186 



Domestic Science 



123. Gases exhibit conducting power, but to an 

 extremely limited extent as compared with solid 

 bodies. 



Advantage is taken of the poor conducting power 

 of air in many simple household contrivances. To keep 

 a tea-pot warm it is surrounded by a "cosy". This 

 is made of loosely packed woollen material. The wool 

 is itself a bad conductor of heat, but 

 its effect is greatly assisted by the air 

 entangled between the threads of the 

 wool. This is a worse conductor than 

 the wool, and being prevented by the 

 network of wool-threads from moving 

 readily, cannot cause loss of heat by 

 the method of transmission usual in 

 gases, to be described later. The 

 cotton-wool packing used in Experi- 

 ments 57 and 58 has a similar function. 

 The layer of snow which frequently 

 covers the ground in severe winter 

 weather is of great service as a means 

 of preventing the underlying soil from 

 losing so much heat that the plants 

 growing in it suffer serious damage. 

 'The small crystals of ice, of which 

 snow consists, are of low conducting 

 power and the air between the crystals 

 is again of very considerable assistance 

 in preserving plant life. 



Empty space is a perfect non-conductor of heat. The 

 "Thermos" flasks (see Fig. 57) which have come so 

 much into use of late consist essentially of a double- 

 walled glass vessel, the space between the walls being 

 rendered as completely vacuous as possible. Hot 



Fig. 57. 



