1886.] 



MICEOSCOPICAL JOUKNAL. 



149 



do this is to place them for a few 

 minutes in a bleaching solution which 

 may be chlorine water Labarraque 

 solution, or any such active agent. 

 No acid is required. In the course 

 of fifteen minutes the frustules will 

 probably be quite w'hite and, owing 

 to the air contained in them, they will 

 form a perfectly pure layer floating at 

 the top of the fluid. It is then only 

 necessary to remove the solution be- 

 low by means of a pipette or syphon, 

 wash several times with water, draw- 

 ing itoftMn the same way, and finally 

 collecting the diatoms in a bottle with 

 some alcohol for preservation. They 

 are now perfectly clean, and white as 

 snow. 



To prepare a dry mount select a 

 clean cover-glass and place a suffi- 

 cient number of the cleaned diatoms 

 with water upon it to form a perfectly 

 even layer of the diatoms over the 

 central part of the cover. As the 

 water evaporates the frustules will 

 gather close together and form a com- 

 pact mass in a single, uniform layer, 

 perfectly adapted for a display slide. 

 An exceedingly thin and clear solu- 

 tion of gum may be used in this 

 operation to attach the frustules more 

 securely. When thoroughly dry ce- 

 ment the cover-glass over a ring just 

 deep enough to protect the diatoms, 

 preferably with a dead black bottom. 



This particular diatom, however, 

 is a far more brilliant object when 

 mounted in balsam and viewed with 

 a dark field. It is likewise one of the 

 most difficult to mount in balsam, 

 owing to the persistence with which 

 the air is retained within the frustules. 

 A mount in balsam of the diatoms 

 attached to the seaweed as they grow 

 can be made by the method devised 

 by the late Charles Stodder. Select- 

 ing a perfectly dry specimen, place it 

 in chloroform for a short time, and, if 

 necessary in order to remove all the 

 air, heat the latter gently. In this 

 way the frustules become filled with 

 the liquid. Then place some drops 

 of chloroform on a slide, transfer 

 the specimen selected for mounting 



to this, and keep it covei-ed with the 

 liquid. It is well to put on a cover- 

 glass to prevent rapid evaporation of 

 the liquid. Then add chloroform 

 balsam and let it run under the cover 

 and follow the chloroform as it evap- 

 orates from the frustules, aiding the 

 operation with gentle heat. In this 

 way the hollow frustules can be com- 

 pletely filled with balsam without 

 difficulty, and the mounts thus ob- 

 tained are very fine. 



In mounting the free frustules in 

 balsam we have adopted a plan some- 

 what different in detail, in order to 

 obtain a perfecti}' flat and even layer 

 of frustules against the cover-glass. 

 The cleaned specimens in considera- 

 ble abundance were first placed in 

 chloroform in a small vial, and raw, 

 hard balsam added until a not very 

 thick solution was obtained, which 

 thoroughly permeated the shells. 

 The solution was poured upon a 

 cover-glass resting on a mounting table 

 with a spirit-lamp beneath. In a short 

 time the frustules settled down upon 

 the cover-glass and formed an even 

 layer. The closer they are the more ef- 

 fective the result. Heating now, very 

 gently indeed, the balsam becomes 

 slowly hardened without disturbing 

 the diatoms. If necessary, more bal- 

 sam can be added, but if possible a 

 sufficient qviantity should be put on at 

 first, as the addition of more is likely 

 to disarrange the specimens. The 

 balsam must be thoroughly hardened, 

 without heating enough to discolor 

 it. We now have the frustules nicely 

 mounted in the balsam on the cover- 

 glass, and the latter may now be 

 turned over and attached to a ring on 

 a slide, and the mount thus finished. 

 It will be greatly improved, however, 

 by the well-known process of backing 

 with black varnish. First put on a 

 layer of shellac over the balsam to 

 protect it from the action of turpen- 

 tine, and then apply an opaque layer 

 of black varnish. When this is 

 thoroughly dry, mount the cover- 

 glass on a ring and it will make one 

 of the finest objects in any cabinet. 



