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PHYSICAL GEOGRAPHY. 



PART FIFTH. 



Modes of Occurrence, and Uses to Man of the Diatomace.«. 



And now it will be desirable to say something with regard to the 

 various modes of occurrence of the diatomaceae, specifying the particu- 

 lar habitats or kinds of situations or water (salt, fresh, or brackish), for 

 the use of such as may wish to know more concerning these beautiful 

 atomies than can be told within the limits of a short sketch like the 

 present. Thereafter it will be well to say something of the importance 

 to the geologist of knowing the life-history of the diatomaceae, and, 

 finally, their uses to man in the arts and otherwise, as they occur in 

 mass in various parts of the world. And in this connection we are 

 reminded of the words of the poet, who says, — 



Naught so vile that on this earth doth live, 

 But to the earth some special good doth give ; 



for we find that the diatomaceas have in past ages played, and, in truth, 

 still are playing, a most important part in the grand drama of nature ; 

 and their minute dimensions is no excuse for the neglect to which they 

 have been subjected by scientific and capable observers. 



Diatomaceae are to be looked for in both fresh and salt water, as well 

 as in that which can be included under neither of these heads, the brack- 

 ish water of seaside marshes, where the springs of to-day are overflowed 

 by the rising tide of to-morrow. In general, the piece of water in which 

 they are looked for must be permanent, for authors tell us that it is use- 

 less to expect to find them in the transient pools left by the rain. This 

 is, however, not strictly the case, for, on one occasion, I found diatoms in 

 a pool formed by the drainings of a stable-yard, and even little collec- 

 tions of water only two or three days old have occasionally yielded forms 

 when carefully searched, but in such cases they were only few and mi- 

 nute. Prof. Gregory found them in moist earth about the roots of plants, 

 and others have collected them from between the branches of mosses 

 which clustered upon tree barks or house-tops. Even the dust which 

 has fallen upon the sails and decks of vessels, far out at sea, have been 

 found to contain them, by the German microscopist, Ehrenberg, who 

 therefrom has deduced certain supposed facts with regard to atmospheric 



