist some- 
* The original memoir by Prof. H. Rose will be found in Poggendorff’s 
Annalen, September 1859. The present abstract is translated from the 
Bibliotheque Universelle for Sept. 20th, 1859. 
