PREPARATION OF DIATOMS. 69 



River, O., sent to me, but although it had been chlorated, still, when a 

 mounting was made, not more than one or two frustules would be in 

 the field of view, the great mass being either smaller forms, or fine frag- 

 ments of silex ; by careful watching and testing the time when the 

 different sizes would remain suspended, I have made from this a pre- 

 paration, which will show hundreds where before were scarcely any, and 

 which would never be recognised as the same gathering. Supposing now 

 a trial shows us the diatoms tolerably abundant, the trial being made 

 by heating in the manner presently to be described ; the phial is filled 

 with alcohol and water, half and half. Some samples of alcohol leave 

 behind a scum after evaporation, especially noticeable after burning in 

 the mode presently to be described, and water which will leave crystals, 

 or any scum, must be avoided. The beauty of the preparation will 

 largely depend upon being particular in this matter. 



" For mounting diatoms I invariably place a drop of the fluid con- 

 taining them upon the cover, never on the slide. The alcohol and water 

 will spread out on the slide, but will remain heaped up on the round 

 cover, like a plane convex lens. I prepare a little stand, of quite fine 

 wire (so as not to conduct off too much heat), bent at right angles and 

 inserted into a base ; the free end is bent into a ring, and upon this 

 ring is placed a square plate of very thin iron, (') (such as is used for 

 the so-called "tin-types" in photography, with the Japan burned off), 

 held in place by bending the corners of the square over the ring, loosely, 

 to allow expansion, without bending when heated ; upon this plate the 

 cleaned cover is placed, and then, by means of a pipette, a drop of the 

 alcoholic liquid with the diatoms is placed upon it, and the spirit lamp 

 applied below. The alcohol takes fire and is allowed to burn out ; the 

 flame of the lamp is then placed beneath, and the rest gently boiled, 

 the remaining alcohol escaping during this ebullition causes the diatoms, 

 by this very act, to distribute themselves very evenly over the cover, and 

 all matting is effectually prevented. It is better, after one perceives that 

 this even distribution has taken place, not to push the heat so as to 

 make large bubbles again, but to slowly evaporate until dry, after which 

 the full power of the flame must be applied until the iron plate and the 

 glass cover are red hot ; at first the mass of diatoms, etc., will become 

 black, but as the organic contents and debris burn away there will finally 

 remain only the silex nearly white. I invariably burn in this manner 

 on the cover; even the specimens which have been prepared with acids, 



( ' ) I prefer to use a small square piece of platinum foil, with the corners folded back, and 

 held tightly with a pair of forceps. H.V.H. 



