84 PREPARATION AND MOUNTING 



sam is used with them ; but very few are acted upon by this oil. 

 To use it, it must be dropped in a sufficient quantity to cover the 

 crystal or object to be preserved with a thin coating of oil. It 

 may be necessary sometimes to spread it with a needle or other 

 instrument. The thin glass should then be carefully placed upon 

 it, so that all air may be excluded ; and should any oil be forced 

 out, owing to the quantity used being too great, it must be re- 

 moved with blotting paper. When the edge of the thin glass 

 cover and the surrounding parts of the slide are as clean as possi- 

 ble, a coating of sealing-wax varnish or liquid glue must be applied 

 and allowed to dry. A second or even a third coating may be 

 required, but not before the previous cover is quite dry. These 

 varnishes, however, are very brittle, and it is much safer, as a 

 finish, to use one of the tougher cements gold-size, for instance 

 which will render it doubly secure. 



The above are the principal liquids, &c., used for preserving 

 objects in cells. The different cells may be here mentioned ; and 

 it is recommended that these should always be kept some time 

 before use in order that the cement may become perfectly dry ; 

 and care must bo taken that no cement be used on which the 

 preservative liquid employed has any action whatever. 



CEMENT CELLS. Where the object is not very thick, this kind 

 of cell is generally used. They are easily made with the turnta- 

 ble before described ; but when the objects to be preserved are 

 very minute, these cells need not be much deeper than the ordi- 

 nary circle of cement on the slide. When, however, a compara- 

 tively great depth is required, it is sometimes necessary to make 

 the wall of the cell as deep as possible, then allow it to dry and 

 make another addition. Of these cements gold-size is one of the 

 most trustworthy, and may be readily used for the shallow cells. 

 The asphaltum and india-rubber, before noticed, I have found 

 very durable when well baked, and exceedingly pleasant to work 

 with. It may be used of such a thickness as to give space for 

 tolerably large objects. Black japan also is much used. Many 

 cements, however, which are recommended by some writers, are 

 worse than useless, owing to the brittleness which renders their 

 durability uncertain, as sealing-wax varnish, liquid-glue, &c. 



The student may feel himself at a loss in choosing the cement 



