MOUNTING AND PEEPAEATION OP OBJECTS. 61 



disturbed. They are next placed in a shallow saucer 

 and covered with a piece of muslin which lies in 

 immediate contact with the mud, while a film of 

 water lies above it. The saucer is now exposed to 

 daylight and the diatoms creep through the muslin, 

 collecting in a consistent film on its upper surface. 



If desirable, they may be mounted in this con- 

 dition in distilled water and camphor, but if the 

 silicious skeletons alone are required, the gelatinous 

 envelope must be removed by boiling in nitric acid, 

 the sediment well washed till all trace of acid has 

 disappeared, and the residue mounted dry. 



Sections may be cut by freezing, to be hereafter 

 described, using gelatin jelly instead of gum, and 

 mounting direct in glycerine on a slide. 



When contained in other deposits two ounces of 

 the mixture are placed in two gallons of water, after 

 violent shaking allowed to stand two hours, and the 

 supernatant fluid removed. This process is repeated 

 till the water is no longer coloured. As much fluid 

 as possible being removed, the deposit is treated 

 with strong hydrochloric acid, and allowed to stand 

 for an hour. The acid is then poured off and a fresh 

 supply added, the whole being boiled for one hour 

 and washed in the manner first described, until all 

 traces of acid are removed. The process is repeated 

 with strong nitric acid, and after subsequent washing 

 to free it from acid, the deposit is placed in a tum- 

 bler full of water. Now comes the most trying part 

 of the preparation. 



The tumbler is allowed to stand five minutes, at 



