PREPARATION OF DIATOMS. 



65 



If these facts are not accidental, but are confirmed by repeated and 

 independent observations, they will need a good deal of explanation. 





Fig. 22. Parasite infesting PI. Anqulatum. 



We give here (fig. 22) a representation of the parasite infesting the 

 Pkurosigma attenuation in three different phases of its evolution. This 

 figure is copied from the photograph sent us by Air. Gill. 



7. Preparation of Diatoms. 



After a gathering of diatoms has been made, they must be separated 

 from the mud which often accompanies them, then deprived of their 

 endochrome, and subsequently prepared either in balsam or dry. 



To isolate diatoms from mud, the entire mass is placed in a plate, 

 a quantity of water just sufficient to cover them is then poured on to it, 

 and the whole is exposed to the light in a well-illumined spot. After 

 an interval, sometimes of a few hours, at others of a day or two, the 

 diatoms may be seen to have come out of the mud and to be coating 

 the surface of the water. The water surrounding them is then carefully 

 drawn off and the diatoms thus exposed can be removed by means of 

 a camel's hair pencil if the stratum is thin, or by means of a scraper 

 or blade if the bed is thick. 



Thus obtained, the diatoms (') are placed in small tubes which are 

 filled up with alcohol, and they can then be either prepared at once 

 by a process of calcination on a cover-glass, which we shall afterwards 

 describe, or they may be previously treated with acid. Fresh water 



( r ) It is assumed that fresh water diatoms are being dealt with. In the case of marine 

 diatoms it would be necessary to wash them two or three times in distilled water, so as to rid 

 them of every trace of sodium chloride. 



