152 SCIENCE OF COMMON THINGS. 



Temperature of trees. Flannels and furs not really warm. 



Several degrees above that of the surrounding atmo- 

 sphere. 



983 What occasions this difference between the temperature of the sap 

 of a tree and the temperature of the surrounding atmosphere 1 



The vital action of the tree. 



It is also a noticeable fact that sap drawn from a tree will freeze at the 

 same temperature as water, while the sap circulating in the tree, nnder 

 .the influence of vital agency, will not freeze until reduced seventeen degrees 

 below the freezing-point of water. 



983 Why in a frozen pond or lake is the ice always thinner^ and often 

 entirely wanting, in those parts where springs exist upon the bottom 1 



Because the spring water, coining from a point in 

 the earth below the influence of the frosts, is elevated 

 in temperature, and by imparting its heat prevents an 

 accumulation of ice upon the surface above. 



984 Is there in reality any positive warmth in the materials of cloth- 

 ing? 



No ; but we consider clothing warm or cool accord- 

 ing as it impedes or facilitates the passage of heat to or 

 from the surface of our bodies. The thick cloak which 

 guards a Spaniard against the cold of winter is also in 

 summer used by him as a protection against the direct 

 rays of the sun; and, while in temperate climates flan- 

 nel is the warmest article of dress, we cannot at the 

 same time preserve ice more effectually than by inclos- 

 ing it in its softest folds. 



985 Does fine or coarse woollen cloth make the warmest clothing? 



TliQfaier the cloth, the more slowly it conducts heat. 

 Fine cloths, therefore, are warmer than coarse ones. 



98S Is silk a good conductor of heat f 



~No ; it is a bad conductor of heat. Spun silk allows 

 the heat of the body to pass off more quickly than wool ; 

 but raw silk confines it more than wool. 



987* Tlie sheets of a bed feel cold and the blankets warm. : is there any 

 difference in the respective temperature of these articles f 



No ; the temperature of both the sheets and the blan- 

 kets is always exactly the same. 



988 Why, then, does one feel colder than the other f 



Sheets feel colder than the blankets because they are 



