150 METHODS OF MICROSCOPICAL EESEARCH. 



the requisite number of covers should be cleaned, 

 and three of them laid upon each slip, previously 

 breathed upon to cause the covers to rest firmly 

 upon it, and the slips laid upon the brass table ; a 

 drop of water containing the diatoms should then 

 be allowed to fall upon each cover, as previously 

 recommended, and the covers can all be dried 

 together ; a sufficient quantity of the turpentine- 

 balsam should then be put upon the covers, which 

 should be placed in a cabinet with trays, or under a 

 glass shade, so that all dust may be excluded, for 

 twenty-four hours. By this means the absolute 

 permeation of the forms by the balsam, and the 

 consequent removal of all air are secured before 

 the final completion of the mount, an advantage so 

 obvious that further comment is unnecessary. The 

 covers should then be replaced upon the brass table 

 and subjected to gentle heat from a " Bunsen " 

 burner until the balsam is hard ; the cover should 

 then be taken up by means of forceps, and a slip 

 being made hot the cover should be placed precisely 

 in the centre of the slip, 1 which should then be held 

 over the "Bunsen" burner until the balsam runs 

 to a neatly bevelled edge round the cover. No air- 

 bubbles will trouble the mounter if this process is 

 carefully practised. This is the so-called " exposure- 



1 In order to ensure the placing the cover exactly in the 

 centre of the slip, either of the three following devices will 

 be found convenient : 



Firstly. Upon a small white tile draw, in china painting 

 colours (which should be burnt in), a 3 in. by 1 in. space (repre- 

 senting a slip), with its precise centre strongly marked. This 

 tile will be found of great use in suddenly cooling balsam, when 

 that may be desirable. 



Secondly. Cover a 3 in. by 1 in. slip with white paper, rule 



