44 ELEMENTS OF /IPPLIED MICROSCOPY. 



since this substance sets like a rock and has a high index 

 of refraction, which makes opaque objects very distinct. 

 It is commonly used dissolved in xylol, the natural fir 

 balsam being mixed with an equal volume of the oil; 

 the solution after filtering is concentrated to a syrupy 

 consistency by evaporation. It is used like glycerin 

 jelly, being placed on the object, covered and set aside, 

 when the xylol evaporates and the balsam becomes 

 firmly set. 



With an object of firm texture, not containing water, 

 the preparation of a balsam mount is very simple. The 

 wing or leg of an insect, for example, may easily be 

 mounted in this way. With softer specimens, especially 

 those made up largely of water, it is necessary to resort 

 to some preliminary treatment which shall harden and 

 dehydrate the tissues. 



7. Fixing and Dehydration. — The first step in the prep- 

 aration of a soft plant or animal tissue for balsam mount- 

 ing is to treat it with some agent which shall fix the cell 

 structures in exactly the condition in which they occurred 

 in life, preventing the disintegrating changes which nor- 

 mally follow the death of protoplasm, and which shall 

 at the same time so act chemically upon the cell constitu- 

 ents as to harden and protect them from the action of 

 the chemical agents to be applied later. Flemming's 

 mixture is one of the best of these fixing agents; it con- 

 tains .25% chromic acid, .1% osmic acid, .1% glacial 

 acetic acid dissolved in water, and should be allowed to 

 act for half an hour or more. A saturated solution of 

 corrosive sublimate is often useful. Absolute alcohol 



