108 THE VICTORIAN NATURALIST. 



tion — is first passed through the coarse sieve, thus separating the 

 coarse material from the fine. This latter is next passed through 

 the fine sieve, an operation requiring plenty of water and 

 patience. The portion retained in the strainer may now be 

 examined, and if much mud, &c, still appears to be present we 

 had better boil it with a little soda in a pint of water, using, 

 preferably, an ordinary kitchen pan and fire or gas stove. After 

 this we again pass it through the fine strainer. The residue this 

 time will probably consist of fine sand, diatoms, and other 

 insoluble material. We now transfer a portion of this to the 

 circular glass dish or rotary glass, keeping it in motion for a while 

 by a circular movement of the hand. The sand, &c, will quickly 

 make its way to the bottom, while the upper stratum, still in 

 motion, may be withdrawn by means of the syringe and placed in 

 the beaker glass. Sufficient material may be dealt with in this 

 manner to give about an inch in depth in the beaker after settling. 

 All the water having been drawn off, we carefully pour in an equal 

 bulk of sulphuric acid, mix thoroughly, and boil for ten or 

 fifteen minutes, or even longer, adding more acid if needful. 

 When the liquid has become thoroughly blackened chlorate 

 of potash may be added, very cautiously, so as to avoid 

 explosions, until the mixture appears of a yellowish colour. 

 After cooling somewhat, we may pour the liquid into one of the 

 jugs, fill up with water, and pass through the fine strainer as before. 

 It may sometimes be desirable to set aside the water which passes 

 through the fine strainer after treatment with acid, in order not 

 to lose the finer diatoms ; but usually a sufficient proportion of 

 these are retained within the strainer. After all trace of acid has 

 been got rid of, we repeat the rotary process with the glass dish, 

 in order to eliminate any further sand or fine silt not previously got 

 rid of, transferring the white floating material, consisting now 

 mainly of diatoms, to the beaker, in which it must be again 

 allowed to settle. The water having been withdrawn to within an 

 inch of the sediment, four or five crystals of soda, about the size 

 of peas may be throv\n in, and the whole reboiled. Care and 

 judgment are required as to the length of time this boiling should 

 continue. Delicate species, like Pleurosigma, will not with safety 

 bear more than three or four minutes, while in the case of the 

 larger and heavier kinds it may be continued for ten if needful. 

 The object of this boiling in an alkaline solution is to thoroughly 

 free the diatoms from all flocculent matter yet adherent, after 

 which they may be finally strained and transferred to a suitable 

 phial with distilled water preparatory to mounting. The treatment 

 of a fossil deposit differs from the foregoing only in regard to the 

 presence or absence of lime, or in respect to its degree of hardness. 

 If lime is present, treatment with nitric acid may be first resorted 

 to. If the deposit is hard and compact, but free from lime, as is 



