1897-9^-] On the Diatomaceaz. 365 



being mounted the pipette must be thoroughly cleaned between 

 each mounting. 



A very shallow cell of the requisite size is now made upon 

 a slide, and when thoroughly dry one of the charged cover- 

 glasses is taken in the forceps and placed upon the cell, with 

 the diatoms of course next to the slide : it is then pressed 

 down so as to make it adhere. Any pressure must be applied 

 to the part immediately above the ring of the cell. It may 

 then be put aside until a number are ready for being finished 

 upon the turntable, by having a course or two of cement run 

 round the edges, slightly overlapping the edge of the cover- 

 glass. The slide is now duly labelled, and the process of 

 mounting is finished. 



■ Mounting in a Medium. — The object of mounting in a 

 medium is by embedding the diatoms in a substance of a 

 higher refractive power than that of the silex of the diatom 

 to bring them into greater relief under the microscope, and 

 thus to show their details more clearly. There are a large 

 number of mounting media. I have used Canada balsam, 

 styrax, glycerino-gelatine, cassia oil, and a solution of aniline 

 and sulphur. Of all these I prefer the Canada balsam or the 

 styrax, and as the styrax is of a somewhat higher refractive 

 index I generally use it. 



Styrax is the natural juice of the Styrax orientalis, a tree of 

 Asia Minor. It is best to purchase it ready prepared. My 

 preparation was obtained from Messrs Rousseau et Fils, 42 

 Eue-des-Ecoles, Paris. A small quantity of it lasts a long time. 

 The only objection to it is its dark brown colour, but in the 

 thin layer which is necessary this is not at all harmful. It is 

 diluted to the consistency of a thin syrup by pure benzole. 



The process in this mounting is the same as in the pre- 

 ceding, up to the point where the diatoms are left dry upon 

 the cover-glass. A drop of benzole is then put upon them 

 which spreads over all, fills the crevices, and thus expels the 

 air. Before it has dried, sufficient styrax to cover the diatoms 

 with a thin layer is dropped upon them. The cover-glass is 

 now left under the bell-glass from twelve to twenty-four 

 hours, according to the time at your disposal. This allows 

 the benzole to evaporate. When you are ready to put the 

 cover-glasses on to the slides the slides are arranged on the 



vol. in. 2 B 



