CELLS. SILICA. 
57 
defining qualities of a microscope, but, in some instances, 
their minuteness is such as even to defy the highest 
magnifying powers we at present possess, to render 
them visible. 
The Diatomacece inhabit both salt and fresh water ; 
the Desmidiece fresh water only ; these last have lately 
been classified and arranged by a member of our own 
profession, Mr. John Ralfs, and in his excellent work you 
will find all that relates to their structure and mode of 
development. If the Diatomacece be animals, the 
Desmidiece are certainly vegetables, Mr. Ralfs having 
detected in them the presence of starch. 
The Diatomacece possessing siliceous skeletons, 
require a brief notice ; a good example is the Isthmia 
obliquata found on our coasts, attached to fuel , and 
remarkable for the elegance of its form and mark- 
ings. A single shell is represented by c in Fig. 43. 
The Isthmia , although having a siliceous shell 
or covering, is claimed by the botanists, and the first 
figures of it were given in Sowerby’s “ British Bo- 
tany.” Many of the Diatomacece , on account of their 
silica, are not only employed largely in the arts, as a 
material for polishing metal and other hard structures, 
but in the form of a light white powder, resembling 
magnesia, are said to be used as an article of food, being 
known in Germany as the berg-mehl , or mountain-meal. 
The berg-mehl consists principally of the remains of 
naviculce , one of which is represented by B, in Fig. 43. 
In certain parts of the world, as in Germany, and 
