CF MICROSCOPIC OBJECTS. 97 
they may be dispersed over any space desired. When they 
are thoroughly dry, drop a little balsam on one side, and 
exclude the bubbles. The slide may then be warmed to 
such a degree that the balsam, by lifting the glass at one 
end, will be carried over the specimens, which may then be 
covered with thin glass, made warm as before described. 
Where the objects are quite dry, and loose upon the glass, 
it requires great care in placing the cover upon them, other- 
wise they are forced to one edge, or altogether from under 
it, in the wave of the balsam. For this reason, Professor 
Williamson adds a few drops of gum-water to the last 
washing, which causes them to adhere sufficiently to the 
glass to prevent any such mishap. 
Mr. T. G. Rylands’s method differs in some degree from 
the above, and is, to use his own words, as follows :—Thick 
balsam is preferable, and the burnt covers (see Chapter ITI.) 
to be mounted are laid in a convenient position, with the 
diatoms upwards. The slides required having been care- 
fully cleaned and marked on the under side with a ring of 
ink about half an inch in diameter by the aid of a turntable 
to point out the centre, a drop of benzole is applied by a 
large pin to the diatoms on the cover, so as to exclude the 
air from the valves and frustules. The slide is then held 
over the lamp, and when warn, a sufficiently large drop of 
balsam is put upon it, and heated until it begins to steam. 
If small bubbles appear, a puff of breath removes them, 
The slide being held slightly inclined from the operator, and 
the drop of balsam becoming convex at its lower edge, the 
cover is brought in contact with it at that point, gradually 
laid down, pressed with the forceps, and brought to its 
central position. When cool the superfluous balsam (if 
any) is removed with a heated knife-blade, the slide cleaned 
with a little turpentine, and finished by washing in a hand- 
basin with soap and water. In this process there is no 
delay if the balsam be sufficiently thick, as the slide may be 
cleaned off almost before it is cold. 
It is now well known that from common chalk it is an 
u 
