THE DIATOMS. 



57 



acid, or sulphuric acid and potassium bichromate, the or- 

 ganic portions will be entirely removed, and the silicious 

 shells will be left uninjured. These shells, as well as the 

 living Diatoms, have been objects of great interest to 

 microscopists at all times on account of the great variety 

 of beautiful markings. The more delicate forms are also 

 highly esteemed as test objects for microscope objectives. 

 Besides the living forms there are numerous deposits of 

 the silicious shells of Diatoms in various parts of the 

 world. The accumulation of sediment formed by their 

 successive production and death in the waters of oceans, 

 lakes, and rivers gives rise to deposits that may attain 

 considerable thickness, and these, by subsequent changes of 

 level, may come to form parts of the dry land. 



FIG. 29. 1, Surirella constricta, side view; 2, Pinnularia viridis ; 3, part of 

 a filament of Fragilaria pectinalis ; 4 and 5, side and front views of Terp- 

 sinoe musica, the so-called musical Diatom ; 6, Navicula. 



Certain deposits in Norway and Sweden are known as 

 Bergmehl (mountain meal). They are used by the peas- 

 ants to mix with the flour in making bread. Fossil Dia- 

 toms are sold under various names as polishing powders, for 

 which purpose they are excellent, and are extensively used. 



Diatoms may be obtained for examination from these 

 deposits, or the living forms may be secured by stripping 

 the yellowish-brown coating from the stems of water 

 plants, or by gathering the thin film of black mud from 

 the bottom of springs, ponds, and streams. 



