ART. 15. COLLECTIlSrG AND PREPARING DIATOMS MANN. 5 



liantly clear. But where the organic matter is very high, as it is 

 in very muddy material, the washed residue must be passed through 

 the sulphuric acid process. If this is needed, the water is very care- 

 fully poured away so as to remove as much of it as possible. The 

 sediment is transferred to a small distilling flask, or if this is not 

 available, to a porcelain evaporating dish. Sulphuric acid is care- 

 fully added to the wet material and the residue in the beaker is 

 washed over into the distilling flask with sulphuric acid. The heat 

 generated by the union of sulphuric acid and the material is high 

 and care must be taken to avoid explosion. The mass is now boiled 

 over a sand bath. It generally becomes perfectly black or a dark 

 brown. The boiling should continue an hour or two, as the sul- 

 phuric acid evaporates very slowly in the evaporating dish and not 

 at all in the distilling flask. There is now cautiously added to the 

 boiling mass minute particles of sodium nitrate. This is accom- 

 panied by violent sputtering, oxygen is liberated, the organic mat- 

 ter is entirely oxidized and destroyed, and the mass becomes per- 

 fectly white or pale straw color. After it has become cold it is 

 poured into a large beaker with a capacity of at least one liter, 

 half filled with water, and water then added to fill the beaker. Bat- 

 tery jars will not do for this step, because the heat generated by the 

 union of the sulphuric acid with the water is liable to crack the jar. 

 The material is now freed from acid by settling and decantation. 



Whether the double or three-fold acid process has been used, the 

 matter now reaches its final treatment to free it from undesirable ma- 

 terial. This will consist principally of sand and perhaps large radio- 

 laria. A little of the clean material is put in a porcelain evaporating 

 dish with a diameter of about 4| inches, water poured in so as to stir 

 up the mass and the whole rotated in such a way as to give a slight 

 whirling motion to the contents. It is difficult to describe this 

 process, though easy to demonstate it. It may help to render clear 

 the idea of the motion desired by saying that if we imagine an ink 

 spot to be placed on the under side of the evaporating dish exactly in 

 its center, the motion consists in rotating this ink spot around a tiny 

 spot on the table in such a waj^ that the circle of rotation will hare a 

 diameter of about a quarter of an inch. The effect of this peculiar 

 motion is to roll the rounded particles of sand and the radiolaria into 

 a little mound at the bottom of the dish, while the diatoms, which are 

 mostly angular or flat, spread out in the water. After the rotation 

 has continued for about a minute, the contents are quickly- poured 

 into a clean beaker, taking care that the sand, etc., remain in the 

 dish. Fresh water is now added and the process is repeated until it 

 is found that the residue in the dish is free from diatoms. This sand 

 separation is not always necessary, in fact is never necessary unless 



