BELL'S CEMENT. 211 



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 glycerin mounts. 



409. 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 Bell's or other cement on it. 



For greater safety, the gelatin may of course be treated 

 with bichromate according to Marsh's plan. 



410. Comparative Tenacity of Cements (see BEHRENS, Zeit. f. 

 wiss. Mik., ii, 1885, p. 54, and AUBERT Amer. Mon. Mic. 

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

 Behrens gives the palm to amber varnish ; Aubert places 

 Miller's cauotchouc cement at the head of the list, Lovett's 

 cement coming half way down, and zinc white cement at the 

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

 chouc cement. 



411. Bell's Cement. Composition unknown. May be ob- 

 tained from the opticians or from J. Bell and Co., chemists, 

 338, Oxford Street, London. 



This varnish flows easily from the brush, and sets quickly. 

 For glycerin or other fluid mounts, the cover should be ringed 

 as above described with glycerin jelly before applying the 

 varnish. This precaution is especially necessary with glycerin. 

 This is the best varnish for fluid mounts known to me. It is 

 soluble in ether or chloroform. It is not attacked by oil of 

 cedar. 



