32 FIRST PRINCIPLES. 



powers of certain substances may be studied with advan- 

 tage : 



Copper 3225 



Iron 477 



Slate 16 



Glass 6.6 



Water 5.8 



Fire-bricks ....... 5.1 



Olive Oil 1.83 



Oak 1.7 



W 7 ool .32 



Air .16 



" The conductivity of snow is about fth that of moist clay " 

 (Ganof). The order of superiority in conductive power 

 possessed respectively by linen, cotton, silk, and wool, is the 

 same as that in which they are here written. The conductivity 

 of hair may be regarded as the same as that of wool ; for the 

 latter substance is merely a variety of the former which has 

 undergone a modification of shape, but not of composition or 

 structure. 



The great difference between the respective conductive 

 powers of snow and water seems to be due chiefly to the 

 large quantity of air contained in snow. Bark has less con- 

 ductive power than wood, apparently from the fact of its 

 containing more air. 



Ordinary manufactured silk conducts heat better than pure 

 silk, owing to its adulteration with certain agents such as salts 

 of iron and tin ; and common horse rugs are better conductors 

 of heat than those which are " all wool," on account of their 

 being made partly of cotton. The presence of water and air 

 modifies the apparent powers of conduction of many sub- 

 stances. For instance, the fact that a woollen cloth holds 

 imprisoned between its fibres a larger quantity of air than a 

 cotton cloth of similar size and weight would do, makes the 

 former material a worse conductor of heat, as compared to the 

 latter, than the knowledge of the respective conducting powers 



