CEMENTS AND VARNISHES. 289 



Journ., 1885, p. 227; Journ. Roy. Mic. Soc., 1886, p. 173). 

 BEHRENS g'ives the palm to amber varnish ; AUBERT places 

 MILLER'S caoutchouc cement at the head of the list, LOVETT'S 

 cement coming halfway down, and zinc white cement at the 

 bottom, with less than one quarter the tenacity of the caout- 

 chouc cement. 



453. The Paper Cell Method. According to my experience, 

 the best way to make a fluid mount safe is the following : 

 By means of two punches I cut out rings of paper of about 

 a millimetre in breadth, and of about a millimetre smaller in 

 diameter than the cover- glass. Moisten the paper ring with 

 mounting fluid, and centre it on the slide. Fill the cell thus 

 formed with mounting fluid arrange the object in it ; put 

 the cover on ; fill the annular space between the paper and 

 the margin of the cover with glycerin jelly (a turn-table may 

 be useful for this operation) ; and as soon as the gelatin has 

 set turn a ring of gold-size on it, and when that is quite dry, 

 varnish with BELL'S cement. 



For greater safety, the gelatin may, of course, be treated 

 with bichromate, according to MAKSH'S plan, next . 



454. Gelatin Cement (MARSH'S Section-cutting, 2nd ed., p. 

 104). Take half an ounce of NELSON'S opaque gelatin, soak 

 well in water, melt in the usual way, stir in 3 drops of crea- 

 sote, and put away in a small bottle. It is used warm. 



When the ring of gelatin has become quite set and dry, 

 which will not take long, it may be painted over with a 

 solution of bichromate of potash made by dissolving 10 grains 

 of the salt in an ounce of water. This should be done in the 

 daytime, as the action of daylight is necessary to enable the 

 bichromate to render the gelatin insoluble in water. The 

 cover may then be finished with BELL'S cement. 



This process is particularly adapted for glycei'in mounts. 



455. ROUSSELET'S Method for Aqueous Mounts (op. c^., 

 Close the mount with a ring of a mixture of two parts 

 of a solution of damar in benzol and one part gold-size. 

 When dry, put on three or four thin coats of pure gold-size 

 at intervals of twenty-four hours, and finish with a ring of 

 WARD'S brown cement. 



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