OF MICROSCOPIC OBJECTS. 9] 
found to be as hard as resin, the softened and warm solution 
may be now poured into a bottle, and when cool the benzole 
added in sufficient quantity to make it of a desirable thick- 
ness. Ten objects out of eleven contain air, or at least are 
full of minute holes which are necessarily filled with it; so 
that if they should be immersed in any liquid of thick 
consistency, these cells of air would be imprisoned, and 
become bubbles. The air, then, must be removed, and this 
is usually accomplished by soaking for some time in turpen- 
tine, the period required differing according to the nature of 
the object. In some cases, the turpentine acts upon the 
colour, or even removes it altogether, so that it must be 
. watched carefully. Often, however, this is an advantage, 
as where the structure alone is wanted, the removal of the 
colouring matter renders it more transparent. There are 
objects, however, which retain the air with such tenacity 
that soaking alone will not remove it. If these will bear 
heat without being injured, they may be boiled in turpentine, 
or even in baisam, when the air will be partly or totally 
expelled. But where heat is objectionable, they must be 
immersed in the turpentine, and so submitted to the action 
of the air-pump. Even with this aid, sometimes days are 
required to accomplish it perfectly, during which time the 
air should be exhausted at intervals of five or six hours, if 
convenient, and the objects turned over now and then. 
Many complaints are made concerning turpentine, both 
as to its cleanliness and penetrating power. Most of these 
spring from the fact that few substances in the market vary 
so much as turpentine in purity; all sorts of rubbish are 
sold under this name, and now benzole is employed by many 
in all cases where turpentine alone was once used. 
Sometimes the objects are so minute that it is impossible 
to submit them to any soaking, and in this case they must 
be laid upon the slide at once, and the turpentine applied 
to them there. Butit must not be forgotten that there are 
some few which are much better mounted in such a way 
that the balsam may thoroughly surround, and yet not 
