226 MOUNTING OF OBJECTS. 



these mischances may be avoided, by not attempting to take up 

 on the rod more than it will properly carry ; and by holding it 

 in a horizontal position, after drawing it out from the bottle, 

 until the slip on which it is to deposit the balsam, is just beneath 

 its point. The Author has himself been in the habit of employ- 

 ing, instead of a hollow cap to his balsam jar, a disk of light 

 wood, simply lying on its mouth ; the centre of this disk is per- 

 forated with a hole, just large enough to allow the glass rod to 

 fit it rather stiffly ; and the rod is to be passed into the jar, only 

 so far as to dip by its point into the balsam, being pushed fur- 

 ther down as the level of the balsam is lowered. In this manner, 

 a small quantity of balsam may be taken up with far less risk of 

 messing it, than when the rod dips into it for two or three inches; 

 if due care be taken not to allow the balsam to come into con- 

 tact with the lip of the jar, the cover never sticks to it, but is 

 readily lifted otf upon the rod ; and although it might seem an 

 incumbrance in the use of the rod, yet it is not practically found 

 to be so. If the balsam be kept too long, it becomes, through 

 the loss of part of its volatile oil, too stifl'for convenient use, and 

 may be thinned by mixing it at a gentle heat with pure oil of 

 turpentine ; this mixture, however, does not produce that 

 thorough incorporation of the constituents which exists in the 

 fresh balsam ; and it is consequently preferable to use in other 

 ways the balsam which has become somewhat too stiff, and to 

 have recourse to a fresh supply of liquid balsam for mounting 

 purposes. When Canada balsam is to be employed as a cement, 

 as for attaching sections, &c., to glass slides (§ 109), it should be 

 in a much stifter condition ; since, if it be dropped on the slide 

 in too liquid a state, it will probably spread much wider, and 

 will lie in a thinner stratum than is desirable. This hardening 

 process may be carried to any extent that may be desired, by 

 exposing the balsam in an uncorked jar (the mouth of which 

 however, should be covered with paper, for the sake of keep- 

 ing off dust) to a continual gentle heat, such as that of a water- 

 bath. 



127. In mounting objects in Canada balsam, it is convenient 

 to be provided with cei'tain simple instruments, the use of which 

 will save much time and trouble. For the heat required, a 



Fio. 61. 



Slider Forceps 



Spirit lamp is by far the best source ; both as admitting of easy 

 regulation, and as being perfectly free from smoke. Where a 

 number of objects are being mounted on the same occasion, it 

 will be found convenient to employ either a water-bath covered 



