38 A TEXT-BOOK OF BOTANY. 



original size, after which it develops a new wall, the larger valve 

 forming first (Fig. 19, H, I). 



DIATOMACEOUS EARTH, also known commercially as " In- 

 fusorial Earth," or " Kieselguhr " (meaning siliceous marl), 

 occurs in extensive deposits, some of these, as the stratum at 

 Richmond, Va., extending to a depth of 18 feet. These deposits 

 consist of the siliceous walls of the Diatoms, which, owing to their 

 composition, are practically indestructible, and are accumulated 

 in those localities which have favored the growth of the organ- 

 ism. The natural deposit is mined and usually calcined to de- 

 stroy the organic matter, after which it is washed and dried. 

 The article used in pharmacy is further purified by boiling with 

 diluted hydrochloric acid, washing, and calcining. This purified 

 product is known as Terra Silicea Purificata, and occurs in the 

 form of an almost whitish, or light grayish, or light brown powder. 

 It is odorless, insoluble in water and in mineral acids or dilute 

 solutions of the alkalies. Under the microscope mounts made in 

 water or solutions of hydrated chloral show the frustules or 

 siliceous walls of the Diatoms. In the better grades of Diatoma- 

 ceous Earth the entire skeleton with the characteristic markings is 

 present. Material coming from various localities shows a differ- 

 ence in genera of Diatoms. The exact naming of the species 

 requires the assistance of specialists. In order to avoid confusion 

 it is necessary to bear in mind that there are two and sometimes 

 even three views which may be obtained of the same Diatom. 



Diatomaceous Earth consists of about 85 per cent, of SiO 2 , 

 10 per cent, of water, and 5 per cent, of clay, iron oxide, magnesia, 

 lime, and organic material. Owing to the fact that Diatomaceous 

 Earth is made up of the hollow shells of Diatoms, it has the 

 property of absorbing by capillarity gases and liquids. For this 

 reason it is used in the preparation of dynamite ; the highly ex- 

 plosive nitroglycerin being absorbed by the diatomaceous shells, 

 render-ing the product capable of being handled. When calcined, 

 it will absorb its own weight of water. It is used in pharmacy for 

 filtering and as a diluent for powdered extracts, etc. Among the 

 technical uses may be mentioned : polishing of metals and woods, 

 insulating steam pipes and electrical insulators, packing of caustic 

 and inflammable liquids, and the manufacture of glass, paper, and 



