118 The Preparation of Microscope Slides 



transfer the objects from their clearing medium to a tube or dish of the 

 solution of balsam in whatever hydrocarbon has been selected and then 

 permit the hydrocarbon to evaporate. When the balsam that remains has 

 reached a good consistency for mounting, take each specimen, together 

 with a drop of balsam, place it on a slide, and add a coverslip. By this 

 method large numbers of slides can be made in a short time. It is not 

 necessary to use solutions of dried balsam, and the author prefers, for this 

 purpose, to dilute natural balsam with benzene. Mounting large objects 

 in a deep cell in Canada balsam is not to be recommended because the 

 balsam becomes yellow with age and, when in thick layers, tends to ob- 

 scure the specimen. A wholemount of a 96-hr chicken embryo, for ex- 

 ample, is of extremely doubtful value, but if it has to be made it is best 

 first to impregnate it thoroughly with a fairly thin dilution of natural 

 balsam. It is then placed in the cell, piling the solution up on top, and left 

 in a desiccator. The cell is refilled as the evaporation diminishes the con- 

 tents; when completely filled with solvent-free balsam, it is warmed on a 

 hot table. The coverslip is applied directly. 



Finishing Balsam Mounts. If a mount has been made correctly with 

 natural balsam, and if the size of the drop has been estimated correctly, 

 no finishing is required since no balsam will overflow the edges of the 

 coverslip. Natural balsam is very thick when cold, so that the coverslip 

 will not become displaced if the mount is handled before the balsam is 

 fully hard. The hardening may be aided by heat, but care must be exer- 

 cised in heating thick mounts, particularly where the coverslip is not 

 supported by a cell, that the liquefaction of the balsam does not cause the 

 coverslip to tip sideways. Despite the fact that drying time is sometimes 

 prolonged, natural balsam should always be used for thick mounts be- 

 cause, if a solution of dried balsam and xylene is employed, the evapora- 

 tion of the solution will cause huge air bubbles to be drawn under the 

 coverslip. When it is sufficiently hard, the slide should be cleaned, first, 

 by chipping off any excess balsam with a knife and, secondly, by wiping 

 away any chips with a rag moistened in 90 per cent alcohol. This will 

 leave a whitish film over the surface of the slide, which may then be re- 

 moved with a warm soap solution. The slide should be polished before 

 being labeled. 



With regard to labeling, it may be pointed out that no power on earth 

 will persuade gum arabic, customarily used for attaching labels, to adhere 

 to a greasy or oily slide. The portion of the slide to which the label is to be 

 attached should therefore be cleaned more carefully than any other. 

 The author prefers to moisten both sides of the label, press it firmly to the 

 glass, and write on it only after it is dry. 



