156 THE WORLDS LUMBER ROOM. 



manure to supply the lack of silica. Some of these earths, 

 whether of animal or vegetable origin, have been mixed 

 with clay and lime, and made into " floating bricks," one- 

 sixth the weight of ordinary bricks, and fireproof ; some are 

 added to sealing-wax, paper, soap, indiarubber and model- 

 ling-clay, to give consistency, and, finally, the once harmless 

 little vegetables are mixed with nitroglycerine and con- 

 verted into dynamite ! 



Nature acts more kindly by them, and is believed to 

 have turned some of them into semi-opals. 



One of the most important ends served by the diatoms, 

 however, is that of being food for many of the lower 

 animals, especially the Protozoa, among which we may 

 mention the beautiful little Noctiluca, or " night-light," 



30,000 of which may be 

 contained in one cubic 

 inch of water. They are 

 themselves shell-less, 

 moulded in the shape of 

 melons, the largest hardly 

 bigger than the head of a 

 minikin pin, and are bril- 



Fig. 34.-NOCTILUCA MILIARIS. liantl y phosphorescent. 



(Fig. 34.) At certain 



seasons they crowd the waves in such multitudes that one 

 will be found in each drop of water. Phosphorescence in 

 the open sea is, however, said to be produced chiefly by a 

 minute plant (Pyroristis) of the [size of a pin's head, which 

 is very abundant far from land and, like the diatom, also 

 has a thin casing of silica. 



