294 
ELEMENTARY CHEMICAL MICROSCOPY 
clamp open when removing or changing the object slide serving 
as a cover. 
Instead of holding the watch glass and cover, at the edges, 
between the thumb and finger as described above, the clamp 
shown in Fig. 154 may be used, or two watch glasses with ground 
edges selected to fit edge to edge may be clamped together. In 
certain instances either one of these watch glass methods may 
prove to be more practicable than the crucible. In all cases, 
however, the clamp support is far superior to the fingers. 
Although the device just described may be satisfactorily 
applied to the fractional distillation of small amounts of volatile 
liquids, small distilling tubes will be found in certain cases to be 
somewhat safer for very volatile substances. These are readily 
made from small glass tubing of thin wall as shown in Fig. 155. 
The finished distilling tube is shown in A. To introduce the 
liquid to be distilled a rubber pipette cap r is slipped over the 
large end of the tube (Fig. 155 B); the tube is inverted as shown, 
the drawn-out end of the tube is dipped into the liquid to be 
distilled and the rubber bulb is compressed just enough so that 
when released the liquid will rise into the bulb in sufficient 
volume to not quite half fill it. The tube is then again turned 
to the position A, the bulb surrounded by ice and the drawn-out 
tube sealed off in the flame of a blast lamp or blow pipe. The 
bulb is removed from the ice, wiped dry and the apparatus 
arranged as shown in Fig. 155 C. The liquid may now be heated 
