EVAPORATION DRYING. 



the vapour with -which it is charged. In the former case it is 

 under- charged, and therefore readily receives the evaporation from 

 the roads and footways, which accordingly become dry ; in the 

 latter it is surcharged, and is at or near its state of saturation ; 

 it can receive no more vapour ; no evaporation is possible, and the 

 roads remain wet although no rain fall. 



14. Washerwomen who spread linen in the air to be dried, well 

 know that the facility of drying it varies on different days. 

 Some days have no drying power, the air being saturated with 

 vapour. Others dry the linen easily and quickly. Then the 

 air is little charged with vapour, and is far below the point of 

 saturation. Between these there are many degrees in which the 

 facility of drying varies. 



15. Wind stimulates evaporation, and therefore expedites drying. 

 This is easily explained. So fast as the stratum of air over water 

 becomes charged with vapour and raised towards its point of satu- 

 ration, it is swept away, and a fresh portion of dry air is brought 

 into contact with the wet surface. This in its turn is swept away, 

 giving place to another dry portion of air, and so on. In this way, 

 all moist objects exposed to wind or currents of air are speedily dried. 



"Wet objects are quickly dried when exposed to artificial heat, 

 the moisture they contain being rapidly evaporated. 



16. Water when absolutely pure is without taste, and insipid. 

 But in its natural state water never is pure. Spring water raised 

 from inferior strata of the ground has always various earthy and 

 saline matters dissolved in it. In fact, every constituent of the 

 strata from which it has been raised, or through which it may 

 have passed, which is soluble in water, is necessarily dissolved in 

 it in greater or less quantity. River water contains more or less 

 of all the soluble constituents which it encounters either at its 

 sources or on the beds and banks of the channels through which it 

 has passed, besides the soluble parts of various dead animal and 

 vegetable matter which it inevitably receives in its course. 



17. All water in its natural state contains more or less fixed 

 air mixed with it. This is most commonly carbonic acid. This 

 gas, which is the same as that which effervesces in soda water, 

 lemonade, champagne, and bottled malt liquors, gives to the 

 flavour of water a certain agreeable pungency. 



18. Water acquires very various flavours and other qualities, 

 according to the nature of the substances which it holds in solu- 

 tion. Spring water, in general, even when it is most pure, holds 

 lime ana silicious earths in solution. It is from these that it 

 acquires the quality popularly called hardness. It will not easily 

 mix with soap, and it is not suited to culinary purposes. 



19. Water which is free from this quality, and which holds but 



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