62 THE MICROSCOPIST. 



"When objects of a cellular nature have to be mounted, if they 

 are such as heat will not much injure, they may be boiled in the 

 balsam; otherwise numbers of air-bubbles will be left in the 

 cells, and the true structure cannot then be made out satisfac- 

 torily. The extra degree of heat will expand the air and 

 cause it to escape, and the balsam will take its place. 



Some object of a tubular nature, such as the tracheae of 

 insects, are better seen if air be contained in the tubes; they 

 will then exhibit the spiral fibre in their interior; but a tra- 

 cheal tube filled with balsam does not show the fibre at all, the 

 balsam having made all the parts transparent. Small insects, 

 such as fleas, and the parasites of animals, when not over- 

 heated, show the ramifications of the trachea, but those which 

 have been soaked long in turpentine, or have had the air ex- 

 pelled by heat, do not exhibit the spiral markings except under 

 polarized light. 



When air is to be got rid of, the heat must be high ; other- 

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

 balsam kept low, and the mounting accomplished quickly. 



The best way to heat the balsam on the slide is to place the 

 slide on a small table made of iron or tin, to which a spirit- 

 lamp is applied, as first suggested by Dr. Goadby; yet with 

 careful management a spirit-lamp will do alone. 



Some persons keep their balsam in a tin vessel that can be 

 warmed so as to melt it. A drop of the fluid can then be 

 taken out and put on the object upon the slide. This plan is 

 attended with little or no risk of air-bubbles. The cover 

 should be warmed on its under surface before it is laid on 

 the balsam, and if necessary, a small amount of heat applied 

 to the under side of the slide, to make the balsam flow more 

 rapidly. 



When animal structures, such as parts of insects, or injec- 

 tions, have to be mounted, the heating of the balsam must be 



