60 A HALF-HOUK WITH THE 



diffused than the desmids. The lattei are decom- 

 posed, and their bodies perish when they die ; but 

 from the fact that the diatoms deposit silex in 

 their structure, they are almost imperishable. They 

 are found in great abundance in the mud of rivers, 

 ponds, and lakes. They are also present in those 

 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, 

 ihe 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 

 hydrochloric 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 transferred 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 deposit 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 allow- 

 ing the heaviest particles alone to subside, these 

 will be generally found to contain the sand and 

 larger diatoms. By repeating this process suc- 

 cessively, the deposits consist gradually of smaller 

 and smaller diatoms, which may be examined with 

 gradually higher powers, in proportion to their 

 minuteness. Some are perfectly round, as in the 

 case of the genus Coscinodiscus, a species of which 

 is figured at 38, plate 2. It is marked beautifully 

 over their surface ; others are triangular : some are 



