H. NORLAND ON MOUNTING DIATOMACEJE. 327 



the slip remain a short time on the hot block, when they will 

 gradually disappear. The iron block must be placed level, as 

 otherwise the cover-glass is apt to slide on one side when the 

 styrax softens under the heat. When once the cover-glass is 

 in position it must never be moved nor touched until after the 

 styrax has been hardened off, for fear of displacing the arranged 

 diatoms. These slides are hardened and finished off in the same 

 manner as described for "spread" slides. Should it be deter- 

 mined to employ balsam instead of styrax, the same will have 

 to be applied cold, and not hot, as with styrax, because 

 a thin skin or pellicle forms on the surface of the balsam, 

 which skin is almost certain to displace the diatoms in the final 

 act of mounting. In using balsam to " arranged " diatoms 

 without fixing cement it would be as well to use the pure, natural 

 balsam thinned down with benzole, and not the baked balsam 

 similarly thinned down, as this latter kind rapidly forms a 

 pellicle on exposure to the atmosphere. 



Should the " selected " diatoms be large and heavy, or 

 strongly hollowed out, or beset with spines or processes, it is 

 then necessary that they be fixed down onto the cover- 

 glass with gum or other cement, as otherwise they would be 

 certain to get displaced when the balsam was applied. I never 

 use styrax to diatoms which have been fixed with gum, etc., as 

 not only would the gum be almost certain to be apparent, but 

 because the styrax runs out at the edges to a considerable extent 

 when heated under small covers, such as I generally use, thereby 

 allowing the cover to sink down so close to the slip as to 

 endanger the safety of the diatoms between them. Personally, 

 I fix my diatoms with gum, because I can do this under the 

 microscope and watch the process, which with me is very similar 

 to the method I have just described, the only differences being 

 as follows : — In the first place, I apply the minutest drop of gum 

 arabic dissolved in water, by means of a glass rod, onto the 

 centre of my cover-glass, after the same has been fixed on the 

 ruled disk of the "mounting slip " by means of the breath, as 

 already described ; this drop of gum is allowed to dry, and is 

 then examined under the microscope. Any specks of dirt or 

 fibres in the gum can now be removed by means of a stiff 

 mounted bristle, on the gum being softened by being breathed 

 upon through the " breathing tube." It is also as well to shade 



