2 THE MICROSCOPE. 
to which diatoms may adhere—it should be at once subjected to acid 
treatment, with, however, a thorough washing of salt. In washing 
great care must be exercised not to lose the diatoms which adhere to 
floating matter. Very often it is necessary, and always well, to 
sacrifice a little time and use the filter. For acid treatment I very 
greatly prefer the sulphuric acid and bi-chromate of potash, cold 
method. Place the wet material in a porcelain vessel (granite iron 
cups are serviceable and cheap); add about half as much powdered 
bi-chromate of potash as there is material; while stirring pour in 
sulphuric acid slowly, but with increasing rapidity as the intensity 
of chemical action will allow it, and in most cases, if the proportion 
of bi-chromate is correct, when the action has ceased, the destruction 
of destructible matter will be complete. Allow the acid to cool and 
pour into a gallon jar of filtered water. After thorough settling 
draw off the liquid with a siphon. Never pour off water at any 
time during the process. Repeat this until the acid is entirely 
removed. If the acid cleaning has been complete there will now 
remain undesirable matter of three kinds—coarse sand, fine sand, 
and fine amorphous matter—which must be removed in three ways: 
coarse sand by centripetal force, fine sand by friction, and amorphous 
matter by gravity. 
Place a proportionate quantity of the material in a small 
tumbler—a liquor glass with slightly convex bottom is good,— 
between the thumb and finger take a glass rod about ten inches 
long, suspend with lower end in the glass, and, by giving the hand a 
rotary motion in a small circle, cause the lower end of the rod to 
travel round the periphery of the bottom of the glass with the 
utmost possible speed, giving contents of glass the utmost possible 
rotary motion. This will, of course, heap up the coarse sand in the 
center, and the remainder may be drawn off before settling 
with a siphon applied to the edge of bottom of glass. Kefill the 
glass and repeat the process until nothing but sand remains, which 
may be thrown away. Take the settlings which have been saved in a 
larger vessel, return them to the glass, and repeat the rotating 
process as before. Do this until, with the utmost speed of rotation, 
no sand can be collected in the center of the glass. 
Now place the material in a wide-mouthed vial of suitable size 
and dispose of the amorphous fine matter. Fill the vial two-thirds 
full of filtered water and shake. Shake as you never shook before. 
The fine matter adheres to the diatoms and is carried down with 
them in settling. A quantity of material in this stage that after a 
