MOUNTING OBJECTS IN CANADA BALSAM. 307 



centre of the cover, and the cover to the middle of the slide ; 

 when this is accomplished, the sUde may be put aside to cool 

 in a horizontal position. But supposing that numbers of air 

 bubbles are present, the balsam must be made to boil, the air 

 bubbles \?iU then be seen to go from the centre to the circum- 

 ference, where they mostly burst; if not, the slide may be 

 turned over (with the balsam downwards) upon the flame of 

 the lamp, and the heat then being applied directly to them, 

 they will speedily disappear. When the balsam is too fluid 

 for the slide to be turned over and heated, the bubbles may be 

 got rid of by drawing them with a clean needle-point away 

 from the centre towards the circumference, that they may be 

 out of the field which the thin glass will cover ; the needle- 

 point is then to be made warm in the lamp, and the bubbles 

 touched with it, when they will burst and disappear as in the 

 former methods. 



Should the balsam be too hot when the section is put into 

 it, the latter will probably curl up and numbers of small air 

 bubbles arise; when this is the case, time will be saved if 

 the section be removed and placed either in turpentine or 

 ether, and a fresh slide taken, new balsam put on it, and the 

 process gone over again, instead of using balsam with an 

 infinity of small bubbles in it. Should the slide on which the 

 balsam has been boiled not be wanted again immediately, it 

 may be placed in a convenient vessel with some turpentine, 

 which will dissolve all the hard balsam, and the slide will be 

 perfectly cleaned and ready for use again in a few days. 



Some persons keep their Canada balsam in a tin vessel that 

 can be warmed so as to melt the balsam ; a small quantity of 

 this may be taken out when fluid and dropped upon the object 

 previously arranged upon the slide, this plan is attended with 

 little or no risk of air bubbles. The cover should be warmed 

 on its under surface before it is laid on the balsam, and, if 

 necessary, a small amount of heat may be applied to the 

 under side of the slide to make the balsam flow more readily. 



Animal Structures. — When animal structm-es, such as parts 

 of insects or injections, have to be mounted, the heating of the 

 balsam must be carefully managed, and the balsam itself be 

 20* 



