68 PREPARATION OF DIATOMS. 



cohering. And I have many bottles of preparations for mounting, 

 which are nearly as clean as though they had been treated with acids. 

 And many of the most interesting preparations which I have were 

 never boiled in acids. Of course, very much depends upon the skill 

 and carefulness of the gatherer, and a little patience and judgment 

 will enable any one to obtain the crude material tolerably pure. Only 

 a few days ago I made a gathering of JVitzschia, in which I have 

 the frustules almost as free from foreign matter as though they had passed 

 through the most elaborate acid and chlorate of potassa treatment. 



" Supposing, then, that one has before him a phial which will hold 

 a considerable quantity of water compared with the sediment in it, 

 the latter composed more or less of diatoms. We proceed thus, and 

 if it has stood for some days, perfectly undisturbed, so much the 

 better. The bottle is twirled rapidly, and the lighter material rising 

 up in the axis will soon diffuse itself throughout the water. 



" Allowing it to settle for two or three seconds, until to the eye 

 the grosser portions have just been deposited, all that remains floating 

 is now poured off into another phial, and it is from this stock that 

 we are to separate the diatoms and sand from the clay and organic 

 matter. The material poured into this second bottle is allowed to settle 

 until the water simply appears milky or cloudy ; the time will vary 

 according to the minuteness of the diatoms, and can only be judged 

 of from experience, say one minute, when all that remains floating must 

 be poured off, and thrown away, unless there are very minute forms 

 which it may be desirable to separate. The phial is again to be filled 

 with rain, or distilled, water, (hard or lime water should be strictly 

 eschewed) and again shaken up. As soon as the heaviest deposit 

 touches the bottom, the rest should be poured off into a third phial, 

 leaving say about one-fourth the amount behind in the second phial. 

 This third phial will now consist mainly of sand and diatoms, with 

 lighter organic matter and pure clay; the last two can be removed by 

 elutriation ; for this purpose, fill the phial No. 3 with water, and after 

 well shaking allow it to settle two to five minutes, pour off and throw 

 away the slightly milky water, and repeat the operation, allowing it to 

 settle a somewhat longer time ; the operation may be repeated a third time, 

 when particles, suspended after an interval of eight or ten minutes, may be 

 poured off. Often, after the first settling of bottle No. 2, the diatoms will 

 rise more pure in the mass by twirling the bottle than by shaking it up. 



"A little practice and care will enable anyone to separate certain diatoms 

 according to size. I had a gathering of Plcurosigma Spencerii from Scioto 



