38 



THE ART OF MAKING MICROSCOPE SLIDES 



Diatoms 



other extraneous material. Many of the 

 more interesting diatoms, however, are 

 found in hard aggregates which must be 

 broken up before the frustules can be 

 separated. Many methods of doing this 

 have been described, but undoubtedly one 

 of the most useful is the technique of 

 Swatman {Microscope, 7:132). 



This technique utiUzes the expansion 

 and contraction which takes place on the 

 sudden crystallization of a supersaturated 

 solution of sodium acetate. The rock, or 

 hard aggregate, containi ng the diatoms is 

 roughly broken into j'i-inch pieces and 

 placed at the bottom of an Erlenmeyer 

 flask. Two or three times its own bulk of 

 sodium acetate is then added, and thor- 

 oughly mixed in, before adding water to 

 the extent of about 5 % the total weight of 

 the sodium acetate. The flask is then very 

 carefully warmed, the flame being first ap- 

 plied to the sides and not to the bottom, 

 until the sodium acetate is molten. Heat- 

 ing should then be continued until the 

 material commences to boil and it should 

 be maintained in a hot condition for as 

 long as is required to cause the penetra- 

 tion of this supersaturated solution to 

 every part of the aggregate. The flask is 

 then cooled slowly, care being taken to 

 avoid jarring, and when the solution is 

 cold a single crystal of sodium acetate is 

 dropped into it, which causes instant 

 crystallization. As the flask will heat up 

 greatly during crystallization, it is then 

 recooled in water. The mass is remelted, 

 recooled, recrj^stallized, and so on until a 

 sufficient disintegration of the rock has 

 taken place. Another method of arriving 

 at the same result is to soak the pieces of 

 material in water, to freeze them very 

 rapidly (either in a freezer unit or in dry 

 ice) then to drop them into warm water, 

 refreeze them, and so on. This process is 

 no more effective, however, and is usually 

 much more trouble to carry out, than the 

 sodium acetate procedure outlined. When 

 the mass has sufficiently disintegrated, it 

 is strained through a coarse sieve to get 

 rid of the lumps and the diatomacious ma- 

 terial allowed to form a sludge at the 

 bottom. 



Whatever method has been employed, 

 one has now, from either fresh or fossil 



material, a sludge which should be trans- 

 ferred to a flask. This sludge contains dia- 

 toms together with various organic and 

 inorganic impurities. The first thing is to 

 get rid of any carbonates which may be 

 present by adding hydrochloric acid cau- 

 tiously (if there is a great deal of carbonate 

 present effervescence may rise above the 

 neck of the flask and cause a loss of mate- 

 rial) until no further gas is evolved. The 

 flask is then filled with water, the undis- 

 solved material allowed to settle, the water 

 poured off, and the process repeated until 

 all the soluble chloride has been removed. 

 If there is any appreciable amount of clay 

 present, it will also have been removed by 

 this process, since even the smallest dia- 

 toms will settle relatively rapidly com- 

 pared to the fine particles of clay. It is 

 next necessary to remove an}^ organic 

 matter which may be present, and many 

 methods have been proposed for this. The 

 conventional method, also described by 

 Swatman (loc. cit.), is to get the diatoms 

 into concentrated sulfuric acid which is 

 then heated to about 120°C. This chars 

 the organic matter which is then oxidized 

 by dropping small crystals of potassium 

 chlorate into the hot acid. It should per- 

 haps be emphasized that only exceedingly 

 small crystals should be added, and that 

 those who do not normally wear glasses 

 should use some form of protection against 

 the chance of spurting acid. If there is 

 much organic matter present, the heated 

 acid will be from black to dark brown in 

 color, and chlorate is added until the color 

 is reduced to yellow. The only safe method 

 of removing the acid is to wait until it is 

 entirely cold and then pour it in a slow and 

 steady stream, while constantly stirring, 

 into a relatively large volume of water. 

 The diatoms settle out on the bottom. 

 Some iron may still be present, either de- 

 rived from a fossil deposit or from the 

 chlorophyll of the plant debris. This is best 

 removed by suspending the diatoms in 5 % 

 sodium hydroxide and bringing them to the 

 boil. If there is any iron present it will ap- 

 pear as a brownish ferric hydroxide 

 through which the diatoms will settle 

 readily and which may then be poured off. 

 After repeating this process several times, 

 the diatoms are treated with hydrochloric 



