\ 
OF MICROSCOPIC OBJECTS. 43 
underneath, and worked with a small handie 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 here- 
after, 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 
mounting objects for the microscope. Of these will be given 
the composition where it is probable the young student can 
use 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 manu- 
facture than from paying the maker’s profit. 
Amongst these, Canapa Batsam may, perhaps, be termed 
the most necessary, as it is generally used for the preserva- 
tion 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 com- 
paratively 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 
