OF MICROSCOPIC 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 versé. By this means you 
remove as much turpentine from under the cover as is 
necessary; then drop the balsam, thinned with chloroform 
(see Chapter IT.), 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 transfer . 
ring after being once spread out and dried. 
