250 
THE GEOLOGIST. 
degrees of tiiieness, is rctjuired. (See figures.) This sieve may be of 
block-tiu or ziac, the latter is preferable. The best consists of two 
cylinders abont three inches across, and rather more liigh, one fitting 
within the other (a ande^; each having its lower edge iu-turncd for 
about an eighth of an inch (h and f), so that, by pressing down the 
internal cylinder, a circular plate of perforated zinc (c and d), made 
to fit the interior of tlie larger cylinder, is firmly held between the 
bottom rims of the two cylinders. There should be seven or eight 
perforated plates of dilFerent degrees of fineness ; such sieves may be ob- 
tained of Mr. Snow, zinc-worker, 50 Millbank-street, Westminster, at 
a. cost of 33. each. In using the sieve, especially in M ct siftings, care 
should be taken to keep the apparatus wiped clean and dry, so that 
none of the fine particles of the different materials should become mixed 
with each other; and special attention is required to prevent the fino 
sand getting between the two cylinders, which would render it very 
difficult to separate them. 
The muds and sands obtained by dredgings and soundings, as well as 
clays, &c., brought up by the anchor, may be prepared in a manner 
similar to that recommended for the manipulation of other clays. 
Picked specimens from clays and sands may be sometimes further 
cleaned by being boiled in a weak solution of potash, in the proportion 
of about one part of liiiuor potassa, with nineteen parts of water, or more 
of the potash if required, according to the coarseness and foulness of the 
specimens, up to one part in four, in a flask over a spirit or gas-lamp for 
ten minutes or more. 
To prepare chalk for microscopical examination, if only small quan- 
tities are to be treated, the best plan is to select a piece of soft white 
chalk, — that which has been kept some time in a cabinet is most easily 
•worked, especially chalk from the interior of an Ananchyte or Galcrite, 
and to wash it Avith a moderately soft nail-brush in a hand-basin half 
full of water, keeping the chalk and brush just under the surface, so 
that the loosened particles should all fall in the water. To prevent 
the chalk from being worn into longitudinal furrows, the part under 
the brush should be constantly moved round. It should also be looked 
at occasionally with a hand-magnifier, so that any larger specimens of 
brj'ozoa and foraminifera may not be destroyed by rough brushing, but 
be more carefully picked out with a needle or penknife, or separated by 
the careful use of the brush. When a sufficient quantity has been 
brushed down, the water should bo stirred and the chalky portion 
poured away, or water may be allowed to run into and overflow the 
basin, until the water remains clear over the sediment. The latter should 
then be placed in a round-bottomed tea- cup or gallipot, and rubbed gently 
with the finger in water, until, by further washing, some more of the 
chalk has been removed, when the residue should be carefully dried in 
an oven, or otherwise, without the least disturbance. Y>''hcn dry it 
may be sifted and picked; but if it be still " chalky," further mani- 
pulation by gentle rubbing in water will be necessary, especially if the 
chalk-dust has been put by for some time after having been prepared. 
On a larger scale, chalk may be prepared by being broken up in a 
mortar, or roughly ground vvith a brick, stone, or roller, on a pavement, 
and thou washed iu water, the larger fragments being excepted. 
