302 SUMMARY OF CUERENT RESEARCHES RELATING TO 



If a more stable mounting is desired, proceed as before, and then 

 cover the specimen with a drop of carbolated gum, after which the 

 cover- glass is put on. This medium is preferable to glycerin jelly, as 

 it is manipulated cold. The gum is made as follows. Best white gum- 

 arabic 5 grm., distilled water 5 grm. After the gum has dissolved add 

 10 drops of carbolated glycerin and warm gently until the fluid clears. 



The author states that with a little practice very good sections can 

 be made by merely placing the object moistened with water on the 

 thumbnail of the left hand and chopping at it with a razor. 



The sections are put in a drop of carbolated glycerin on a slide 

 between two cover-glasses, and covered with a third cover-glass, so that 

 the latter is supported by the two former. This renders their manipu- 

 lation easy. 



For bringing out the details of the structure of the peristome, and 

 to distinguish certain cell-walls, the author uses a dilute solution of 

 perchloride of iron (officinal solution of perchloride 1 part, distilled 

 water 9 parts). 



Clearing recent Diatomaceous Material.* — The preparatory clear- 

 ing, says Mr. F. W. Weir, must of course vary with the nature of the 

 material. A poor gathering, requiring a quart or two of material to 

 commence with, and consisting chiefly of coarse sand, should be placed 

 in a large pail of water, and stirred with a very rapid rotary motion, 

 allowed to settle a moment, poured off and saved. This process should 

 be repeated until the portion saved is sufficiently concentrated to be 

 suitable for further treatment. If the collection is comparatively rich, 

 and consists of the usual marsh deposit, it should be at once subjected 

 to acid treatment, with, however, a thorough washing with salt. In 

 order not to lose any diatoms it is often necessary to use the filter. For 

 acid treatment the author prefers sulphuric acid and bichromate of 

 potash. Place the wet material in a porcelain vessel ; add about half as 

 much powdered bichromate of potash as there is material ; while stirring 

 pour in sulphuric acid slowly, but with increasing rapidity. Allow the 

 acid to cool, and pour into a gallon jar of filtered water. When 

 thoroughly settled draw off the liquid with a siphon, repeating the 

 process until the acid is entirely removed. If the acid clearing have 

 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 centrifugal force, fine sand by 

 friction, and amorphous sand by gravity. 



Place a proportionate quantity of the material in a small tumbler ; 

 between the thumb and finger take a glass rod about 10 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. 

 This keeps up the coarse sand in the centre, and the remainder may be 

 drawn off before settling with a siphon applied to the edge of the bottom 

 of the glass. Eepeat the process until nothing but sand remains. Take 

 the settlings and go through a similar process until sand no longer 

 collects in the centre of the glass. 



Now place the material in a wide-mouthed vial of suitable size. 

 Fill the vial two-thirds full of filtered water and shake vigorously. Allow 



* The Microscope, ix. (1889) pp. 1-4. 



