38 MATERIALS AND PROCESSES OF SLIDE-MAKING 



taking due precautions against fire. At intervals a small drop should be removed 

 on the end of a glass rod and placed on an ice-cold sheet of metal where it 

 will harden. The process of evaporation should be stopped when the test spec- 

 imen shows the desired characteristics. This dried balsam is usually dissolved 

 in the proportion of 60 Gm. of balsam to 40 Gm. of xylene, although some 

 people, including the author, prefer to substitute chloroform for xylene. 



A good many attempts have been made to produce a synthetic resin which 

 will have the desirable qualities of balsam without having the disadvantage 

 of becoming yellow or turning acid with age. At the present time none of 

 these is altogether satisfactory. There is also the grave risk, in specifying a 

 formulation of this nature, that the proprietary resins mentioned will no 

 longer be on the market when one endeavors to prepare the recommended 

 mountant. At least to the beginner it is strongly recommended that he con- 

 fine himself to natural balsam for wholemounts and a solution of dried balsam 

 in a volatile material for mounting sections. 



