HISTOLOGICAL METHODS. 187 



mine or logwood. It is suitable for mounting tissues that 

 have beeen stained with silver nitrate or gold chloride. For 

 tissues containing vessels injected with Prussian blue, use 

 two drops H Cl in i oz. glycerine, to preserve the colour. 

 It is somewhat difficult to mount things in glycerine. Air 

 bubbles are difficult to eliminate, and if the glycerine pass 

 outside the cover-glass, it is difficult to get rid of it. It can 

 be removed most readily by sucking it up with a fine glass 

 pipette, Fig. 45). It is often necessary, however, to transfer 

 the preparation to a new slide, on which a smaller drop of 

 glycerine has been placed. The faults of glycerine are i, 

 a tendency to render the tissues too transparent ; 2, a diffi- 

 culty in getting rid of superfluous fluid; 3, a difficulty in 

 getting rid of air bubbles. 



349. Farrants' Solution is an excellent substitute for 

 glycerine in many instances indicated in the preceding 

 demonstrations, because of its feebler tendency to render 

 the tissues transparent. It consists of " equal parts of gum- 

 arabic, glycerine, and a saturated solution of arsenious acid " 

 (frey). Owing to its containing gum it becomes hard, so 

 that the cover-glass becomes fixed at its margin. In 

 mounting preparations with this fluid, the covered object is 

 allowed to lie for a day before the varnish is applied, so 

 that the cover may be fixed and thereby prevented from 

 shifting during varnishing, as so often happens with gly- 

 cerine. 



350. Glycerine Jelly is a good preservative agent for 

 many tissues, such as lung, blood-vessels, tooth, bone, 

 cochlea, etc. The tissues must be steeped in weak spirit 

 ( 35 2 ) previous to mounting. They cannot be transferred 

 from glycerine to glycerine jelly directly. Fungi are very 

 apt to grow in some kinds of glycerine jelly that are sold. 

 Rimmington's glycerine jelly is the best. Its composition 

 is unknown, but it appears to contain glycerine, gelatine or 

 isinglass, and carbolic acid. 



351. Potassium Acetate. A saturated solution of this 

 substance has long been used by botanists. It is the best 

 medium for osmic acid preparations (Sc/mltze). Glycerine 

 renders these too transparent. The fluid may be prepared 



