COLLECTION, EXAMINATION, ETC. 317 



of the piece of pumice stone must be kept always 

 very plane by means of a fine file. 



' Suppose that we desire to obtain a section of a 

 BUocidina. The first object to be attained is the 

 filling of all its chambers, to the very centre or 

 primordial chamber, with a medium sufiiciently re- 

 sisting to prevent the shell being crushed by the 

 action of the rubbing stone. To effect this the 

 Biloculina must be dropped into a stoppered tube 

 with a little chloroform, upon which at first it will 

 most probably float, but the fluid penetrating little 

 by little into the inner chamber will drive out the 

 air, and the test will sink to the bottom of the tube. 

 After letting it remain there for a while it is picked 

 out and set in a drop of Canada balsam, placed upon 

 the centre of a cover slip. The chloroform being a 

 rapid and perfect solvent of Canada balsam, the 

 latter will soon penetrate to the inmost chambers ; 

 but it is desirable, if one is not pressed for time, to 

 wait a day or two before continuing the operation, in 

 order that the chloroform may disappear from the 

 chambers by evaporation as completely as possible ; 

 this prevents the inconvenient presence of bubbles of 

 air in the subsequent stages of the operation. 



' When one judges that the balsam has suffi- 

 ciently penetrated all the chambers of the test, one 

 warms the cover glass over the spirit lamp to drive 

 off the volatile constituents of the resin, and before 

 the drop of balsam containing the test has become 

 cold and hard the test is placed in the position 

 required for making the section by means of a hot 



