366 On the Diatomacece. [Sess. 



mounting-plate, the cover-glasses are taken up in the forceps 

 and placed one by one upon their respective slides, with the 

 styrax face of course downwards. The spirit-lamp is now lit 

 and the plate gently heated, when the styrax will spread in 

 an even layer under the cover-glass. As the cover-glass has a 

 tendency to slip to one side it must, by delicate manipulation, 

 be kept in the centre of the slide. The slides are now laid aside 

 to cool, when a ring of cement may be run round the edge, and 

 after the slide is duly labelled the mounting is complete. 



Mounting selected Diatoms. — To some this mode of mounting 

 has great fascination. It requires a steady eye, a firm hand, 

 and above all great patience. One of the most celebrated 

 mounters by this mode was the late Herr J. D. Moller, Wedel 

 in Holstein. His " Typen-Platten " are famed, many hun- 

 dreds of diatoms being arranged on one slide ; on the one in 

 my possession there are upwards of 400. The substance he 

 used as a fixative was, I understand, kept a secret. His 

 mounting medium was monobromide of naphthalin, which has 

 a high refractive index. 



The most accomplished mounter of my personal acquaint- 

 ances is the Eev. Dr Davidson, minister of the parish of 

 Logie-Coldstone, whom I have before mentioned in connection 

 with this study. The most essential thing is to get a good 

 fixative for the diatoms. As I have said, that used by the 

 late Herr Moller is not generally known. The following may, 

 however, be used with advantage : White shellac dissolved 

 in pyroligneous spirit to a thin solution and then filtered. A 

 very small quantity of this is spread by means of a clean quill 

 in the centre of the cleaned cover-glass which is to receive 

 the diatoms, and allowed to dry. This cover-glass, with the 

 smeared side uppermost, is placed on a slip of glass, to which 

 it may be made to adhere by a small quantity of moisture 

 under it. ' It is then placed under the mounting microscope. 

 On the same plane on the glass slip is placed a drop of the 

 cleaned diatoms to be selected, and beside it a drop of dis- 

 tilled water. With a bristle from a cat's whisker cut to the 

 requisite degree of firmness, and fastened in a quill, the 

 diatoms selected under the mounting microscope are separated 

 from the mass and transferred to the drop of distilled water 

 to be thoroughly washed and freed from all extraneous matter. 

 When cleaned they are pushed to the side and allowed to dry. 



