AMONGST THE SEAWEEDS. 31 



beautiful frustules provide valuable material for 

 mounting for the microscope. Their movements 

 seem to be wholly mechanical, and there is no 

 trace of any self-directive power. They are of a 

 jerky nature, and are quite aimless and uncertain. 



So prolific are these tiny plants that their ac- 

 cumulated valves make large deposits in some places, 

 as is the case at Dolgelly, Mull, and elsewhere in 

 Britain. The entire city of Richmond, in Virginia, 

 is built upon a bed of these diatoms eighteen feet 

 in thickness. In Bohemia there is a deposit of 

 them fourteen feet thick, from which is obtained 

 that fine material used for delicate castings. I 

 have a tin of diatomaceous earth from Hanover, 

 known as Kieselguhr, or flint mud, which con- 

 sists of very little else than the frustules of 

 diatoms. This is the material which is used in 

 the manufacture of dynamite, since it has the 

 property of absorbing three times its weight of 

 nitro-glycerine, thus presenting a remarkable illus- 

 tration of combined feebleness and power. Tripoli 

 powder and rotten stone, used for polishing, are 

 similarly constituted. It is no exaggeration to say 

 that, notwithstanding their extreme minuteness, 

 these organisms have added more to the fossiliferous 

 contents of the earth than all the great mammals 

 put together. 



The external covering or shell of diatoms is made 

 up of two portions or valves, usually called frustules ; 

 but there is no hinge connection, as in the case of 

 molluscous bivalves. The two frustules are con- 

 nected by a layer of silica, which runs round their 



