ART. 15. COLLECTING AND PREPARING DIATOMS MANN. 7 



an ideal herbarium specimen and its value for purposes of identifica- 

 tion is great. 



There remains tlie subject of the preparation of fossil diatoms. 

 So-called diatomaceous earth is generally free from organic matter 

 and is only subjected to acid treatment when it contains iron or other 

 substances that can be dissolved out by acids, or especially when it 

 contains calcareous matter. Diatomaceous earth of this last kind is 

 easily disintegrated by treatment with hydrochloric acid ; as the lime 

 is thereby dissolved and the mass falls into a powder. But most 

 diatomaceous earth is not so easily handled, being composed entirely 

 of silica remains and often hardened into a stony condition. The 

 breaking up of such samples can not be done by pulverizing, as this 

 would shatter nearly all of the diatoms. The mass must be gently 

 brought into a powdery condition. The best way of accomplishing 

 this is to first break the material up into small pieces about the size 

 of a pea, using for this purpose not a hammer, but a stout needle, 

 which cracks off small particles without breaking many of the 

 diatoms. The pieces having thus been reduced in size are boiled in a 

 beaker with a weak solution of some mild alkali, like sodium car- 

 bonate. A solution of borax also sometimes works satisfactorily. 

 The material is boiled until the liquid begins to look milky by the 

 slow breaking away of the diatoms from the lumps. The liquid is 

 then poured into a larger beaker, fresh alkali water added to the 

 lumps, and again boiled. The process is kept up until by this gentle 

 method the lumps are slowly broken down. After the combined 

 boilings poured together have been allowed to settle, the liquid is 

 poured off and the sediment washed by decantation until all trace of 

 the alkali is removed. 



In cases of extremely resistent fossil material, where neither hydro- 

 chloric acid nor long boiling in weak alkaline solutions breaks down 

 the lumps, this disintegration may sometimes be effected by soaking 

 the lumps in strong sodium carbonate, quickly replacing with hy- 

 drochloric acid, returning again to sodium carbonate, and so alter- 

 nating until the violent chemical reactions set up within the lumps by 

 these alternations have mechanically broken them down. 



If the repeated washings necessary with any of the foregoing 

 processes are properly timed they will also accomplish the removal 

 of clay or minute broken particles of diatoms that are in the sedi- 

 ment. Where the fossil substance contains sand, the final process of 

 rotating this in an evaporating dish will remove it, as in the case of 

 the living diatom material previously described. The cleaned dia- 

 toms are then put up in bottles with 35 per cent alcohol, as in the 

 case of the living material. 



It sometimes happens with fossil material, more rarelj^ with fresh, 

 that a fine flocculent residue is mixed with the cleaned diatoms and 



