278 The Microscope, 



closely -stoppered bottle. Balsam thus prepared is of a lighte:? color, 

 hardens somewhat more rapidly and does not become so much dis- 

 colored on exposure to light; though in a mount, even with the 

 extemporaneous mixture, years are required to bring about this 

 discoloration. 



Dammarlack. — This gum comes in hard, friable masses and may 

 be prepared like the balsam l^y dissolving in chloroform or benzol. 

 It makes a beautiful, colorless medium, thus making it valuable for 

 photo-micrographic work; but is unfortunately not permanent. In 

 several hundred specimens mounted in this medium by the writer 

 some ten years ago, fully two-thirds were found destroyed by a 

 cloudy, granular precipitate. The remaining one-third are in a 

 tolerably good state of preservation. A good preparation of dammar 

 recommended by Gibbes may be made as follows: 



1. Gum dammar | ounce. 



Turpentine 1| ounces. 



Pulverize the dammar and dissolve in the tiu'pentine, and filter. 



2. Gum mastic ^ ounce. 



Chloroform 2 ounces. 



Dissolve and filter. 



Mix the two solutions, filter again and keep in close bottles. A 

 little of this may be kept in a dropping- bottle for use. If it thickens 

 from evaporation a little chloroform may be added. This prepara- 

 tion, however, is not permanent. 



Dr. F. L. James in his excellent little work " Elementary 

 Microscopical Technology," recommends a preparation of dammar,, 

 which, he thinks, will prove permanent in character. It is made as 

 follows: "To the clarified solution in benzol (which is made by 

 dissolving the gum in sufficient benzol to make it thin enough to pass 

 through a filter-paper) add alcohol of 90*^ until a precipitate is no 

 longer formed. Remove the precipitated gum and wash with dis- 

 tilled- water and afterwards with alcohol; let dry thoroughly and 

 redissolve in pure benzol. This resin, when dry, is exceedingly 

 brittle, falling into an impalpable white powder upon the slightest 

 pressure. The addition of twenty drojis of poppy or nut-oil, while 

 imparting a faint yellowish tinge, corrects the brittleness." 



Glycerin. — When the use of resins is inadmissible, glycerin, 

 pure or in combination is almost universally employed. Indeed, 

 some microscopists prefer it to balsam. For use in animal histology,, 

 however, the writer does not favor it. Tissues preserved in it do not 



