Shake the bottle in order that the surfaces of the covers may be fully exposed to the action of the acid, and set aside for several hours. Decant the solution, add water repeatedly until all traces of the mixture are removed, and keep the circles in the bottle in fifty-per cent. alcohol. When needed, take out a circle with forceps and dry on a linen cloth. The slides may be treated in the same way, or they may be easily prepared by immersion in a solution of washing soda, and then washed and dried. This process may be used in cleaning the balsam or styrax from old slides. _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.