282 ELEMENTARY CHEMICAL MICROSCOPY 



however, very small quantities of liquids can be handled, and the 

 high speed obtainable will throw out even a precipitate whose 

 specific gravity differs but little from the liquid in which it is 

 suspended. 



A convenient form of tube for use at high speeds may be made 

 as follows: An ordinary glass tube of proper size is drawn out 

 to a point in the flame of the blast lamp, and then, by continued 

 heating, the glass is allowed to thicken a little at the end; the 

 end is pressed, while still soft, against a piece of asbestos board, 

 or a piece of charcoal, to flatten it sufficiently to lit well in the 

 hematokrit frame. The tube is then cut the proper length, and 

 the upper end smoothed with a file or rounded in the lamp flame. 

 The turbid liquid to be treated is introduced into the tube by 

 means of a pipette with long capillary end, and the tube is then 

 placed in the frame; a similar tube is filled with water to the 

 same height, and is placed in the other side as a balance. Thus 

 arranged, the machine is turned at such speed and for such a 

 time as may be necessary to yield a clear liquid. 



The treatment to which the sedimentation tube is then sub- 

 jected will depend upon whether the liquid or the sediment (or 

 both) is wanted. When the clear supernatant liquid is required, 

 it is removed by means of a pipette with long capillary tip. But 

 when the precipitate alone is needed the clear liquid is most con- 

 veniently removed by capillary tubes, made by drawing out odds 

 and ends of glass tubing. With such tubes it is only necessary 

 to touch the liquid, which will immediately be drawn up by 

 capillarity; the tubes filled as far as the force will raise the liquid 

 are thrown away. One tube after another is inserted until the 

 liquid is lowered to a point just above the sediment. Distilled 

 water is introduced, and if the precipitate is to be washed, the 

 contents of the tube are mixed well with a platinum wire, and the 

 tube is again whirled to effect a separation; for most purposes 

 one washing is sufficient. The wash water is removed as before, 

 and if the amount of sediment is very small, the tube is cut off 

 just above it to enable easy removal of the solid material. The 

 upper part of the tube is not wasted, but serves to make capillary 

 tubes. These small sedimentation tubes are easily and quickly 



