THE PREPARATION OF DIATOMS FROM THE LONDON CLAY. 179 
of the water with such particles as will rise with it. Blow this out 
of the pipette on to a slide, or into another watch glass, and dry off 
the water. The partially transparent diatoms will be found chiefly 
on the surface. 
“These light forms are not nearly so abundant as the more 
thoroughly mineralized ones, and are not always to be found in 
clay containing the others.” 
This subject will be found of extreme interest to the chemist, 
as it is left for him to determine whether the pyrites has been 
deposited upon the siliceous frustule, or, what is very improbable 
from a chemical point of view, replaced it. Mr. Kitton thinks the 
latter. He writes :— 
“This metallic appearance is undoubtedly caused either by the 
deposition of iron pyrites (ferric bisulphide) upon the siliceous 
skeleton, or it has replaced atom by atom the original silica of the 
diatom frustule. I was at first inclined to the former supposition, 
and which the appearance of the valve seemed to confirm ; the 
lustre-less somewhat granular character of the exterior, and the 
smooth polished interior surfaces forcibly reminding one of the 
electro-galvanic deposition of metals. Acting on this supposition, 
I thought it might be possible to divest the silica of its metallic 
coating, and thus be able to ascertain the species with greater 
certainty. To effect this, I submitted some of the material to the 
action of boiling nitric acid. ‘This very effectually removed the 
pyrites, and was equally effectual in the destruction of the diatom. 
Supposing that this treatment was too violent, I placed some 
valves and frustules in an excavated slide with some very dilute 
acid, and placed over them a thin cover glass, and watched the 
action under a $ objective. In the course of a short time I saw 
the valves become more and more transparent, and at last dis- 
appear, leaving only a faint yellow stain in the acidulated water. 
I may here observe that I could detect no symptom of effervescence 
upon the surface or around the edges of the valves or frustules. 
This experiment, I think, fully justifies the supposition that the 
silica has been replaced by, rather than that a deposit of pyrites 
had taken place.” 
The diatom frustule is an exceedingly thin and fragile body, 
and it may be that a coating of pyrites has been deposited upon it 
by the reducing action of the organic contents upon a sulphate of 
iron. The coating may, perhaps, have adhered so firmly as to 
break up the valves into innumerable fragments when digested 
with nitric acid. It is evidently a question for the chemist, and 
one which is worthy of solution. 
