_Preparation of Strewn Mounts._--Place several covers on the mounting stand. With a dipping tube, cover each circle with distilled water, and add a small drop of the prepared diatoms, being careful to avoid any vibration of the stand. Heat the stand until small bubbles begin to appear, remove the lamp, and allow the water to evaporate. If the above method is carefully followed, the diatoms will be deposited in an even layer, provided the material is not too dense. Take a slide, centre it, and place a small amount of styrax on the centre. Invert the prepared cover, and gently place it upon the styrax. Heat the slide { 134}on the mounting stand until the styrax bubbles and then allow to cool. If bubbles still remain, heat again until they disappear. It is well to mount several slides more than required, as some may be imperfect.
_Preparation of Selected Mounts._--Take a slide, place a minute quantity of beeswax on two places at a distance apart nearly equal to the diameter of the cover used. Place a cover on the wax and press it down flat, or sufficiently to keep it in position. Dip a fine needle into the following cement:
Glacial acetic acid 12 drachms Gelatine 2 drachms Alcohol 1 drachm
This is made by adding the acid to the gelatine in a water-bath and then the alcohol, and filtering. Apply the moistened needle to the centre of the cover and spread as small a quantity as possible in a thin layer. Now place the slide upon the turn table, centre it with respect to the position of the gelatine, and with the finest sable brush draw a circle about a tenth of an inch in diameter around the gelatine in water-color (Windsor), blue or vermilion, or in India ink. Instead of the water-color, a circle of tin-foil the size of the cover and pierced with a hole in the centre may be used, but the colored circle is to be preferred, as, when brought into view, it indicates exactly the focus required for observing the diatom.
The bottle containing the cleaned material, which has been kept in water and alcohol, should be refilled with distilled water and well shaken, when a small portion may be taken up with a dipping tube and evenly distributed over a portion of a slide and then dried. By the use of a mechanical finger, fitted with a small piece of finely spun glass attached by wax to the holder of the finger, when the microscope is focussed until the glass thread touches the diatom selected, it will adhere to the thread. Raise the body of the microscope, remove the slide containing the spread material, or move it to another part of the stage, and place the slide with the prepared cover in the same position. Now carefully lower the body-tube of the instrument until the diatom rests upon the gelatine, breathe gently upon it, remove the cover from the slide, invert it over another slide containing a drop of styrax and proceed by heating to mount as before. The size of the diatom, the amount of gelatine, and several other factors, will enter into the question of success or failure. I have, however, employed the above method and have mounted thousands of slides of selected diatoms successfully. It is necessary to avoid any air current which will cause the diatom to fall from the thread. On very cold days the glass thread sometimes becomes electrified and the diatoms will not stick; on sultry days in August in our locality the diatoms will stick too closely.