CURRENTS IN A HEATED LIQUID. 777 



When heat is applied to the lower part of a vessel 

 containing any liquid a series of ascending and descend- 

 ing currents are produced in the liquid, in consequence 

 of the difference between the specific gravity of the 

 hotter portions of the liquid and that of the colder. 

 The portion nearest to the source of heat becomes 

 heated first, this makes it lighter, and it ascends, while 

 its place is supplied by colder and heavier liquid, 

 which descends, becomes heated in its turn, and as- 

 cends in like manner. These 

 ascending and descending cur- 

 rents cause a circulation of the 

 liquid, which goes on until the 

 whole mass has the same tem- 

 perature, as may be observed 

 very easily by means of a test- 

 tube filled with water and held 

 in the edge of a flame, as in 



,. 00 ~m j?-u J FIG. 386 (% real site). 



fig. 386. The current of heated 



water ascends on the upper side of the tube and the 



current of cold liquid descends on the lower side. 



Water, unless specially purified, contains nearly always a suffi- 

 cient number of floating particles to show the circulatory movement, 

 when the water is heated. Or a minute bit of blotting-paper, about 

 }mm m g] - ze) should be rubbed down in a mortar with a few drops 

 of water ; the fibrous shreds, when thrown into a test-tube filled 

 with water, will show the circulation very distinctly. 



If two communicating tubes contain the same liquid, 

 and the liquid in one tube has a higher temperature 

 than that in the other, the surface of the liquid will have 

 a somewhat higher level in the warm tube than in the 

 colder one. The difference of level is most easily seen if 



