35© REAGENTS AND THEIR PREPARATION [Ch. X 



For educational and other public institutions the U. S. government 

 grants the privilege of using ethyl alcohol without paying the revenue 

 tax, but for private institutions and for individuals it would be a 

 great relief if the denatured alcohol could be mixed in all proportions 

 with water without the formation of precipitates. 



§ 643. Balsam, Canada balsam, balsam of fir. — This is one of the 

 oldest and most satisfactory of the resinous media used for mounting 

 microscopic preparations. 



The natural balsam is most often used; it has the advantage of 

 being able to take up a small amount of water so that if sections are 

 not quite dehydrated they will clear up after a time. 



§ 544. Xylene balsam. — This is Canada balsam diluted or thinned 

 with xylene. It is recommended by many to evaporate the natural 

 balsam to dryness and then to dissolve it in xylene. For some pur- 

 poses, e.g. for mounting glycogen preparations, this is advantageous; 

 but it is unnecessary for most purposes. Xylene balsam requires 

 a very complete desiccation or dehydration of objects to be mounted 

 in it, for the xylene is immiscible with water. 



The hydrocarbon, xylene (CsHio) is called xylol in German. In 

 English, members of the hydrocarbon series have the termination 

 "ene," while members of the alcohol series terminate in "ol." 



§ 545. Filtering balsam. — Balsam is now furnished already filtered 

 through filter paper. If xylene balsam is used it may be made thin 

 and filtered without heat. For filtering balsam and all resinous and 

 gummy materials, the writer has found a paper funnel the most sat- 

 isfactory. It can be used once and then thrown away. Such a 

 funnel may be easily made by rolling a sheet of thick writing paper 

 in the form of a cone and cementing the paper where it overlaps, or 

 winding a string several times around the lower part. Such a funnel 

 is best used in one of the rings for holding funnels, so common in 

 chemical laboratories. The filtering is most successfully done in a 

 very warm place, like an incubator or an incubator room. 



§ 546. Neutral balsam. — All the samples of balsam tested by 

 the author have been found slightly acid. This is an advantage for 

 carmine and acid fuchsin stain or any other acid stain. Also for 

 preparations injected with carmine or Berlin blue. In these cases 



