PREPARATION OF DIATOMS. Si 



diatoms on the prepared cover-glass and to arrange them conveniently, 

 in order to avoid all risk of their being carried away by the bristle 

 or brush. 



"Diatoms should be selected under a compound microscope with a 

 magnification of So to ioo diameters ; they are placed on the prepared 

 cover-glass by means of a lens or doublet, which is quite sufficient to 

 give a general view of what is being done. 



" D. Fixing. The diatom having been placed in position, the brush 

 is slightly soaked in the solution of Gum-Tragacanth, and, after moistening 

 the cover-glass with the brush, the diatom should be gently touched with 

 the brush, which should be glided over the cover-glass. If there is any 

 fear of disarranging the placed diatom it may be sufficient to place some 

 gum completely round it, and to moisten it thoroughly with the breath ; the 

 gum spreads and fixes the diatom, but the first procedure is preferable and 

 surer. By passing the brush between the lips, its point is re-made, and 

 it is ready for selecting. (This is the method of Mr. H. Dalton). 



" E. Imbibition. One or more diatoms having in this manner been 

 placed and fixed, the prepared cover-glass is put under the microscope 

 and examined, to see if the diatoms are exactly the same species, 

 and whether they are suitably arranged. When these conditions are 

 fulfilled and after every trace of moisture has disappeared (heat gently 

 for greater security) a drop of the imbibing solution is deposited on 

 the cover-glass, and the progressive absorption of air bubbles can be 

 observed under the microscope, further liquid being added when the 

 previous dose is on the point of being evaporated, but there is no 

 cause for alarm if it should trickle slightly over the sides on to the slip. 



" F. Mounting. When the air bubbles have disappeared and before 

 the imbibing solution has completely evaporated, a drop of styrax is 

 added ; it should be left a few moments to allow the balsam to penetrate 

 the diatoms so as to take the place of the imbibing solution (without 

 this the bubbles may reappear), then it is carried to the heating table. 

 It is heated until the styrax commences to smoke ; should any 

 bubbles be seen on the cover-glass, they can be burst by bringing 

 the flame of a lighted match close to them ; then making the cover- 

 glass slide to the edge of the slip with the end of a pair of forceps 

 it is taken hold of and turned over on to the centre of the slip. 

 By taking the slip under the microscope while it is still warm, the 

 cover-glass may be turned round in such a manner that the diatoms 

 are in the best position. It now only remains to let it cool and to 

 remove all surplus balsam with a piece of linen saturated with alcohol 



G 



