Diatoms 



FLUID WHOLEMOUNTS IN NONAQUEOUS MEDIA 



39 



acid to remove the last of the iron chloride 

 and again washed by decantation. 



It will probably happen, howe\-er, that 

 many of the finest markings on the dia- 

 toms are still filled with finely divided clay 

 which must be removed by treating the 

 diatoms in the cold with a 10 "^t, dilution 

 of ammonia. The frustules will be dam- 

 aged if a hot or strong solution is used, and 

 it is best to leave the diatoms in cold 

 ammonia for two or three days, shaking at 

 intervals, before washing them by de- 

 cantation. The final stage in cleaning the 

 diatoms is now to wash them with re- 

 peated changes of filtered distilled water 

 until all traces of dissolved salts have been 

 removed. 



Some workers, after treating the dia- 

 toms with ammonia, repeat the sulfuric 

 acid-potassium chlorate treatment as a 

 final precaution. Another variant is to 

 precede the original treatment with sul- 

 furic acid and potassium chlorate by treat- 

 ment with a hot mixture of two parts of 

 sulfuric acid with one of nitric acid. This 

 treatment is recommended when the orig- 

 inal collection contains very large quan- 

 tities of vegetable matter in addition to 

 the diatoms. Swatman (loc. cit.) points 

 out that if diatoms are collected from 

 mud containing coal dust this will not be 

 satisfactorily removed by any of the pre- 

 ceding processes and recommends that the 

 diatoms be fused in a platinum crucible 

 with pure potassium nitrate for removal 

 of this contaminant. 



It will have been observed, either in 

 theory or practice, that many of the proc- 

 esses just described result in the produc- 

 tion of noxious vapors, so that they cannot 

 be properly carried out by anyone not 

 ha\dng access to a chemical hood. To meet 

 this objection Hendey 1938 (11360, 58:49) 

 has devised a most ingenious apparatus 

 which will permit any of the processes 

 described to be used in a living room. 



All the methods so far described pre- 

 sume that the collector has been working 

 close to his laboratory and has, therefore, 

 not been faced with the problem of trans- 

 l)orting large quantities of vegetable 

 matter. A rough method of field cleaning 

 (Swatman 1941: 11479, 1:191) may be 

 used to concentrate diatoms. As much 



water as possible is drained from the rough 

 sludge and replaced with 10% sulfuric 

 acid. Potassium permanganate is then 

 added, with constant stirring, until the 

 solution remains pink after standing for a 

 few minutes; then enough oxalic acid is 

 added to dissolve the brown oxide sludge. 

 The clear solution may be poured off and 

 the diatoms roughly washed before being 

 transferred to a tube. 



By whatever method the diatoms have 

 been cleaned, they are now presumed to 

 be accumulated in clean distilled water. 

 They should be roughly sorted into their 

 kinds, since diatoms are much easier to 

 handle under the surface of water with the 

 aid of a fine pipet than they are when dry. 

 The different kinds are then stored in 

 small vials of distilled water to which a 

 trace (about one-tenth of 1 %) of formalde- 

 hyde is added with a view to discouraging 

 organic growth. Larger quantities of form- 

 aldehyde should not be used, or a fine de- 

 posit will be found on the surface of the 

 diatom when it is subsequently dried. 



To mount a strewn slide of diatoms, it is 

 only necessary to take a drop of the dis- 

 tilled water with the diatoms suspended 

 in it, to let this evaporate on a coverslip, 

 to dry the coverslip with heat, and then 

 to mount it in the manner to be described 

 subsequently. It may be presumed, how- 

 ever, that the worker wishes to prepare a 

 slide in which the diatoms are arranged in 

 some given order on the coverslip. It must 

 not be thought for one moment that this 

 method of arranging diatoms on the cover- 

 slip is of necessity confined to the produc- 

 tion of artistic pictures. It is true that the 

 method was developed by those who 

 wished to build pictures, but it can also be 

 used to line uj) in correct ranks all of the 

 species found, for example, in one locahty. 



No method of arranging diatoms on, 

 and attaching them to, a coverslip will 

 compare with that of BelHdo 1927 (11360, 

 47:9). The description cited is one of 

 very considerable complexity and goes 

 into details not possible in the present 

 place. It consists essentially, however, of 

 coating a chemically clean covershp with 

 an exceedingly thiu film of Bellido's ce- 

 ment (Chapter 28, V 11.1 Bellido 1897) 

 which is then dried. Bellitlo recommends 



