LOW CONDUCTIVITY OF WATER. 



279 



tube in a sloping position (fig. 132), and taking care that the 

 flame of the lamp does not heat the bottom part ; the water in the 

 upper part of the tube will soon begin to boil, so that ice cold 

 ivater and solid ice are contained at the bottom of the tube, whilst 

 the water at the top is boiling vigorously ; the heat of the boiling 

 water is so slowly conducted downwards that the lump of ice will 

 remain for some time unmelted. If, however, the bottom part of 

 the test-tube be warmed instead of the top, the ice will rapidly 

 melt, and will all disappear long before the water in the upper 

 part of the tube becomes heated to any considerable extent. 



Fig. 1.32. Low Conductivity of Water. 



Fig. 133. Low Conduc- 

 tivity of Water. 



Instead of applying a flame to the outside of the tube, a better 

 plan is to place in the upper part of the test-tube a thin spiral 

 wire of platinum, through which an electrical current can be passed 

 so as to heat the wire considerably ; in this way the upper portion 

 of the water can be made to boil without the risk of heating the 

 lower part of the tube by currents of warm air, &c. ; so that if 

 the bottom part be wrapped in dry flannel or cotton wool, &c., to 

 keep off the heating action of the air of the apartment, a much 

 longer time will elapse before the ice becomes melted. Fig. 133 



