36 Introduction. 



a rain of several days, contains more water than this and 

 green wood much more, sometimes as high as 50 per cent., 

 while well seasoned sheltered wood may contain less than 

 half that amount. 



It is frequently urged that when some green or wet wood 

 is burned with that which is dry there is a saving of fuel. 

 There is some truth in this, especially in stoves having too 

 strong a draft and too direct a connection with the chim- 

 ney and if the radiating surface is small or poor. The 

 evaporation of the water prevents so high a temperature 

 from occurring in the stove, which makes the draft less 

 strong, and this gives more time for the heat to escape 

 from the stove before reaching the chimney, and hence 

 less is lost in this way. Then as the fire burns more 

 slowly there is not the overheating of the stove, at times, 

 which may occur with lack of care when very dry wood 

 is used, and a considerable saving occurs in this way. 

 These statements apply more particularly to heating 

 stoves than to cooking stoves. Dry wood is best for the 

 kitchen stove under most circumstances, the slower fire 

 being secured when needed by using larger sticks and by 

 controlling the draft. 



SURFACE TENSION, SOLUTION AND OSMOSIS. 



49. Surface Tension. The free surface of any liquid be- 

 haves much as though it were covered by an elastic mem- 

 brane and it is this surface action which draws the rain- 

 drop into the form of a sphere as it falls through the air. 

 It is surface tension that causes water to form into spheres 

 on a dusty floor, on a hot stove or on cabbage leaves. The 

 dewdrop owes its shape to surface tension, and it is this 

 which is employed to mould the melted lead into perfect 

 spheres as it falls from high towers, cooling into solid shot 

 before reaching the bottom. 



The cause of surface tension is the cohesive attraction 

 of the molecules for one another. This attraction extends 



