Examination and Study hy means of the Microscope. 233 



and covered with nitric acid, and boiled. While it is boiUng, a few 

 crystals of bichromate 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 aramoniacal 

 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 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 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 hydro- 

 chloric acid. Occasionally a certain amount of flocculent matter 

 will be left, which it wiU 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 Suh-Plutonic Deposits, being stony, and 

 possessed of very much the same physical characters, are manipu- 

 lated in the same manner. A small lump of the deposit is placed 

 in a test-tube, and covered with a strong solution of caustic potash. 

 It is then boiled for a few minutes, and usuaUy it immediately begins 

 to break up and fall down in the shape of a soft mud-hke material. 

 At once the liquid, with the suspended 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 

 down, it is again boiled in the alkaline solution and in water alter- 

 nately, until it has all been disintegrated. It is then all permitted 

 to settle for at least three hours, when it is thoroughly washed and 

 boiled in hydrochloric acid for about half an hour. There is then 

 added an equal amount of nitric acid, and the boiling continued for 

 a short time. It is then washed and heated in sulphuric acid, with 

 the addition of bichromate of potash and hydrochloric acid. 



All mixed gatherings of Diatomaceas, and particularly all muds 

 and deposits, should be separated into densities, so that for the most 

 part the larger forms are collected together, free from sand, and 

 separate from the smaller species and broken specimens. This is 



