CONDUCTING POWER OF LIQUIDS. 237 



made to fill the same space. Ashes, being more po- 

 rous, are found to be still better. Tan, which is simi- 

 lar to saw-dust, is well adapted to filling in the walls 

 of stables and poultry -houses, where more than usual 

 warmth in winter is required. Confined air is a very 

 poor conductor of heat ; hence the advantage of double 

 walls and double windows, provided there are no crev- 

 ices for the escape of the confined air. This principle 

 has been lately applied in the manufacture of hollow 

 brick for building the walls of dwellings. 



The light and porous nature of snow renders it emi- 

 nently serviceable as a clothing to the earth in the 

 depth of winter, preventing the escape of the heat from 

 below, and protecting the roots of plants from injury 

 or destruction. Hence the very severity of the cold 

 of the Northern regions, by producing an abundance of 

 those beautiful feathery crystals which form snow, be- 

 comes the means of protecting from its own effects the 

 tender herbage buried beneath this ample shelter. 



CONDUCTING POWER OF LIQUIDS. 



Liquids are found to conduct heat very slowly, and 

 they were for a long time considered perfect non-con- 

 ductors. Some interesting experiments have been per- 

 formed in illustration of this property. A 

 large glass jar may be filled with water (Fig. 

 199), in which may be fixed an air thermom- 

 eter, which is always very quickly sensitive 

 to small quantities of heat. A shallow cup 

 of ether, floating just above the bulb, may be 

 set on fire, and will continue to burn for 

 some time before any effect can be seen upon 



