216 MOUNTING OBJECTS IN CANADA BALSAM. 



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. — In mounting minute Balsam-objects, such as Diatoms, 

 Polycystina, or Sponge-spicules, and even objects of larger size, 

 provided they be not of unusual thickness, great advantage will 

 be obtained from following the plan suggested by Mr. James 

 Smith, for which his Mounting Instrument ( Fig. 99 ) is specially 

 adapted. The slide being placed upon its slide-plate, and the 

 object having been laid upon the glass in the desired position, the 

 covering-glass is very gently laid upon this, and the ivory knob is 

 to be brought down so as by a very slight pressure on the cover to 

 keep it in its place. The slide is then to be very gently warmed, 

 and the Balsam to be applied (which may be most conveniently 

 done by means of the glass Syringe, § 158) at the edge of the cover, 

 from which it will be drawn-in by capillary attraction, leaving no 

 bubbles if too much heat be not applied. In this manner the 

 objects are kept exactly in the places in which they were at first 

 laid ; and scarcely a particle of superfluous balsam, if due care 

 has been employed, remains on the slide. The solution of Canada 

 Balsam in Chloroform (§ 158) may be applied in the same manner 

 without heat. — If the object contain numerous large air-spaces with 

 ree openings, and be one whose texture is not injured by heat, 

 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 entrance 

 of balsam ; and then, by a second heating, the balsam being boiled 

 within the cavities, its vapour expels the remaining air, and, on 

 the condensation of the vapour, 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 altogether 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 the 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 

 re-admission of the air, the balsam will be forced by its pressure 

 into the place which they occupied. Some objects, however, retain 

 the air with such tenacity as to require the repetition of the ex- 



* Small Air-pumps, with a plate and receiver specially adapted for 

 mounting purposes, are made by Mr. Baker and Mr. Collins. 



