124 THE MICROSCOPE. 



weeds, and on the mud at the bottom of the sea. 

 You may also procure numerous forms from the 

 stomach of various sea-creatures, such as oysters, 

 sea-cucumbers, sea-squirts, soles, and other flat-fish. 

 It is a distinctive character of this group to have 

 encircling their various forms an external coat of silex, 

 which would appear to be almost indestructible. We 

 have seen how an accumulation of them give rise to 

 deposits of considerable thickness ; and guano, it is 

 well known, contains many forms, some of extreme 

 beauty. If you wish to collect Diatomacese from 

 guano, you should wash a portion several times in 

 water, and stir it well ; then let it rest for some hours, 

 so as to give the lighter forms time to sink; then 

 pour off the water, and if necessary give the sediment 

 another washing. You must now use strong acids; 

 the deposit is to be placed in a test-tube with hydro- 

 chloric acid, and gently heated. After the sediment 

 has subsided, pour off the acid, and heat it with a 

 fresh dose ; pour off again, and heat with nitric acid 

 two or three times, and apply heat for three or 

 four hours of about 200; then wash the sediment 

 till the acid is removed. "The separation of siliceous 

 sand, and the subdivision of the entire aggregate of 

 diatoms into the larger and the finer kinds, may be 

 accomplished by stirring the sediment in a tall jar of 

 water, and then, while it is still in motion, pouring oft 

 the supernatant fluid as soon as the coarser particles 

 have subsided ; this fluid should be set aside, and as 

 soon as a finer sediment has subsided, it should again 

 be poured off; and this process may be repeated 

 three or four times at increasing intervals, until no 

 further sediment subsides after the lapse of half an 

 hour. The first sediment will probably contain all 

 the sandy particles, with perhaps some of the largest 

 diatoms, which may be picked out from among them ; 



