On the elementary Particles of certain Crystals. 61 
» Examination by dissection. ~The sac was formed by an 
expansion of the membrane of the spinal marrow, passing 
between the spinous processes of the vertebra. The nerves 
passed from the spinal marrow across the cavity of the sac, 
and were distributed by many filaments on its internal sur- 
face. There was not any inflammation on the sac or 
nerves. 
The bones of the head were much separated; and a pint 
and a half of a very limpid and pale-coloured serum was found 
in the lateral ventricles. There were no other marks of 
disease either in the brain, its membranes, or blood-vessels. 
When did the hydrocephalus take place? Was it at the 
time in which the child’s health began to decline, i. e. 
fourteen days before his death ? or, Was the morbid con- 
dition Of the brain and spinal marrow coeval ? 
Greville Street, Hatton Garden, June 13, 1813. 
XIU. The Bakerian Lecture. On the elementary Particles of 
certain Crystals. By WitLt1am Hypz Wo ttasron, 
M.D. Sec. R.S.* 
Aone the known forms of crystallized bodies, there is 
no one common to a greater number of substances than the 
regular octchedron, and no one in which a corresponding 
difficulty has occurred with regard to determining which 
modification of its ‘form js to be considered as primitive; 
since in all these substances the tetrahedron appears to have 
equal claim to be received as the original from which all 
their other modifications are to be derived. 
The relation of these solids to each other is most di- 
stinctly exhibited to those who are not much conversant 
with crystallography, by assuming the tetrahedron as pri- 
mitive, for this may immediately be converted into an oc- 
tohedron by the removal of four smailer tetrahedrons from 
jts Solid angles. (Plate I. fig. 1.) : 
‘The substance which most readily admits of division by 
fracture into these forms is fluor spar; and there is-‘no dif- 
ficulty in obtaining a sufficient quantity for such experi- 
ments. But it is not, in fact, either the tetrahedron or the 
octahedron, which first presents itself as the apparent pri- 
mitive form obtained by fracture. 
If we form a plate of uniform thickness by two succes- 
sive divisions of the spar, parallel to each other, we shall 
find the plate divisible into prismatic rods, the section of 
-* From the Philosophical Transactions for 1813, part i. 
which 
