PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS. 209 



others are brought into view, its special action being upon horny textures, 

 -whose component cells are thus rendered more clearly distinguishable. 



h Ether dissolves Resins, Fats, and Oils; but it will not act on these through 

 membranes penetrated with watery fluid. 



i. Alcohol dissolves Resins and some Volatile Oils; but it does not act on ordi- 

 nary Oils and Fats. It coagulates Albuminous matters, and consequently renders 

 more opaque such textures as contain them. The opacity, however, may be re- 

 moved by the addition of a small quantity of Soda. 



205. Preservative Media. We have now to consider the various modes 

 of preserving the preparations that have been made by the several methods 

 now indicated; and shall first treat of such as are applicable to those 

 minute Animal and Vegetable organisms, and to those Sections or Dis- 

 sections of large structures, which are suitable for being mounted as 

 transparent objects. A broad distinction may be in the first place laid 

 down between resinous and aqueous preservative media; to the former 

 belong only Canada Balsam and Dammar; whilst the latter include all 

 the mixtures of which Water is component. The choice between the 

 two 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 pene- 

 trated by paraffin e, wax, or cacao-butter, 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 durability 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, which is specially liable to 

 happen with Dammar. When it is desired to mount in either of these 

 media Sections of structures that have been imbedded in gum or gelatine, 

 these substances must first be completely dissolved-out by steeping in 

 water; the sections must then be * dehydrated ' by subjecting them to 

 mixtures of spirit and water progressively increased in strength to absolute 

 alcohol; and after this has been effected, they are to be transferred to tur- 

 pentine, and thence to benzole. In this process much of the staining is 

 apt to be lost; so that stained sections are often more advantageously 

 mounted in some of those aqueous preparations of Glycerine, which 

 approach the resinous media in transparance and permanence. When 

 Canada balsam was first employed for mounting preparations, it was em- 

 ployed in its natural semifluid 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 benzole or chloroform, the solution being made (with 

 the aid of gentle heat) of such viscidity as will allow it to ' run ' freely 

 when slightly warmed. Either of these solvents evaporates so much more 

 quickly than turpentine, that the balsam left behind hardens in a com- 

 paratively short time. 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 

 14 



