186 PRACTICAL HISTOLOGY. 



Place some pure Canada balsam in a saucer or other shallow 

 vessel ; cover the vessel with bibulous paper to exclude 

 dust ; dry it in an oven at a temperature not above 65 C., 

 until, when it cools, it becomes as hard as ice ; dissolve this 

 crystalline balsam in benzole. The old-fashioned method 

 of mounting things in balsam was so troublesome, that one 

 avoided it as much as possible. Undried balsam was taken 

 and rendered quite liquid by heat. The slide, the cover- 

 glass, and the tissue to be mounted, all required to be heated. 

 It was difficult to get rid of air bubbles and to avoid over- 

 heating the tissue. But by using a limpid solution of per- 

 fectly dried balsam, all difficulty disappears. 



c. Dammar is now much employed as a substitute for 

 Canada balsam. It renders the tissues more transparent 

 than balsam. The following is an excellent method for 

 preparing the dammar fluid : Dissolve \ oz. dammar resin 

 and \ oz. gum mastic in 3 oz. benzole, and filter. If rapid 

 filtration be desired, use twice as much benzole, and then 

 evaporate the fluid to half its bulk. Chloroform and tur- 

 pentine are by some recommended as solvents of these resins, 

 but the dammar fluid so made is turbid. If, however, it be 

 prepared as above recommended, it is perfectly clear. Bal- 

 sam or dammar is suitable for mounting unsoftened bone 

 and tooth, hair, brain, and spinal cord, and indeed most 

 tissues that have been hardened in alcohol or chromic acid, 

 which require to have their transparency much increased. 

 It renders softened bone and tooth too transparent. Speak- 

 ing generally, it is more suitable for mounting stained than 

 unstained tissues. The latter are often rendered too trans- 

 parent. The tissues very perfectly preserved in either of 

 these substances, because all water is removed. 



348. Glycerine is not adapted for white fibrous tissue 

 and blood-vessels, unless they have been hardened in 

 chromic acid. Otherwise it causes the white fibres to swell 

 up and lose their normal features, but it is well suited for 

 tissues that are to be examined by very high powers. Liver, 

 lung, alimentary canal, and skin, after having been hardened 

 in chromic acid or alcohol, show better in glycerine than in 

 balsam or dammar, unless they have been stained with car- 



