NATURAL HISTORY OF THE DIATOMACEjE. 495 



After a while it is poured into a plentiful supply of clean water and 

 washed therewith several times, care being taken to permit all of the 

 diatoms to settle. As soon as the wash-water is only slightly colored, 

 the guano is transferred to a good sized evaporating-dish, and covered 

 with nitric acid, and boiled. While it is boiling, a few crystals of bichro- 

 mate of potash are dropped in, and the material washed as in the case 

 of muds. Thereafter the diatoms are boiled in sulphuric acid with 

 bichromate of potash and hydrochloric acid, as before described. 



Phosphatic guanos, as that from Brazil, 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. Thereafter nitric acid 

 and sulphuric acid and bichromate of potash must be employed, as in 

 the other case. 



Lacustrine Sedimentary Deposits. For the most part these are pul- 

 verulent, 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 or mica will often serve to clean some specimens, 

 but it will, in general, be found best to boil in nitric acid with a little 

 bichromate of potash, and subsequently in sulphuric acid and bichromate 

 of potash, with the after addition of hydrochloric acid. Occasionally a 

 certain amount of flocculent matter will be left, which it will be necessary 

 to remove with 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. 



Marine Fossil and Sub-PIntonic Deposits, being stony and possessed 

 of very much the same physical characters, are manipulated in the same 

 manner. A small lump of the deposit is placed in a test-tube, and cov- 

 ered with a strong solution of caustic potash. It is then boiled for a few 

 minutes, and usually it immediately begins to break up and fall down in 

 the shape of a soft mud-like material. At once the liquid, with the sus- 

 pended fine powder, is poured off into a large quantity of clean hot water, 

 and if the whole of the lump has not broken down into a powder, what 

 remains has a little water poured over it in the test-tube, and it is again 

 boiled. It will be found that a little more will now crumble off. This is 

 added to the rest in the large vessel, and if the lump has not now broken 



