518 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



Canada balsam and dammar, while the latter include all the mix- 

 tures of which water is a component ; while partly dehydrating 

 media, such as glycerin and alcohol, occupy an intermediate position. 

 The choice between the three kinds of media will partly depend 

 upon the nature of the processes to which the object may have been 

 previously subjected and partly upon the degree of transparence 

 which may be advantageously imparted to it. Sections of substances 

 which have been not only imbedded in but penetrated by paraffin, 

 and have been stained (if desired) previously to cutting, are, as a rule, 

 most conveniently mounted in Canada balsam or dammar ; since 

 they can be at once transferred to either of these from the menstruum 

 by which the imbedding material has been dissolved out. The dura- 

 bility of this method of mounting makes it preferable in all cases to 

 which it is suitable, the exception being where it renders a very 

 thin section too transparent. In such cases sections or other 

 objects may sometimes be more advantageously mounted in some of 

 those aqueous preparations of glycerin which approach the resinous 

 media in transparence and permanence. "When Canada balsam was 

 first employed for mounting preparations it was employed in its 

 natural semi-fluid state, in which it consists of a solution of resin in 

 volatile oil of turpentine ; and unless a large proportion of the latter 

 constituent was driven off by heat in the process of mounting 

 (bubbles being thus formed of which it was often difficult to get 

 rid), or the mounted slide was afterwards subjected to a more 

 moderate heat of long continuance, the balsam would remain soft, 

 and the cover liable to displacement. This is avoided by the method 

 now generally adopted of previously getting rid of the turpentine by 

 protracted exposure of the balsam to a heat not sufficient to boil 

 it, and dissolving the resin thus obtained either in xylol, benzol, or 

 chloroform, but far preferably the former, the solution being made 

 of such viscidity as will allow it to ' run ' freely. Either of these 

 solvents evaporates so much more quickly than turpentine that the 

 balsam left behind hardens in a comparatively short time. Xylol- 

 balsam is now preferred by most mounters. It is made of equal 

 volumes of xylol and balsam. The natural balsam, however, may be 

 preferably used (with care to avoid the liberation of bubbles by 

 overheating) in mounting sections already cemented to the slides 

 by hardened balsam, and also for mounting the chitirious textures 

 of insects, which it has a peculiar power of rendering transparent, 

 and which seem to be penetrated by it more thoroughly than they 

 are by the artificially prepared solution. The solution of dammar 

 in xylol is very convenient to work with, and hardens quickly. 



The following are the principal aqueous media whose value has 

 been best tested by general and protracted experience : 



a. Fresh specimens of minute protophytes can often be very well 

 preserved in distilled water saturated with camphor, the complete 

 exclusion of air serving both to check their living actions and to 

 prevent decomposing changes. When the preservation of colour 

 is not a special object about a tenth part of alcohol may be added, and 

 this will be found a suitable medium for the preservation of many 

 delicate animal textures. 



