124 THE DIATOMACEyE. {JuHC, 



form sKmy pellicles, of a brownish or greenish colour, on mud, 

 stones, pieces of wood, and other submerged bodies. In com- 

 pany with Alffee they cover the entire sides of rocks, conduits, 

 etc., and are found swimming freely in tufts of Conferv(e and 

 similar plants ; or they live, parasite-like, on water-weeds, often 

 in such numbers as to cover them Hke Aphides. Sometimes, by 

 being connected end to end, they form brown chain-like pro- 

 cesses, several feet in length, resembling the filamentous Alg(B 

 (differing from these, however, by immediately becoming discon- 

 nected when touched) . Lakes and stagnant waters are often full 

 of them, and they are occasionally raised, by escaping gases, se- 

 veral inches above the surface of the water in frothy heaps. Only 

 a few species appear to have any choice as to locality, the majo- 

 rity having just as little regard to this as to thermal differences. 

 Many species are distributed over the whole surface of the earth ; 

 found from the poles to the equator, in the waters of the glaciers 

 and in the hottest springs. The sea has its characteristic forms, 

 differing widely from those of fresh-water localities. Diatoms 

 met with in salt inland waters are mostly referable to the latter. 



But the distribution of the Diatomaceee goes still further : we 

 may assume their invariable presence in dust. If we consider 

 how hea'^'y bodies are sometimes blown about and transported 

 from place to place by the wind, we can easily conceive the same 

 to be the case Avith the little Diatom that, far minuter than the 

 smallest particle of dust, frequently remains suspended long after 

 the aerial strata again become calm. If the atmosphere where- 

 in they are suspended be not sufficiently loaded with moisture, 

 they die a natural death, and soon only their indestructible ske- 

 leton remains. But in the atmosphere in damp weather perfect 

 Diatoms are not unfrequently discovered, their contents fully 

 coloured. Once taken up by the breeze, they are found on the 

 castle turret, the mountain-top, the dust that settles on our 

 books, and, in short, everyivhere. 



The mode of collecting the Diatomacecs is most simple, and 

 needs no particular instruction. A case of from twelve to eigh- 

 teen inches long, more or less, wide-mouth stoppered bottles, each 

 capable of holding from two to four drachms of water, is the 

 first requirement. Every bottle should be numbered on the 

 stopper or on some convenient place, and according to these num- 

 bers the locaUty of each gathering is entered in a note-book. A 

 spoon for skimming purposes completes the outfit. 



