292 MANIPULATION. 



which they rarely are, but because a ground surface is cal- 

 culated to give firmer hold to the cement than one which is 

 polished. 



The author prefers this form of cell to any other, even for 

 the most delicate tissues, and both cell and cover can be made 

 so thin, that an eighth of an inch object-glass can be used. 

 The flat cell, however carefully prepared, is almost certain 

 to leak after a time, and, from its containing such a small 

 amount of fluid, a very short period will elapse before the 

 object within it is found perfectly dry ; but in the thin glass 

 cell there is more fluid, and any leaking is made evident by 

 the formation of an air bubble; when the bubble gets very 

 large, the cover can easily be removed, and the object re- 

 mounted without its having first been allowed to get dry. 



When thicker objects, such as injections or other opaque 

 animal structures, require to be mounted, it is necessary to 

 have a much deeper form of cell than any of the preceding ; 

 these may be made of all depths and diameters by having 

 transverse slices cut from glass tubes, and may be denominated 

 the tube cells; one of them is shown in fig. 184, and when 

 cemented to a slide in fig. 185. The tube should be rather 



Fig. 184, Fig. 185. 



thick, at least one-eighth of an inch, in order that the cells 

 may hold firmly to the bottom glass. These cells may be 

 made of all diameters between the one-fourth of an inch and 

 an inch-and-a-half — they may be even made larger, if required; 

 the author has had some cut from stout bottles and from the 

 necks of decanters, that are as much as two inches -and-a-half 

 in diameter. 



Some exceedingly good and useful cells may be made from 

 glass which has been moulded or cast into rectangular tubes ; 

 slices cut transversely from these, of the shapes and sizes 



