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being as light." We can now arrange according to densities, and 

 in this way often separate most of the species contained in the 

 gathering. Several processes of elutriation are mentioned in the 

 London Microscopical Journal, but I have found that using 

 always two inches of water, and separating the thirty minutes 

 into five or six periods, answer all purposes. In this we shall 

 get all the larger Triceratia, Pinnularice, &c., in the heavier den- 

 sities, and the small Achnanthidia and Naviculce in the lighter. 

 Often one application will not separate the densities sufficiently, 

 when it may be tried three or four times. If the gathering be 

 not divided into densities before " acidizing," it will have to be 

 done after ; that is to say, if it be not a comparatively clean one, 

 consisting of a majority of one species. The different densities 

 are now each placed, in small quantities, in a test tube, and about 

 three quarters of an inch of nitric acid added, and boiled. This 

 will remove all but silica, and separate the valves and connecting 

 membranes ; but if we wish the valves to remain united, we must 

 use weaker acid, as I have mentioned before. When the sedi- 

 ment has been well washed, so as to remove all the acid, we may 

 proceed to mount it, which is done as follows. A small portion 

 is taken up with a dipping tube, or a glass rod, (the first being 

 the best,) and spread with a little water on the centre of a clean 

 slide. Only a small quantity should be used, otherwise the Dia- 

 toms will be crowded and overlap each other. It is always bet- 

 ter to have too few than too many specimens on a slide. We 

 now heat the slide cautiously over the flame of the spirit lamp 

 until it is perfectly dry, and while it is warm a small quantity of 

 Canada balsam is dropped on it, and the heat is continued (but 

 not so violently as to boil the balsam) until we consider it ready 

 to receive the cover, and this point can only be ascertained after 

 some experience. The cover, previously cleaned, is now placed 

 on the balsam, one side being placed down first, so that a wave of 

 semi-fluid balsam is driven before it ; we thus prevent the intro- 

 duction of bubbles, which would be extremely difficult to get rid 

 of afterward. Balsam mounting is always to be preferred when 

 it can be done so as to show the species, and when we have gone 

 through the above steps we have finished the mounting part. 

 The slide only requires to be put aside until the balsam is per- 

 fectly hard, which will sometimes not be for a week, The ex- 



