216 Modern Microscopy 



must now proceed to eliminate the whole, or nearly the 

 whole, of the sand and as much as possible of the other 

 debris by means of two operations ' floating ' and ' rock- 

 ing.' If the quantity of material to be operated upon is 

 small, it may be treated off-hand, but if there is much, it 

 is well to sift it out into varying degrees of fineness by 

 passing it through a series of sieves. This will simplify 

 the floating operations by insuring that the particles are 

 approximately of similar weight. 



The floating operations must be performed at a sink, 

 and, if possible, in daylight, the process being more un- 

 certain by artificial light. The finest sieve (120 wire or 

 150 silk) is thoroughly wetted and rested on the tripod. 

 The glass jar is then filled with water nearly to the brim, 

 and a few spoonfuls of sand slowly poured into it. If the 

 material is coarse the sand sinks instantly, and in the 

 course of a few seconds most of the foraminifera follow 

 suit. By holding the jar to the light the course of the 

 falling particles can be followed, and at the proper moment 

 a sudden tilt empties the whole of the water and most of 

 the foraminifera into the sieve, the sand and a few of the 

 heaviest ' forams ' being left in the jar. The purity of the 

 material in the sieve, which is usually called ' floatings,' 

 will depend upon the skill and judgment of the operator, 

 and is largely a matter of practice. The residuum in the 

 jar must be washed out into a basin for further treatment, 

 and the operation repeated with more sand and water until 

 the whole of the gathering has been treated. The time 

 allowed for subsidence will vary with the fineness of the 

 sand, so that in the case of the finest siftings nearly a 

 minute may be required. The actual time can only be deter- 

 mined by watching the falling material in a strong light. 



In the case of very fine sand, the tension of the surface 

 film of water is so great that the sand grains float almost 

 as readily as the foraminifera. This difficulty may be 

 overcome by shaking up the contents of the jar, covering 

 up the top with one's hand while so doing. 



