DRYING. 



tinue to expand for an hour or two, when exposed to water at 

 the temperature of the air. At high temperatures, the diffusion 

 of vapour into air is more rapid ; but still it is not at all instan- 

 taneous. Hence, in such a drying stove, means ought to be 

 taken to repress rather than to promote the exit of the hot 

 air ; otherv/ise a loss of heat will be occasioned by the escape 

 of the air, before it is saturated with humidity. The greatest 

 advantage has been derived from closing such a stove as per- 

 fectly as possible at the top, and only opening it after the goods 

 are dried and about to be removed, in order to allow of a re- 

 newal of the air in the chamber between each operation. In 

 evaporating water by heated air, the vapour itself carries off 

 exactly the same quantity of heat as if it were produced by 

 boiling the water at 212, while the air associated with it like- 

 wise requires to have its temperature raised, and therefore occa- 

 sions an additional consumption of heat. Hence water can 

 never b'e evaporated by air in a drying stove with so small an 

 expenditure of fuel as in a close boiler. 



When bodies to be dried do not part with their moisture 

 freely, but in a gradual manner, as is the case with roots, and 

 most organic substances, the hot air to dry them may be greatly 

 economized by a particular mode of applying it, which is prac- 

 tised in the madder-stove. The principle of this drying stove 



is illustrated by the annexed fi- 

 gure, in which a b represents a 

 tight chamber, having two open- 

 ings, one near the roof, by which 

 hot air is admitted into the cham- 

 ber, and another at the bottom, 

 by which the air escapes into the 

 tall chimney c. The chamber 

 contains a series of stages, from 

 the floor to the roof, on the 

 lowest of which sacks, half filled 

 with the damp madder roots, 

 are first placed. In proportion as the roots dry, the bags are 

 raised from stage to stage, till they arrive at the highest stage, 

 where they are exposed to the air when hottest and most des- 

 iccating. As the dried roots are removed from the top, new 

 roots are introduced below, and passed through in the same 

 manner. Here the dry and hot air, after taking all the moisture 



G 2 



