386 Transactions of the Societi/. 



a series of black dots were very distinctly visible. These valves, 

 when mounted in balsam, became partially transparent ; the 

 umbilicus and dots were evidently openings or perforations in the 

 valve ; the solid metallic framework was thin enough in some cases 

 to allow the light to pass through it, giving the diatom the aspect 

 of a dark indigo-coloured disk with distant minute apertures. 



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 

 efiect 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 disappear, 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 the supposition that the silica has been replaced by, rather than 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 way in which this substitution of one substance for 

 another takes place has, I believe, never been explained. 



I have occasionally observed, both on the outer and inner sur- 

 faces of the valves, a curious deposit of small globules of pyrites, 

 resembling minute (about ^oVo of ^^ ii^cb in diameter) pewter balls 

 or marbles. 



One of the samples (I think that from Prittlewell) was very 

 rich in diatoms, most of which were in a highly pyritized condition, 

 so much so that the markings are often but indistinctly visible. On 

 submitting them to the action of dilute nitric acid, the diatoms 

 slowly dissolved, but did not become trans])arent. 



Although I have made a careful search in the London clay for 

 diatoms in their original state, I have not succeeded in finding 

 them. In the lightest portions I occasionally find minute frag- 



* I liavc! tried the same cxiiciiinciit upon the pyritized sponge spicules and 

 fragments of wood associated with the diatoms and in tlie same condition, with a 

 fciraihu- result. 



