28 
DR. A. WALLER’S MICROSCOPIC OBSERVATIONS 
are all of the same form, although an irregular one, not completely circular. If a 
surface of balsam be placed aside for a short time and afterwards examined, we find 
that these particles, which before were disposed in a regular manner, appear to have 
altered their position, and that they have grouped themselves in radiating lines 
towards a variety of points. See fig. 4. 
Particles of water may be examined in a solid and crystalline state. A slip of 
glass or a small glass tray, coated with Canada balsam on its under surface, and a 
freezing mixture at the upper, will gradually condense the moisture from the air, and 
the balsam will become covered with the well-known spongy deposit of frozen par- 
ticles. The manner in which the crystallization takes place is remarkable. At first 
the moisture condensed is liquid and globular in form, but as the refrigeration con- 
tinues a sudden molecular change occurs, and the globules are found to assume va- 
rious crystalline shapes. When they are small and numerous, on consolidating, they 
form a kind of areolar net-work, the appearance of which is caused by the reflexion 
of numerous crystalline facets (see fig. 7) ; at other places may be seen small pyra- 
midal crystals covering a solid globular nucleus like the head of a mace ; sometimes 
they assume the form of small aciculse, of hexagonal prisms with various secondary 
facets, of octobedrons, &c. At the moment when this molecular action takes place, 
I have sometimes perceived with the microscope a kind of flitting movement, as the 
globules are assuming their various shapes, like that which occurs in the crystalliza- 
tion of particles from a state of solution*. 
The globules which compose mists or fogs may be condensed by exposing a slip of 
glass coated with balsam in the open air, but as the deposition takes place very 
slowly, it is preferable to employ the rotatory bellows, so placed as to throw the 
current of air at an angle of about 60° on the plate. Steam or other volatile sub- 
stances may be condensed in the same manner. When the current is created by the 
common bellows, 1 have found it much more difficult to fix the globules. Those ob- 
tained by this process are exactly similar to those of steam. In my experiments they 
were between 0‘02 mra and 0 - 03 mm in size. 
Essence of turpentine may be used in the same manner as Canada balsam. 
The globules condensed in this manifest much greater mobility, as might be ex- 
pected from the nature of the liquid, frequently rebounding from each other as they 
come in contact. They disappear very rapidly, either by adhering to the glass, or 
by coalescing together. I have observed them moving in vortices around several 
central points where they rapidly collapsed. Their diameters vary between the 
* The most favourable season for performing these experiments is the winter. In summer, when the air is 
warm and loaded with moisture, the particles on condensing, form liquid or solid globules, or very confused 
crystals, and whenever the freezing mixture is moved aside, so as to expose a portion of the under surface to 
microscopic inspection, a very few moments suffice for the particles to liquefy. Solid globules may be distin- 
guished from liquid ones by their borders being much darker, and from their frequently containing smaller 
globules, as in fig. 8. 
