120 Progress in Science. [January, 
Muds will have to be treated in a somewhat different manner from recent 
gatherings. If the mud is dry it must be broken down by boiling for a few 
minutes in a solution of caustic potash, the strength of which must be appor- 
tioned to the particular specimen under treatment. After it has been broken 
down into a soft mud, all the potash is thoroughly washed away by means of 
clean water, and replaced by nitric acid as in the case of recent gatherings. 
This is boiled and a little bichromate of potash added as before, and the whole 
washed. Diatoms occurring in mud are very seldom sufiliciently cleaned by 
this process, so that it it has to be supplemented by another. The sediment 
is washed into an evaporating dish and allowed to settle, and as much of the 
water poured off as possible. Then sulphuric sufficient to cover the deposit 
is poured in and the vessel gradually and carefully heated. As soon as the 
liquid shows signs of boiling, bichromate of potash is added a very little at a 
time, until the green colour first formed begins to assume a yellowish tint 
when no more is dropped in; but a few drops of hydrochloric acid are per- 
mitted to fall in, and the liquid is allowed to cool. As soon as the liquid has 
cooled a little, water should be added cautiously, as great heat will be 
generated and there will be danger of boiling over. Itis then to be poured 
into a large beaker of water and washed as before. If it be found that the 
precipitate is not quite white, it will be necessary to boil it again in sulphuric 
acid, with bichromate of potash and hydzochloric acid until it is quite clean. 
If upon examination under the microscope much flocculent matter is present 
besides the diatoms and sand, this can be removed by boiling for a few 
seconds in a weak solution of caustic potash and washing quickly with plenty 
of clean water. 
Guanos.—The preparation of these substances is rather tedious, difficult, and 
dirty. The ammoniacal guanos are those which contain most diatoms, that 
from the islands upon the coast of Peru may be taken as a typical specimen. 
As it comes into commerce, it is a moist powder of a light iron-rust colour, 
smelling strongly of ammonia and having scattered through its mass lumps of 
ammoniacal salts of a more or less solid consistency. The guano should be 
thinly spread out on a stiff sheet of paper and exposed to the air at a moderate 
heat for a few days, until the moisture and most of the ammonia have evapor- 
ated, and less acid will be required to clean the guano. It will now have 
become much lighter in colour, and crumble to a dry powder. A tin pan is 
now about half filled with a solution of common washing soda in clean filtered 
water and placed over some source of heat, as on astove. The strength of 
the solution is not a matter of any great moment and must vary with the 
guano manipulated. As soon as it begins to boil the guano is dropped in, a 
llttle at a time, while the liquid is stirred with a glass rod or stick of wood. 
Considerable effervescence takes place, ammonia being given off; therefore it 
must be kept continually stirred, and care exercised to prevent its boiling over. 
It is then poured into a plentiful supply of clean water and washed several 
times, care being taken to permit all the diatoms to settle. As soon as the 
wash-water is only slightly coloured, the guano is transferred to a suitable 
evaporating dish and covered with nitric acid and boiled. While boiling a few 
crystals of bichromate of potash are added, and the material treated as in the 
case of muds. Phosphatic guanos, are somewhat more difficult to treat. They 
are generally drier than the ammoniacal kind, and must be boiled in a large 
quantity of hydrochloric acid as many as three times, and the acid must be 
poured off while still hot. Afterwards, nitric acid and sulphuric acid and 
bichromate of potash must be employed as in former cases. 
Lacustrine Sedimentary Deposits.—For the most part these are pulverulent, 
and easy to clean. Some, as found in nature, are so pure that they require no 
cleaning except washing in clean water. Burning on a plate of platinum will 
often serve to clean some specimens; but it will, in general, be found best to boil 
in nitric acid with a littie bichromate of potash, and subsequently in sulphuric 
acid and bichromate of potash with the after addition of hydrochloric acid. 
Occasionally an amount of flocculent matter will be left, which can be removed 
by very careful heating (not boiling), in a weak solution of caustic potash, and 
immediately pouring into a large quantity of clean water and thoroughly 
washing. 
