158 METHODS OF MICROSCOPICAL EESEAECH. 



dredgings also gave to science innumerable and 

 splendid varieties. 



The polycystinous " earth " should be broken 

 into small pieces, about the size of a nut, and boiled 

 for half an hour to an hour in a strong solution of 

 common " washing soda," the disintegrated por- 

 tions being poured off into a large vessel containing 

 clean water, from time to time, and the boiling in 

 soda repeated, as also the pouring off, until the 

 whole mass is perfectly broken up. When the 

 disintegrated matter in the large vessel shall have 

 thoroughly settled down it should be subjected to 

 several washings, in order to remove the soda, the 

 material being allowed to " settle" thoroughly after 

 each washing. It should then be removed to a 

 beaker or wide-mouthed bottle which should be 

 filled up with water, and after being thoroughly 

 stirred or shaken up, the material should be allowed 

 to settle for thirty seconds only, in a depth of 6 

 inches of water, and the supernatant fluid and its 

 floating particles poured off into a large vessel ; 

 this process should be repeated three or four times, 

 and will give the heaviest density of sand and 

 polycystina. Repeat this process with the matter 

 in the large vessel, allowing it to settle for two and 

 a-half to three minutes, and the density containing 

 the small polycystina will be obtained. Subject 

 the remaining matter to like treatment, allowing it 

 to settle in 6 inches of water for twenty minutes, 

 and the density consisting of the debris of polycy- 

 stina and of diatoms will result. Now boil each 

 separate sediment in nitric acid for fifteen to twenty 

 minutes, remove all trace of acid by repeated wash- 

 ings, and finally boil each density in a weak solution 



