COMMON THINGS. WATER. 



vapour the temperature of 212 than to impart the same tempera- 

 ture to water, and it is in raising, the vapour formed from the 

 water to the temperature of the water itself that the entire quantity 

 of heat received from the fire is absorbed. 



9. Thus it is found that a given weight of water at 212 when 

 it passes into vapour, absorbs as much heat as would be sufficient 

 to raise five and a half times the same quantity of water from the 

 freezing to the boiling-point. 



10. It is not alone when raised to the boiling-point that water 

 is converted into vapour. It is vapcurisable more or less at all 

 temperatures, and it has been ascertained that a vapour is produced 

 even from ice. But the evaporation which takes place from water 

 below the boiling-point, is produced in a different manner, and 

 under different conditions. At the boiling-point, water is con- 

 verted into vapour at all points and at every depth, and most 

 abundantly at those parts where it is in contact with the surface 

 of the vessel upon which the fire acts. But at other temperatures 

 the evaporation is altogether superficial. The vapour is evolved 

 from the surface of the water above, and rises into and mingles 

 with the stratum of air which rests on the surface of the water. 

 This evaporation is also infinitely less rapid and copious than that 

 which is produced by raising the whole mass of water to the 

 boiling-point, and maintaining it at that point. 



11. The stratum of air which rests upon the surface of water 

 may be regarded as a medium which has a certain limited power 

 of absorbing the vapour of the water, exactly as a sponge receives 

 liquid water into its numerous pores. The air, like the sponge, 

 has a limited capacity for vapour, and it may become so charged 

 with vapour as to be incapable of absorbing more. The air in this 

 case is said to be saturated with vapour. 



Evaporation from the surface of water, therefore, takes place 

 more or less freely and copiously as the air is more or less below 

 the point of saturation; and when the air has already attained the 

 point of saturation all evaporation ceases. 



12. The process of drying moist or wet objects is an example of 

 the effects of evaporation. The moisture upon the surface, or in 

 the texture or pores of the object is evaporated by exposure to the 

 air, and the object becomes free from moisture, or dry. This 

 evaporation takes place so much the more rapidly as the air is 

 below the point of saturation, and so much the more slowly as it 

 is nearer to that point. 



13. Every one is familiar with the fact, that wet roads and 

 footpaths will on some days be dried in a few hours, while on 

 others they will continue wet without any marks of drying. 

 These are mere consequences of the state of the air in relation to 



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