[ 192 ] 



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 scopical Socict^'6 1Rote^Boo[^0. 



Plate IX. 



Diatoms. — I send this box out with great dififidence. Special- 

 ists are often tabooed and boxes of their slides set aside. But as 

 some of our members have, in the " Notes," asked me to circulate 

 some Diatoms, I have ventured to do so. These may turn the 

 serious attention of some to this most fascinating study ; if they 

 do, I shall be more than repaid. 



Diatoms are " unicellular Algag," having a vegetable structure, 

 which has the power of secreting the liquid silica which comes 

 down from the mountains or stony plains through the rivers into 

 the sea, and depositing it in themselves. These valves appear 

 usually to consist of two or more layers, marked with the most 

 exquisite lines or points. They have been called " Brittle-worts," 

 from their tendency to break up into fragmentary frustules ; this is 

 also the meaning of their Greek name, " Diatom." Their func- 

 tion is to suck out the carbonic acid there is in the water, using the 

 carbon -for themselves, and setting free the oxygen. By secreting 

 the liquid silica, they make the water purer and more useful. 



They exist, and have existed, in countless millions ; in some 

 parts even the ice is made brown by the quantity of diatoms thrown 

 upon it ; they are found in fresh water, brackish water, and sea 

 water, in marls, in dried-up lakes, and in sedimentary rocks. 

 Though so small, they are of every conceivable shape, with striae 

 that run to 130,000 to the inch, as in Amphipleiira pellucida 

 Upwards of 4,000 forms have already been catalogued. They are 

 found in strata dozens of miles in length, and often many feet in 

 thickness ; indeed, a deposit of diatomaceous earth was found in 

 Victoria Land 400 miles long by 120 wide. The Columbia river 

 runs through two precipices, 500 ft. high, almost entirely diatoma- 

 ceous. The " Polishing Slate " of Bohemia, " Turkey Stone," 

 and " Rotten Stone," are chiefly composed of diatoms. " Kiesel- 

 guhr," used for dynamite, and " Sozodont " tooth powder are 

 mainly diatomaceous. The famous Oamaru deposit in New Zea- 

 land is more than two miles in length ; near its southern end it 



