228 MOUNTING OF OBJECTS. 



substances which are unusually opaque. A long-continued mace- 

 ration in turpentine, moreover, assists in freeing the specimen 

 from air-bubbles ; as it gradually creeps into spaces which are 

 otherwise unoccupied, and, when the object is transferred to the 

 balsam, the free miscibility of the two substances causes its place 

 to be partly taken by the latter. In mounting an ordinary ob- 

 ject, a sufficient quantity of liquid balsam should be laid in the 

 centre of the slide ; this should be warmed, but not boiled ; and 

 any air-bubbles which may make their appearance, should either 

 be caused to burst by touching them with the needle-point, or 

 should be drawn to one side. The object, if it can be held in 

 the fine-pointed forceps, should then be plunged into the drop 

 of balsam ; and, if it be not completely covered, a little more 

 balsam should be applied over it, care being taken as before to 

 prevent over-heating, and to get rid of the bubbles as they rise. 

 If, on the other hand, the object be in a finely divided condition 

 (such, for example, as a collection of fossil Infusoria, of Sponge- 

 spicules, &c.), it is better to place it first in the position it is to 

 occupy on the slide, then to place near it a drop of balsam, which 

 is to be warmed and freed from bubbles as before ; the slide is 

 then to be so inclined, as to cause the balsam to run towards the 

 object, through and over which it will probably diffuse itself 

 without any air-bubbles ; but if any should arise, they may be 

 broken by a heated needle-point. If the object contain numerous 

 large air-spaces with free openings, and be one whose texture is 

 not injured by heat, as is the case with Foraminifera, the air 

 may often be got rid of by boiling it in the balsam ; for the heat, 

 causing the air to expand, drives out a large proportion of it ; 

 this will be replaced, if it be allowed partly to cool, by the en- 

 trance of balsam ; and then, by a second heating, the balsam 

 being boiled within the cavities, its vapor expels the remaining 

 air, and, on the condensation of the vapor, the liquid balsam runs 

 in and takes its place. For this method to succeed, however, it 

 is essential that the balsam be prevented from becoming hard 

 through boiling, by the addition of fresh liquid balsam from 

 time to time ; and it will often be found that, should vacuities 

 remain which boiling does not remove, these contract or alto- 

 gether disappear if the slide be kept for a few days at a gentle 

 heat, the semi-fluid balsam being gradually forced into their 

 place by the pressure of the surrounding air. There are many 

 textures, however, which are extremely injured by a very slight 

 excess of heat, having a tendency to curl up and to become stiff 

 and brittle ; and objects containing these are at once spoiled by 

 boiling them in balsam. In such cases, it is much better to 

 have recourse to the assistance of the air-pump; for by placing 

 the slide, with the object immersed in very liquid balsam, upon 

 a tin or copper vessel filled with hot water, under the receiver, 

 and then exhausting this, the air-bubbles will be drawn forth, 

 and, on the readmission of the air, the balsam is forced by its 



