220 METHODS IN PLANT HISTOLOGY 



covered at high water, or from the mud of filter beds and pumping-worls:s, or 

 in other places. The material is put in a dish of water, and after it has settled 

 the water is decanted. This is repeated until the water will clear in about half 

 an hour. The sediment is then treated with an equal bulk of sulphuric acid, 

 after which dichromate of potash is added until all action ceases. After a 

 couple of hours the acid is washed out. To separate the diatoms, place the 

 sediment in a glass dish with water, and when the water becomes clear give 

 the dish a slight rotary motion. This will bring the diatoms to the top, when 

 they may be removed with a pipette and placed in alcohol. To mount, place 

 a number in distilled water, evaporate a few drops of the mixture on a cover- 

 glass, which is then mounted on a slide in balsam. 



It is better to use a very slight smear of Mayer's albumen fixative 

 to prevent the diatoms from floating to the edge of the cover. 



Many scouring soaps and silver polishes contain large quantities of 

 fossil diatoms, and the diatomaceous earths are particularly rich. 

 Diatomaceous earths from Cherryfield, Maine, and from Beddington, 

 in the same region, are the richest we have seen (Fig. 46). The de- 

 posits at Richmond, Virginia, have long been famous. In some of our 

 western states there are deposits 300 feet thick, with 80 per cent of 

 silica, the silica being the valves of diatoms. 



Break up a small lump of such material and boil it in hydrochloric 

 acid. An evaporating dish or a test-tube is convenient for this purpose. 

 Let the diatoms settle, pour off the acid, and then wash in water. As 

 soon as the diatoms settle, the water should be poured off. The wash- 

 ing should be continued until the hydrochloric acid has been removed. 

 When the washing is complete, pour on a little absolute alcohol, and 

 after a few minutes pour off the alcohol and add equal parts of turpen- 

 tine and carbolic acid. The material will keep indefinitely in this con- 

 dition, and may be mounted in balsam at any time. In making a 

 mount, put a little of the material on a slide and allow it to become 

 dry, or nearly dry, and then add the balsam and cover. If the balsam 

 should be added too soon, the diatoms are likely to move to the edge 

 of the cover. 



We have had excellent results with the following method: After 

 washing in water, keep the diatoms in 5 per cent formahn. To make 

 a mount, smear the slide very slightly with Mayer's albumen fixative, 

 add a little of the material, and heat just enough to coagulate the albu- 

 men. When perfectly dry, add a drop of balsam and a cover. Or, after 

 coagulating the fixative, dip in absolute alcohol and then in xylol be- 

 fore mounting in balsam. Without the alcohol and xylol, some air is 



