OF MICEOSCOPIC OBJECTS. 61 



diatoms upon it. This may be held over the flame of the 

 spirit-lamp until all the surrounding matter is burnt away, 

 and a tolerable idea may thus be obtained as to the quality 

 of our treasure. 



In some cases it is well to use this burning operation 

 alone in mounting specimens of diatoms, when they may be 

 placed in their natural state upon thin glass, burnt for 

 awhile upon the platinum plate, hereinafter described, and 

 mounted dry or in balsam. 



In the preparation and cleaning of Diatomaceae, there is 

 little satisfaction unless these operations have been success- 

 fully performed, as a very small portion of foreign matter 

 seriously interferes 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 most frequently employed is the 

 following: Place the gathering containing the Diato- 

 maces3 in a small glass or porcelain vessel, add strong 

 nitric acid, and, by the aid of a 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 microscope, 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 



