94 
Gregory on Diatomaceous Earth. 
the mass. These fragments form an excellent polishing 
powder, which may be had of various degrees of fineness. I 
find it best, except in the case of the very purest specimens, 
first to ignite the earth over the spirit-lamp in a platinum 
capsule, till the black colour first caused by the action of 
the heat on the organic matter present is burned off, and the 
earth is again nearly white. I then digest it for some hours 
in strong nitromuriatic acid, which removes the iron, and, 
after washing away the acid, press the lumps in water gently 
with the finger till the whole is diffused in the water. It is 
then elutriated as usual, to separate on the one hand the 
coarse sand, if any be present, and, on the other, the com- 
minuted fragments. The slides now offered to the Society 
were prepared in this way from earth of but middling quality, 
my supply of the purest having been very small and long ago 
exhausted ; while the deposit being at present, and for months 
past, flooded, it is impossible to procure a fresh supply of 
the purest earth. 
In endeavouring to identify the species present in this 
earth, I found the greatest difficulty from the want of any 
work containing figures of all the known species. The only 
figures I could procure were those of Ehrenberg*s Atlas, 
1838, and those of the last edition of ' Pritchard's Infusoria.' 
The former, of course, does not contain the very numerous 
species added to the list since 1838, and the latter has sel- 
dom more than one or two species in each genus. I had also 
Kiitzing's ' Species Algarum,' without any figures. But I was 
able, after studying a good many slides of excellent quality, 
to distinguish somewhere about 65 forms, although I could 
not with any confidence name above one half of the number. 
Under these circumstances, I ventured to apply to the Rev. 
W. Smith, to whom I was fortunately able to send an excel- 
lent specimen of the earth. That distinguished naturalist 
had the very great kindness, in spite of his absorbing occupa- 
tions, to examine the earth, and to send me the following list 
of species which he has detected in the specimens sent. The 
names are those adopted in his forthcoming synopsis : — 
Pinmilaria major 
acuminata 
„ oblonga 
„ vmdis 
„ divergens 
„ acuta 
radiosa 
mesolepta 
interrupta 
„ Tabellaria 
„ gibba 
Pinnularia gracilis 
„ lata 
,, alpina 
Navicula serians 
„ rbomboides 
„ ovalis 
„ dicephala 
„ firma 
angustata 
Gomphonema acuminatum 
cruciatum 
