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chloride becomes converted into the very dark red ferric acetate. The 
diatoms are now allowed to settle closely for two or three hours, and 
the excess of iron salt poured off as completely as possible. Next the 
test-tube with the moist diatoms is stood in a small vessel of boiling 
water till all the ferric acetate has passed into the basic state, as 
evidenced by its changing to an opaque buff colour. The diatoms, 
now “ charged ” with the insoluble basic ferric acetate, are shaken up 
with a few drops of water and acetic acid, and poured into not too 
great an excess of a solution of potassium ferrocyanide in acetic acid. 
After standing for some hours with occasional agitation, the excess of 
Prussian blue which has been formed among and around the diatoms 
can be removed (at any rate in great part), by repeatedly shaking up 
with fresh lots of distilled or rain water, as after elimination of the 
soluble salts this body assumes a form which settles very slowly indeed. 
Stirring the settled diatoms with a soft camel’s-hair brush helps to 
remove the precipitate which may be clotted on their surface. 
(2) Platinum Method. — Applicable to all diatoms, but apt to fail. 
To the cleaned and ignited diatoms contained in a small porcelain 
crucible add an alcoholic solution of sodio-platinic chloride, and 
evaporate with extreme slowness and without any approach to boiling. 
Finish the drying with the utmost care, to prevent the formation of 
bubbles of steam within the minute cavities, as this would result in 
ejection of the platinum salt and the consequent failure of the 
preparation. When quite dry raise the temperature very slowly till 
a low red heat is reached, and then throw a few crystals of oxalic acid 
into the crucible and immediately replace the cover. This completes 
the reduction of the platinum salt to metal and sodium chloride, and 
it now only remains to wash away the latter and as much of the 
unattached platinum as possible, and to select any required diatom 
from the residue. 
(3) Mercurous Sulphide Method. — As stated above, this is the 
best method I have hitherto found for “ charging ” all diatoms except 
those having the finest markings. Take a cold saturated solu- 
tion of mercurous nitrate (swfr-nitrate of mercury) and dilute it with 
its own bulk of water in a small test-tube. Add the diatoms and a 
drop of metallic mercury, and keep the whole standing corked up for 
as long a time as can be spared — days are better than hours, and 
weeks better than days. Shake the tube and withdraw the diatoms 
suspended in the liquid by help of a pipette, leaving behind any 
crystals of basic sub-nitrate of mercury which may have formed. 
Allow the diatoms to settle in a small test-tube, and draw off the 
supernatant liquid first by a pipette and then by a moistened thread 
or a very thin strip of filter paper till nothing but a slightly moist 
mass of diatoms remains. Now add several drops of a strong solution 
of ammonium sulphide which has been recently prepared, and which 
is practically free from dissolved sulphur (it should be almost colour- 
less, not yellow), and shake. Fill up the tube with water, cork, and 
