ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 



681 



slide by a very fine brusli, at the spot intended to be occupied by tbe 

 object, which is then deposited on the gelatin by a pencil, and adheres to 

 it directly, and the cover-glass at once placed on. If they do not adhere 

 immediately, the slide may be slightly warmed, and then allowed to cool. 

 Purification of Tolu Balsam for Microscopical Purposes.* — Herr 0. 

 C. Keller who has already advocated the use of tolu for mounting 

 diatoms, gives the following method for purifying the balsam. 1 kilo- 

 gramme of crude tolu balsam is heated in a water-bath until it is com- 

 pletely melted, when an equal quantity (up to 1200 grm,) of pure 

 spirit of at least 95 per cent, is added. The solution is then filtered, 

 and to it are added 500-600 grm. of petroleum ether in small portions. 

 At first a clear solution results, the petroleum ether being taken up by 

 the alcoholic balsam solution, but soon it separates into two layers. It 

 is then shaken up vigorously, and allowed to stand for 24 hours. Two 

 clear layers are then found, the upper yellowish one consisting principally 

 of cinnamic and benzoic acids, the lower brown one being composed of 

 the tolu resin plus much cinnamic and benzoic acids dissolved in alcohol 

 and a little petroleum ether. The two layers are next separated by 

 decantation. The following step consists in heating 4 litres of distilled 

 water in a capacious vessel almost up to boiling point, and when the flame 

 is put out the resinous solution is poured slowly in. As the petroleum 

 ether boils at 65°-75^ C. it disappears, the resin is precipitated, and 

 when cold the cinnamic and benzoic acids crystallize out. The resinous 

 mass is then stirred up several times with boiling water in order to get 

 rid of the last traces of the acid. The resin is best dried over sulphuric 

 acid or by the aid of gentle heat, and dissolved in benzol or chloroform. 

 If, as may happen, when dried by heat, the balsam becomes red or 

 brown-red, it should not be used. 



Fig. 117. 



Hot Plate Apparatus.f — It is useful for microscopists to have at 

 hand an apparatus capable of being heated to different temperatures in 



* Zeitschr. f. Wiss. Mikr., iv. (18S7) pp. 471-4. 

 t Arch, cle Physiol., viii. (1886) pp. 273-5 (2 figs.). 



