102 PREPARATION AND MOUNTING 
if, however, they have been dried, they should be soaked in 
cold water for a day or two before being submitted to the 
following processes :— 
1. After selecting perfect specimens of suitable. size, im- 
morse them in water heated to about 120°, and place them 
under the receiver of an air-pump. Slowly exhaust the 
air, when bubbles will rise and the water appear to be in a 
state of active ebullition. After a few minutes re-admit the 
air and again exhaust, repeating the process three or four 
times. This will displace the air from most, if not all, of 
the class. 
2. Remove the specimens and allow them to drain upon 
blotting-paper for a few seconds; then place them in an 
earthen vessel fitted with a cover, and previously heated to 
about 200°. This heat may be easily got by placing the 
vessel for a short time in boiling water, wiping it imme- 
diately before using, with a thick cloth. The specimens are 
then dropped into this, covered with the lid, and imme- 
diately placed under the receiver of the air-pump, and 
the air rapidly exhausted. By this means they are dried 
completely, and so quickly that the cells have no time to 
wrinkle. 
3. In an hour or two remove them from the air-pump 
and drop them into a vessel of perfectly transparent cam- 
phine. This may be quite cold when the horny, tubular 
polypidoms, as those of the Sertulariz, are used; but should 
be previously heated to 100° when the calcareous, cellular 
Polyzoa are the objects to be preserved. The vessel should 
be covered with a watch-glass and placed under the re- 
ceiver, the air being exhausted and re-admitted two or three 
times. 
4, The slide which is to receive the specimen should be 
well cleaned and warmed so as to allow the balsam to flow 
freely over it. This must be applied in good quantity, and 
air-bubbles removed with the needle-point. Take the poly- 
pidom from the camphine, drain it a little, and with the 
forceps immerse it fully in the balsam. The glass to be 
