DIATOMACEyE 623 



repeated washings in pure water, and by taking advantage, at the 

 same time, of the different specific gravities of the diatoms and of 

 the intermixed substances, to secure their separation. Sand, being 

 the heaviest, will subside first ; fine particles of mud, on the other 

 hand, will float after the diatoms have subsided. The tendency of 

 living diatoms to make their way towards the light will afford much 

 assistance in procuring the free forms in a tolerably clean state ; for 

 if the gathering which contains them be left undisturbed for a suffi- 

 cient length of time in a shallow vessel exposed to the sunlight, they 

 may be skimmed from the surface. Marine forms must be looked 

 for upon seaweeds, and in the^fijae mud or sand of soundings or 

 dredgings ; they are frequently found also, in considerable numbers, 

 in the stomachs of Holothurise, Ascidians, and Salpae, in those of the 

 oyster, scallop, whelk, and other testaceous molluscs, in those of the 

 crab and lobster, and other Crustacea, and even in those of the sole, 

 turbot, and other flat-fish. In fact, the diatom collector will do 

 well to examine the digestive cavity of any small aquatic animals 

 that may fall in his way, rare and beautiful forms having been 

 obtained from the interior of Noctiluca. The separation of the 

 diatoms from the other contents of these stomachs must be accom- 

 plished by the same process as that by which they are obtained 

 from guano or the calcareous * infusorial earths.' Of this the 

 following are the most essential particulars : The guano or earth is 

 first to be washed several times in pure water, which should be well 

 stirred, and the sediment then allowed to subside for some hours 

 before the water is poured off; since, if it be decanted too soon, it 

 may carry the lighter forms away with it. 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 hydro- 

 chloric 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 produced. 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 F. 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 removed ; and, if there have been no admixture of siliceous 

 sand in the earth or guano, this sediment will consist almost entirely 

 of Diatomacece, with the addition, perhaps, of sponge-spicules. The 

 separation of siliceous sand and the subdivision of the entire aggre- 

 gate of diatoms into the larger and the finer kinds, may be accom- 

 plished 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 



