36 PREPARATION AND MOUNTING 



then be repeated until a drop of the liquid when placed under the 

 microscope shows the valves or " frustules" clean. After allowing 

 the diatoms to settle, the acid must be decanted, and pure water 

 substituted. The washing must be repeated as in the former pro- 

 cess until all the remains of crystals or acid are removed, when the 

 specimens may be preserved in small phials as stated above. 



Such are the usual modes of treating the Diatomacese, but there 

 are certain cases in which particular methods are required to give 

 anything like perfect results. Persons of great experience com- 

 bine a variety of treatments, and thus obtain better and more uni- 

 form specimens. Perhaps it will be advantageous to give the 

 young student the process adopted by one of the most successful 

 preparers of these objects ; but I will first state the different me- 

 thods of mounting the cleaned diatoms dry: how to employ 

 Canada balsam and fluid in their preservation will be elsewhere 

 described. 



It was before stated that the diatoms when cleansed might be 

 preserved in small phials of distilled water. When required for 

 mounting, shake the phial, and with a thin glass tube or rod take 

 up a drop of the fluid and spread it upon the surface of the slide 

 in the desired position. This must then be allowed to dry gra- 

 dually, or by the aid of the lamp if necessary, without being 

 shaken or interfered with, otherwise uniformity of dispersion will 

 be prevented. When thoroughly dry, a thin ring of one of the 

 adhesive varnishes gold-size will be found as good as any may 

 be drawn round the diatoms, and allowed to dry in a slight degree. 

 The slide and thin glass cover should then be warmed and the 

 latter gently pressed upon the ring of varnish until the adhesion 

 all round is complete. 



As some of the diatoms require object-glasses of extremely 

 high power, and consequently short focus, to show them, they 

 must be as close to the outer surface of the cover as possible. 

 For this reason they are sometimes placed upon the under side 

 of the thin glass, as follows. Clean the surfaces of the slide and 

 cover, and with the rod or pipe place the liquid containing the 

 diatoms upon the thin glass, and dry as before. Trace the ring 

 to receive the cover upon the slide, and when almost dry, warm 

 both and proceed as above. Whichever of these methods is 



