MOUNTING OBJECTS IN CANADA BALSAM. 225 



and gives them the aspect of perfect smoothness ; this is well seen 

 in the case of sections of Shell, &c., which, when thus mounted, 

 do not require a high polish (§ 110). But, secondly, where the 

 structure, although itself hard, is penetrated by internal vacuities, 

 the balsam, by filling these, prevents that obscuration resulting 

 from the interposition of air-spaces, and from additional internal 

 surfaces of reflection, by which the transmitted rays are distorted, 

 and a large proportion of them lost : this is well seen in the case 

 of the Foramimfera, and of sections of the "test" and " spines" 

 of Echinida, whose intimate structure can be far better made 

 out, when they are thus mounted, than when mounted dry, 

 although their substance is (for the most part at least) itself so 

 dense, that the balsam cannot be imagined to penetrate it; and 

 likewise with dry Vegetable preparations, which are perhaps also 

 affected in the manner to be next described. Thirdly, there are 

 very many structures of great interest to the Microscopist, whose 

 appearance is extraordinarily improved by this method of mount- 

 ing, in consequence of a specific effect which the balsam has in 

 combining (so to speak) with their component elements, so as 

 to render them far more transparent than before: this effect is 

 seen in the case of all dry preparations of Insect structure, espe- 

 cially of such as consist of their hard external tegument or of 

 parts derived from this ; also in the various horny tissues (hairs, 

 hoof, horn, &c.) of the higher animals ; and likewise in many or- 

 ganized substances, both recent and fossil, which are penetrated 

 by calcareous matter in an amorphous condition. Besides these 

 advantages, the mounting of objects in Canada balsam affords 

 one of the easiest methods of fixing and preserving them ; and 

 consequently, it is almost always had recourse to, in the case of 

 such transparent objects as do not need to be preserved in fluid ; 

 save where, in virtue of the action just described, it impairs the 

 distinctness of surface-markings, or obliterates internal cavities 

 or canals, which constitute the most important features of the 

 object. 



126. Canada Balsam, being nothing else than a very pure 

 Turpentine, is a natural combination of resin with the essential 

 oil of turpentine. In its fresh state, it is a viscid liquid, easily 

 poured out, but capable of being drawn into fine threads ; and 

 this is the condition in which the Microscopist will find it most 

 desirable to use it for the mounting of objects generally. The 

 balsam may be conveniently kept in a glass bottle or jar with a 

 wide mouth, being taken up as required by a small glass rod 

 drawn to a blunt point, such as is used by Chemists as a " stirrer;" 

 and if, instead of a cork or stopper, this bottle should be pro- 

 vided with a tall hollow " cap," the glass rod may ^Iways 

 stand in the balsam, with its upper end projecting into the cap. 

 In taking out the balsam, care should be taken not to drop it 

 prematurely from the rod ; and not to let it come into contact with 

 the interior of the neck or with the mouth of the jar. Both 



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