OF MICROSCOPIC OBJECTS. 35 



natural state upon the thin glass, burnt for awhile upon the plati- 

 num plate, hereafter described, and mounted dry or in balsam. 



In the preparation and cleaning of Diatomacese, there is little 

 satisfaction unless these operations have been successfully per- 

 formed, as a very small portion of foreign matter seriously inter- 

 feres with the object. The mode of preparing them varies even 

 amongst the most experienced. It will be found, therefore, most 

 satisfactory to examine the principal of these separately, although 

 it may be at the risk of some little repetition. 



The method which is the most frequently made use of is the 

 following : Place the " gathering" containing the Diatomacese in 

 a small glass or porcelain vessel, add strong nitric acid, and, by the 

 aid of Bunsen's burner or spirit-lamp, boil for some minutes. 

 From time to time a drop of the mixture may be put upon a slide, 

 and examined under the microscope to see if all foreign matter be 

 got rid of. When the valves are clean, the vessel containing them 

 must be filled with water, and the whole left for an hour or two, 

 so that all the diatoms may settle perfectly. The liquid must then 

 be poured off carefully, or drained away by the aid of a syphon, 

 so that none of the diatoms are removed with it. Indeed, it is 

 well to examine the liquid drained off each time with the micro- 

 scope, as the finer forms are frequently lost in the washings. The 

 vessel must then be refilled with pure water, allowed to settle, and 

 drained as before. This washing must be repeated until a drop 

 being placed upon a slide and evaporated leaves no crystals. 

 When it is desirable to preserve the diatoms in this state before 

 mounting (which process will be described in another place), they 

 may be placed in a small phial with a little distilled water. 



There are many cases in which the above method will not effect 

 a perfect cleansing, as certain substances with which the diatoms 

 are frequently mixed are not soluble in nitric acid. For this reason 

 the following method is resorted to : Take a quantity of the 

 matter containing the Diatomacese and wash first with pure water, 

 to get rid of all the impurities possible. Allow this to settle per- 

 fectly and decant the water. Add hydrochloric acid gradually, 

 and when all effervescence has subsided, boil for some minutes by 

 aid of the lamp. When cool and the particles have subsided, 

 decant the hydrochloric and add nitric acid. The boiling must 



