82 DIATOMS. 



forms require a greater depth of water and more ener- 

 getic rotation while the lightest require the least possi- 

 ble rotation and very little water. 



Fossil earths, like this Richmond specimen, are treated 

 more energetically. The specimen is first boiled in weak 

 caustic soda or potash. This disintegrates it and it falls 

 to pieces in the form of a thick mud. This mud is 

 washed in clean water as in the other case, until it settles 

 clear. Then the sediment is boiled in sulphuric acid for 

 ten or fifteen minutes. The organic matter, of which it 

 contains a considerable quantity, is blackened and the 

 mass gets very dark. Then a little chlorate of potash is 

 added very cautiously and the diatoms are bleached. 

 The color changes to light green, when the whole is washed 

 as before, until all trace of acid is gone. Then it is put 

 in an evaporating dish and boiled with carbonate of pot- 

 ash until it begins to spatter, when the whole is turned 

 into a large beaker, in the bottom of which is a layer of 

 sulphuric acid. It is washed again and then sanded, 

 when it is ready to use. The diatoms are kept in vials 

 in equal parts of alcohol and distilled water. If we run 

 across a mass of free-swimming forms, nitzschias or na- 

 viculas for example, they should be carefully scraped up, 

 taking as [little mud or sand as possible, and put in a 

 bottle. This should not be more than half filled, other- 

 wise the plants will die. When they are brought home 

 they are poured out into a saucer and allowed to settle ; 

 when the superfluous water is carefully poured off, a 

 piece of wet cheese-cloth is then put over them smoothly 

 and the saucer set near a window. In a day or two the 

 diatoms will come up through the meshes of the cheese- 

 cloth and can then be removed with a camel' s-hair pencil. 

 In this way I have obtained perfectly clean specimens 

 with little trouble. Of course only the free-swimming 

 species only will answer for this method. When we wish 

 to mount theni for microscopic use we draw off the alcohol 



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