20 PREPARATION AND MOUNTING 



that greasing the place of contact renders it air-tight. The pump 

 is then joined to the metal plate underneath, and worked with a 

 small handle like a common syringe. By turning a small milled 

 head the air may be allowed to re-enter when it is required to 

 remove the bell-glass and examine or perform any operation upon 

 the object. The mode of using this instrument will be described 

 hereafter, but it may be here stated that substitutes have been 

 devised for this useful apparatus ; but as it is now to be obtained 

 at a low cost, it is hardly worth while to consider them. Much 

 time is, in many instances, certainly saved by its use, as a very 

 long immersion in the liquids would be required to expel the 

 bubbles, where the air-pump would remove them in an hour. 



The next thing to be considered is what may be termed 

 CEMENTS, some of which are necessary in every method of mount- 

 ing objects for the microscope. Of these will be given the com- 

 position where it is probable the young student can make use of 

 it ; but many of them are so universally kept as to be obtainable 

 almost anywhere ; and when small quantities only are required, 

 economy suffers more from home manufacture than from paying 

 the maker's profit. 



Amongst these, CANADA BALSAM may, perhaps, be termed the 

 most necessary, as it is generally used for the preservation of 

 many transparent objects. It is a thick liquid resin of a light 

 amber colour, which on exposure to the atmosphere becomes dry 

 and hard even to brittleness. For this reason it is seldom used 

 as a cement alone where the surface of contact is small, as it 

 would be apt to be displaced by any sudden shock, especially 

 when old. In the ordinary method of using, however, it serves 

 the double purpose of preserving the object and fixing the thin 

 glass cover ; whilst the comparatively large space upon which it 

 lies lessens the risk of displacement. By keeping, this substance 

 becomes thicker ; but a very little warmth will render it liquid 

 enough to use even when to some extent this change has taken 

 place. When heated, however, for some time and allowed to 

 cool, it becomes hardened to any degree, which may be readily 

 regulated by the length of time it has been exposed, and the 

 amount of heat to which it has been subjected. On account of 

 this property it is often used with chloroform: the balsam is 



