HEAT 



387 



the index fluid. Or a differential thermometer may be 

 projected. 



FIG. 211 



PIG. 212 



225. Convection Currents. The simplest apparatus for 

 projecting currents produced by the lowering of specific 

 gravity, is a mere test-tube held in a Bunsen holder and 

 heated as in fig. 212. 

 If the tube be flattened, 

 the effect will be better 

 still ; or a flattened flask 

 can be similarly pro- 

 jected. To make the 

 currents more visible, a 

 few particles of exceed- 

 ingly fine sawdust from 

 heavy wood may be 

 placed in the water, or 

 particles of blotting- 

 paper rubbed up in a mortar with water ; Mr. H. G. Madan 

 uses a crystal or two of magenta coated with gum-water and 



o c 2 



FIG. 213 



