MICROSCOPE IN FRESH WATER. 41 



deposits of clay which once formed the bed of 

 rivers and lakes, and which are now dry. In order 

 to procure the diatoms from these deposits, the 

 clay or earth should be well washed with pure 

 water, and the deposit allowed to subside, and the 

 water poured off. This may be repeated several 

 times. The deposit is then to be washed with hy- 

 drochloric acid, and when the effervescence is over, 

 the acid is poured off, and a fresh portion is added. 

 This may be repeated several times, and when the 

 hydrochloric acid ceases to act, nitric acid may be 

 employed in the same manner. When no action 

 occurs by its use cold, the deposit may be trans- 

 ferred to a watch-glass, and kept over a spirit- 

 lamp, at a temperature of about 200, for three or 

 four hours. The deposit must then be well washed 

 with pure water, to remove all the acid. The de- 

 posit will be found now to consist almost entirely 

 of diatoms. If anything else be found, it will be 

 grains of sand. By casting the deposit into a small 

 quantity of water, and allowing the heaviest par- 

 ticles alone to subside, these will be generally found 

 to contain the sand and larger diatoms. By re- 

 peating this process successively, the deposits con- 

 sist gradually of smaller and smaller diatoms, which 

 may be examined with gradually higher powers, in 

 proportion to their minuteness. Some are per- 

 fectly round (Fig. 33, PL 2), and marked beauti- 

 fully in the centre ; others are triangular ; some 

 are square, and attached together (Fig. 32, PL 2, 

 and Fig. 137, PL 5). The most common forms are 

 those which are oval, or boat-shaped (Fig. 31, PL 2). 

 These are again larger at one end than the other 

 (Fig. 30, PL 2). The markings upon the surface 

 are very various. In some forms the markings 

 are exceedingly minute : so small are they, that 

 certain species of diatoms have been used as 



