260 SELECTION AN USK 



balsam soon becomes cloudy, and is very apt to crack. Balsani 

 that is too highly colored may be bleached by exposure to sun- 

 light a process applied by most opticians to the balsam used 

 by them for cementing the lenses of achromatic combinations. 

 Balsam when new is quite fluid, too much so, indeed, for the 

 mounting of most objects. On the other hand, old balsam is 

 thick, and is apt to crack. Microscopists generally keep balsam 

 in wide-mouthed bottles, and take out what is wanted by means 

 of a glass rod. As the process of evaporation, which makes 

 balsam thick and viscid, goes on more slowly in narrow-mouthed 

 bottles, we prefer the latter, and transfer the balsam to the glass 

 slide by means of a fine wire with a small loop at the end. The 

 wire is passed through a cork, or preferably a wooden stopper, 

 and descends to such a depth as to be just below the surface of 

 the balsam. As the latter is used up, the wire is pushed down, 

 and if cemented in its place by the balsam, a little heat soon frees 

 it. The latter remark applies also to the wooden stopper, which 

 is very apt to stick in the neck of the bottle. A very slight ex- 

 posure to the flame of a spirit lamp is sufficient to loosen it. 



SOLUTION OF BALSAM. When the objects that are to be pre- 

 served in balsam would be injured by the heat necessary to 

 melt it, it is advisable to use a solution of balsam in ether or 

 chloroform. The balsam used for making the solution should be 

 old and thick. This solution is frequently sold with the label, 

 " Balsam for use without heat." 



COLOPHONY. Thiersch recommends a solution of resin or 

 colophony in absolute alcohol. The advantage which this ma- 

 terial presents is that the preparation may be placed in it di- 

 rectly from the absolute alcohol, without becoming cloudy, 

 and without prejudice to the durability of the specimen. He 

 advises the microscopist to prepare the colophony himself from 

 Venice turpentine, which is done by dissolving it in an equal 

 volume of ether, filtering it through paper, and evaporating, 

 until, when cold, it breaks with a conchoidal fracture. The 

 material that remains is then to be dissolved in absolute alco- 

 hol until it is of a syrupy consistence. 



