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 "flat, 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 

 FIG. SB- Best Form of Glass can be accommodated m small watch 



or Quartz Evaporator for 



Microchemical Work. 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, 



