CONDUCTIVITY OF LIQUIDS AND GASES. 815 



takes place, and it can then be easily observed that it 

 takes a very long time before the lower particles of the 

 liquid become sensibly warmer. A test-tube is nearly 

 filled with water, as in fig. 398, and some ice, weighted 

 by a piece of wire wrapped round it, is placed in it. By 

 inclining the tube, and heating the surface of the liquid 

 by means of a spirit lamp, the liquid at the top may be 

 made to boil, while the ice at the bottom remains un- 

 melted. On the other hand, if the ice is placed on the 

 top of the liquid, and the test-tube heated as in the 

 experiment, fig. 386, applying only a very small flame, 

 the ice will soon melt, because in this case the water 

 heated at the bottom of the test-tube rises upwards and 

 parts with its heat to the ice. 



The most suitable wire for this experiment is lead wire ; if this 

 cannot be had, copper wire which has been softened in the flame 

 should be wrapped round the ice. A very small flame should be 

 used for heating the test-tube, and it should be held so that the 

 point of the flame is a little below the surface of the liquid, not in a 

 line with it, or the glass is sure to crack. The cracking of the tube 

 may also be prevented by slightly shaking the water, so that the 

 upper part of the tube may be uniformly and gradually heated ; it 

 must of course be done very gently, or the water will be agitated 

 right through the mass, and the object of the experiment frustrated. 



Air and all other gases without exception are bad 

 conductors, and a flow of heat can only take place 

 through them by an actual movement of their mole- 

 cules, which must be produced by heating that portion 

 of a gas which is lower than the remainder; the heated 

 molecules thus become less dense, and rise, while the 

 heavier portion gradually descends and becomes heated 

 in turn. A space filled with air cannot be heated from 

 above, for the heated molecules in the upper portion of 

 the mass remain where they are, while those below 



