CHAP, i.] THE ART OF WARMING. 



TI. WAKMINCI BY MEANS OF FIREPLACES/^^^? 



This is still the method of warming most commonly adopted in 

 England and France. The hearth, where combustion takes place, is 

 formed of a cavity excavated most commonly in one of the princi- 

 pal or bearing-walls of the house. It is surmounted by a cylindrical 

 or prismatic passage, by which the smoke and the other gaseous 

 products of combustion escape, and the outer orifice of which is 

 raised above the roof. 



In ordinary fireplaces combustion takes place at the expense of 

 the air of the room, which th.us loses its oxygen, and therefore 

 requires to be incessantly renewed. This renovation is effected by a 

 process, the phenomenon of which we are all familiar with under the 

 name of a draught. This is nothing else than the ascending motion 

 of the air and warm gases which escape from the grate. When the 

 fire is first lighted the outer air filling the chimney and the air of the 

 room are in equilibrium. The heat of the fire warms the lower 

 layers of air, which become less dense, and therefore tend to rise, and 

 in fact do rise. The colder air of the layers above fills the vacuum 

 thus caused and produces a descending current, which is at first 

 stronger, than the ascending, and so very often the smoke is driven 

 back into the room. 



As soon, however, as the column of warm air rises to the outer 

 opening of the flue and fills the whole chimney the ascending 

 vertical current gains the mastery, and the draught produces its 

 complete effect; but always on one condition, and that is, that the 

 air of the room, in proportion as it yields its o-xygen to the fire, shall 

 be as constantly replaced by fresh supplies. If from any cause this 

 renewal cannot be effected, the force of the draught diminishes by 

 degrees, and with it the activity of combustion. One result of this is 

 that the smoke is driven back, and another, that the air of the room 

 is vitiated, by being deprived of its oxygen, which is replaced by 

 irrespirable or poisonous gases, such as carbonic acid and carbonic 

 oxide. A draught, then, is as necessary for health as for the proper 

 working of the fireplace. 



Now how is this last condition of which we have been speaking 



z 



