152 
ELEMENTARY CHEMICAL MICROSCOPY 
and it is to be regretted that sheets made from cellulose acetate 
of the same thickness as those made from the nitrocellulose 
cannot be purchased in the open market. 
Treatment of material with alkalies or at a high heat must 
be confined to supporting slips made from platinum foil. In 
fact, a small piece of platinum foil 15 to 20 mm. long by about 
7 mm. wide, sufficiently thick to remain flat when heated at a 
corner may be considered as a necessity. The foil must be 
kept ilat, clean and polished. Since it is opaque, the materials 
must eventually be transferred to glass, quartz, or celluloid 
slides for examination after having been subject to the proper 
reagent or heat treatment. When very low magnifications are 
permissible it is possible to examine the material upon the plati¬ 
num foil without transferring, the illumination being either by 
oblique light or by some form of vertical illuminator. 
Watch Glasses. — When volumes of liquid greater than can 
be handled upon object slides become necessary, small watch 
glasses 10 millimeters and 25 millimeters in diameter will be 
found convenient. Only the deep type of watch glass should be 
employed; for example, a 25-millimeter watch glass should be 
from 3 to 5 millimeters deep. Instead of lo-millimeter shallow- 
watch glasses, object slides with a depression ground into them 
will be found better and more convenient. 
Watch glasses are useful for covering preparations, for making 
tiny moist chambers, for microdesiccators, for distilling and 
subliming, and for evaporating solu¬ 
tions to small bulk. 
Most small watch glasses are made 
from soft non-resistant glass, a fact 
which should be borne in mind when 
using them. 
Still larger volumes of liquid than 
can be accommodated in small watch 
glasses are best concentrated in small 
evaporators of transparent quartz or 
Jena glass (Fig. 85). If those with flat bottoms are chosen they 
may be placed upon the stage of the microscope and any crystals. 
or Quartz Evaporator for 
Microchemical Work. 
