ON MOUNTING. 159 



of bicarbonate of soda in a test-tube of large 

 capacity for an hour ; this will remove all flocculent 

 matter, and after repeated washings, perfectly 

 clean polycystina will be obtained. The heaviest 

 density consisting of sand and the largest poly- 

 cystina should be placed in a test-tube with about 

 3 inches of water and subjected to rotatory motion, 

 this will cause the polycystinous shells to rise above, 

 and to free themselves from, the sand, and they 

 can be poured off into a wide-mouthed bottle or 

 beaker, the sand being left at the bottom of the 

 test-tube. This latter process should be carefully 

 conducted and repeated several times in order that 

 no large and perfect shells may be left amongst the 

 sand. Each density should then be bottled in 

 distilled water. Polycystina may be mounted in 

 balsam in the same manner as diatoms or "dry." 

 Beautiful slides can be produced by calcining the 

 shells upon a piece of thick platinum foil, or in 

 a small platinum capsule, and mounting them 

 "opaque." The neatest and best method of pre- 

 paring such slides is to make a disc half an inch in 

 diameter in the centre of a slip, allowing it to 

 harden for some days. A half-inch cover is then 

 to be cleaned and a sufficient quantity of balsam, 

 thinned with turpentine, put upon it; the polycy- 

 stina are then to be placed in the balsam in suffi- 

 cient quantity to form a surface over the cover 

 when evenly spread upon it ; the cover with the 

 polycystina is then to be put aside (as recommended 

 with diatoms) for twelve or twenty-four hours, in 

 order that the balsam may thoroughly permeate 

 the forms and all air escape ; the balsam is then 

 to be hardened by gentle heat, as already described, 



