104 MICROSCOPIC OBJECTS. 



the liquid india-rubber cement ; dry, and finish with Berlin 

 black varnish, then label as usual. Whatever fluid is used, the 

 process is nearly the same ; but when glycerine or chloride of 

 calcium is used the cell must be sealed either with Bell's 

 cement or a saturated solution of gum-dammar in benzole ; 

 when the silicate of potassa is used it is hardly necessary to 

 seal the cell at all, as the fluid dries at the edges and seals 

 itself. 



The next form of mounting is the dry system. A cell 

 made of cardboard, gutta percha, ebonite, &c. is cemented to 

 the centre of a glass slip, sufficient Berlin black is then used 

 to cover the bottom of the cell, a small drop of pure gum or 

 any good cement is placed in the centre; the object, which 

 has also a minute quantity of the same cement on it, is then 

 fixed exactly on the same spot, and the slide is left under a 

 bell-glass to dry, after which a circle or square of thin glass is 

 closely cemented to the top of the cell ; the slide may then 

 be finished with any black varnish, or it may be covered with 

 any of the paper covers. If the specimen is to form a trans- 

 parent object, the Berlin black must be left out of the cell ; 

 and it is best not to cement the object to the glass slide, as 

 the cement often shows through and spoils the appearance ; 

 the object may be fixed by the slight pressure of a thin glass 

 cover in a shallow cell. 



In the mounting of objects, great care must be taken to 

 show the structural characteristics of the specimen; for if 

 this is attended to, a greater amount of valuable information 

 will be obtained even from a "common object." 



Alga, Confervoid fyc. These show well when mounted in 

 a preservative fluid consisting of 1 part alcohol to 7 water, 

 mixed with an equal quantity of a cold saturated solution 

 of camphor in distilled water. There are some seaweeds 

 with their fructification that show best when mounted in 

 balsam ; but if glycerine or any other fluid which causes a 

 strong exosmotic action on the cell-wall be used as the pre- 

 servative fluid, it must be done by its gradual addition to the 

 water in which the Algse are contained, so that its action on 

 the cell-wall and protoplasm will not be so abrupt as to cause 



