Gregory on Diatomaceous Earth. 
97 
may be an immature one, but to what are we to refer it? It 
differs from Himantidium Arcus and Eunotia gracilis in the 
number of striae, and Mr. Smith thinks it must stand near 
Eunotia Arcus, Kvitz.= JVavicida Arcus, Ehr. It is not, how- 
ever, that species, nor is Mr. Smith sure that it is of that 
genus. He is to examine it more fully, and the matter is 
therefore in good hands. I may add, that while it has a 
general resemblance to small specimens of Himantidium 
Arcus, or of other allied species, it does not commonly occur 
where these are abundant. I have looked at a number of 
Diatomaceous earths, in many of which there were all the 
common species of Eunotia and Himantidium, but have only 
seen this form in one, namely, in a slide prepared by Mr. 
Topping, and labelled " from the banks of the Spey." This 
slide has many things in common with the Mull earth. Any 
of the slides sent with this paper will exhibit numerous 
examples of this form. 
I have further to add, that an average specimen of the Mull 
earth, on being analysed, was found, after being dried at 
212°, to be composed of — 
Silica - - - - - - 70*75 
Protoxide of iron, containing traces of phosphoric 
acid and manganese - - - - 15 '04 
Organic matter - - - - - 12-36 
Loss, chiefly water - - - - 1-85 
100-00 
The iron is here stated as protoxide, but if calculated as 
peroxide, would amount to 16-69 per cent. Some of it cer- 
tainly is in the latter form from the action of the air, and the 
brown colour, and this diminishes the loss, but I have stated 
it as protoxide, because I believe it to be in that state before 
the air has access to it. The presence of phosphoric acid, 
which was easily detected in the oxide of iron, by the use of 
molybdata of ammonia, is interesting. It is most probably 
derived from the organic matter of the Diatomaceae, but I am 
not aware that its presence has been yet observed in any in- 
fusorial earth. I have not determined the proportion of phos ^ 
phoric acid, which, although small, is appreciable. The earth 
contains neither lime nor magnesia. 
It is probable that this earth may be useful as a manure 
from the finely divided silica, the organic matter, and the 
phosphoric acid it contains. Professor Bailey ascribes the 
fertility of certain districts in America to the abundance of 
infusorial remains on tlie soil, so that the experiment is worth 
trying. 
