310 MANIPULATION. 



made out; the extra degree of heat will expand the air, 

 and cause it to make its escape, whilst the balsam will occupy 

 its place. Some objects of a tubular nature, such as the 

 tracheaj of insects, are much better seen if air be contained 

 in the tubes ; they will then exhibit the spiral fibre in their 

 interior, but a tracheal tube filled with balsam does not show 

 the fibre at all, in consequence of the balsam rendering all 

 the parts transparent. Small insects, such as fleas and 

 parasites of animals generally, when not over heated in the 

 balsam, show remarkably well the ramifications of the tra- 

 cheae ; but those which have been soaked for a long time 

 in turpentine, or have had the air expelled from the tubes by 

 heat, do not exhibit the spiral markings at all, unless under 

 polarised light, when they may again be rendered visible. 

 These points show the necessity of attending to the manage- 

 ment of the heating of the balsam ; when air is to be got rid 

 of, the heat must be high, and when the air is necessary to be 

 preserved, the use of turpentine must be avoided, the heat of 

 the balsam must be as low as possible, and the mounting 

 accomplished quickly, in order that the air may not have time 

 to expand very much. 



Foraminifera, ^c. — Certain chambered cells, of the order Fo- 

 raminifera, and many of the siliceous loricEe of infusoria, which 

 also have cavities in their interior, are very diflScult to mount 

 in balsam, so as to get rid of all the air from their interior, 

 even boiling wiU not always answer ; for this purpose the aid 

 of an air puinp or exhausting syringe will be necessary. Mr. 

 Matthew Marshall, who has paid considerable attention to this 

 subject, employs a very strong square copper vessel, provided 

 with a stop-cock ; into this, boiling water is poured, and it is 

 then placed upon the plate of an air-pump, the slides containing 

 the objects from which the air is to be abstracted are laid upon 

 the copper vessel, the heat of which is sufficient to keep the 

 balsam very fliiid ; the receiver is then to be placed upon the 

 pump plate and exhausted, air will soon be seen to make its 

 escape in bubbles from the objects and from the balsam, 

 and when again admitted into the receiver, the bubbles will 

 disappear, and the balsam be found to have run into all the 



