Mounts and Mountants 107 



unfortunately, is often carelessly prepared by suppliers, who heat it to the 

 point where the natural plasticizers, as well as the turpentines, are driven 

 off. This results in a very brittle compound, and slides made with it are 

 likely to flake off the coverslip a year or two after they have been mounted. 

 Purchased specimens should, therefore, be examined carefully by press- 

 ing them with the thumbnail. If, on pressure with the thumbnail, the 

 dried balsam cracks into a powdery material, the sample should be re- 

 jected. The desirable consistency is that in which the thumbnail will just 

 manage to mark a piece without any shattering. It is usually worthwhile 

 to prepare one's own dried balsam from the natural balsam by tipping 

 a pound or two into a shallow metal container (a baking dish is excellent) 

 and placing this on a hot plate, 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 specimen shows the desired 

 characteristics. This dried balsam is usually dissolved in the proportion of 

 60 g of balsam to 40 g 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 

 that will have the desirable qualities of balsam without having the dis- 

 advantage of becoming yellow or turning acid with age. At the present 

 time none of these is altogether satisfactory. At least to the beginner it is 

 strongly recommended that he confine himself to natural balsam for 

 wholemounts and a solution of dried balsam in a volatile solvent for 

 mounting sections. 



