128 PREPARATION AND MOUNTING 



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 removed with blotting-paper. When the edge of the 

 thin glass cover and the surrounding parts of the slide are 

 as clean as possible, 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 pre- 

 serving 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 be 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 turntable before described ; but when the objects to be 

 preserved are very minute, these cells need not be much 

 deeper than the ordinary circle of cement on the slide. 

 When, however, a comparatively 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 as- 

 phaltum 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 



