OF MICKOSCOPIC OBJECTS. 105 



them. Large objects require more frequent washing, as it is 

 desirable to remove the potash thoroughly, or crystals are 

 apt to form after mounting. Having placed them on the 

 slides with thin glass covers, tied down with thread,* dry 

 and immerse them in rectified spirits of turpentine ; place 

 the vessel under the receiver of an air-pump, and keep it 

 exhausted until the turpentine has taken the place of the 

 air-bubbles ; they are then ready for the application of the 

 balsam. Larger objects may often with advantage be trans- 

 ferred to a clean slide, as during the drying there is con- 

 siderable contraction, and an outline showing this often 

 remains beyond the margin. When closely corked, they may 

 remain in the spirits two or three months. As you take 

 them from the bottle, wipe as much turpentine off as possi- 

 ble before removing the thread, and when untied carefully 

 wipe again, placing the finger on one end of the cover whilst 

 you wipe the other, and vice versa. By this means you 

 remove as much turpentine from under the cover as is 

 necessary ; then drop the balsam, thinned with chloroform 

 (see Chapter II.), upon the slide, letting the fluid touch the 

 cover, when it will be taken in between the surfaces by 

 capillary attraction; and after pressing the cover down, it 

 may be left to dry, or you may hold the slide over a spirit- 

 lamp for a few seconds before pressing down the cover. If 

 heat is not applied, they are much longer in drying, but are 

 more transparent. If made too hot, the boiling disarranges 

 the objects, and if carried too far, will leave only the resin of 

 the balsam, rendering it so brittle that the cover is apt to 

 fly off by a fall or any jar producing sufficient concussion 

 Never lift the cover up, if possible, during the operation, as 

 there is danger of admitting air. A few bubbles may appear 

 immediately after mounting, but they generally subside after a 

 few hours, being only the chloroform or turpentine in a state 

 of vapour, which becomes condensed." 



* This applies to the more delicate, which will not bear transfei 

 ring after being once spread out and dried. 



