METHODS OF CEMENTING CELLS. 273 



have several sizes in store, that an object may be mounted 

 speedily, for many valuable things are laid by and forgotten 

 when the trouble of making and cleaning a cell has to be gone 

 through before the mounting can be accomplished. 



To Cement Cells with Canada Balsam. — The iron plate, the 

 spirit lamp, and the tools mentioned in the article on cement- 

 ing cells with marine-glue, will all be as requisite for the 

 Canada balsam as for the former material. The plate is also 

 to be warmed, and the cells laid on it, as there described, but 

 upon the sides of the cell some old but semifluid Canada 

 balsam is to be placed. As soon as it shows the least symptom 

 of boiling, which is known by the disappearance of aU air 

 bubbles, the cell may be taken up with the forceps and laid 

 upon the slide, pressed to get rid of the surperfluous balsam, 

 and then set aside to cool ; the superfluous, balsam may be 

 removed with the knife or chisel, and the ceU may be cleaned 

 with a rag, dipped either into turpentine or ether. To get rid 

 of these, the potash or alcohol may be substituted, and when 

 the cell is rinsed out with clean water, and wiped dry with a 

 rag and chamois leather, it is ready for use. Canada balsam 

 is not so good for the cementing of cells as the marine-glue, 

 in consequence of its getting brittle by age ; some consider- 

 able care even is required in handling preparations that have 

 been mounted only a few years, as the least jar or bending of 

 the slide to which the cell is cemented is often attended with 

 a separation of the balsam. 



It should always be borne in mirid that the heated slide to 

 which the cell has been cemented must not be touched with 

 a damp hand, neither should it be laid on metal or anything 

 wet, as the glass, when suddenly cooled, is very apt to crack. 

 If the upper part of the cell be uneven, it should be 

 rubbed on the metal plate described at page 264 until it is 

 perfectly level, as the success of the operation of cementing 

 down a cover will niaterially depend upon this point, 



18 



