308 MANIPULATION. 



very fluid to commence with; it should be sufficiently warmed 

 to expel all air bubbles, and, when nearly cold, the object 

 may be placed in it, and covered over in the usual way; if 

 the heat be great, the object is sure to curl up, and bubbles 

 appear in all parts; it will most hkely be rendered useless, as 

 no manipvdation, however carefully applied, will restore an 

 overheated specimen of animal structure to its former beauty. 



It often happens that opaque objects, such as the elytra of 

 beetles, and thick pieces of injection, require to be mounted 

 in one or other of the cells described in page 294; when this 

 is the case, it becomes necessary to use very fluid balsam for 

 the purpose ; but not such as has been thinned previously 

 with turpentine, as the author has found by experience 

 that«lthough the cells be carefully covered over, without any 

 trace of air bubbles, these will, nevertheless, appear in a 

 few days, and he has ascertained that they are caused by 

 mixing turpentine with the Canada balsam to make it more 

 fluid ; for although they have all the appearance of bubbles of 

 air, which have either gained entrance from without or have 

 escaped from the preparation itself, they are not really such, 

 but are little vacuities in the balsam, occasioned by the tur- 

 pentine not freely mixing with it at first, but after a time 

 doing so; and as the two when united occupy less space than 

 when separate, these little vacuities are the result. Hence it 

 becomes necessary, when objects are mounted in cells with 

 Canada balsam, that the balsam should be new and very 

 fluid, and before the cover is put on, the balsam should be 

 allowed to remain in the cell with the object for some hours, 

 or even days, if necessary, so that all air bubbles may rise to 

 the surface and burst ; when this has taken place, the cover 

 having been warmed on its under surface, may be laid upon 

 the balsam and pressed in the usual manner, in order to 

 exclude all that is superfluous. If the balsam, however, have 

 been thinned with turpentine, the chances are that the vacui- 

 ties wiU appear, and to remove them it becomes necessary to 

 take off the cover and fill up the cell with fresh balsam, which 

 may be avoided if attention be paid to the above directions. 



In warm weather the vacuities become small, or may even 



