COLLECTION OF DIATOMACEiE. 315 



Some kinds of earth have so little impurity that one washing 

 suffices ; but in any case it is to be continued so long as the water 

 remains coloured. The deposit is then to be treated, in a flask or 

 test-tube, with Hydrochloric (muriatic) acid ; and after the first 

 effervescence is over, a gentle heat may be applied. As soon as 

 the action has ceased, and time has been given for the sediment 

 to subside, the acid should be poured off, and another portion 

 added ; and this should be repeated as often as any effect is pro- 

 duced. "When hydrochloric acid ceases to act, strong Nitric acid 

 should be substituted ; and after the first effervescence is over, a 

 continued heat of about 200° should be applied for some hours. 

 "When sufficient time has been given for subsidence, the acid may 

 be poured off and the sediment treated with another portion ; and 

 this is to be repeated until no further action takes place. The 

 sediment is then to be washed until all trace of the acid is re- 

 moved ; and, if there have been no admixture of siliceous sand in 

 the earth or guano, this sediment will consist almost entirely of 

 Diatomacese, with the addition, perhaps, of Sponge-spicules. 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 off 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 sedi- 

 ment 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 ; and the subsequent sediments will consist almost exclu- 

 sively of Diatoms, the sizes of which will be so graduated, that 

 the earliest sediments may be examined with the lower powers, 

 the next with the medium powers, while the latest will require 

 the higher powers — a separation which is attended with great 

 convenience. * It sometimes happens that fossilized Diatoms are 

 so strongly united to each other by Siliceous cement, as not to be 

 separable by ordinary methods ; in this case, small lumps of the 

 deposit should be boiled for a short time in a weak Alkaline solu- 

 tion, which will act upon this cement more readily than on the 

 siliceous frustules ; and as soon as they are softened so as to 

 crumble to mud, this must be immediately washed in a large quan- 

 tity of water, and then treated in the usual way. If a very weak 

 alkaline solution does not answer the purpose, a stronger one may 



* A somewhat more complicated method of applying the same principle 

 is described by Mr. Okeden in the " Quart. Journ. of Microsc. Science," 

 Vol. iii. (1855), p. 158. The Author believes, however, that the method 

 above described will answer every purpose. 



