MODE OF COLLECTING DIATOMACB^. 305 



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

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

 fioat after the Diatoms have subsided. The tendency which the 

 Diatomacese have, 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 un- 

 disturbed for a sufficient length of time, in a shallow vessel ex- 

 posed to the sunlight, they may be skimmed from the surface. 

 The marine forms must be looked for upon sea-weeds, and in 

 the fine mud or sand of soundings or dredgings ; they are fre- 

 quently found also, in considerable numbers, in the stomachs of 

 the oyster, scallop, whelk, and other Mollusks, especially the 

 "bivalves," in those of the crab and lobster, and even in those 

 of the sole, turbot, and other "flat-fish." "Several species," 

 says Prof. W. Smith, " rarely or never occurring in my usual 

 haunts, have been supplied in abundance by the careful dissec- 

 tion of the above mierophagists." The separation of the Diatoms 

 from the other contents of these stomachs, must be accomplished 

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

 Guano or the calcareous Infusoiual 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 colored. 

 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 produced. "When 

 hydrochloric acid ceases to act, strong nitric ftcid should be sub- 

 stituted; 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 

 Diatomaeese, 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, 



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