26^ Mr Tredgold on the Principles and Practice 
fuel only supplies a certain quantity of heat in a given time, it is 
obvious, that, if we expose too much surface at a temperature of 
800®, more heat will be given out than the fuel can supply, and the 
temperature of the fire must be lowered, or will burn dead, as it 
is termed. If the back of the grate containing the fuel be of iron, 
the surface of hot fire must be less than when slow conductors are 
used, because there will be a greater loss of heat through the iron- 
back. It is often attempted to employ the heat which is given 
off by an iron-back to warm air ; but air warmed in this manner is 
burnt, and unfit for respiration, besides creating a great deal of 
dust ; and the loss of radiant heat is nearly equivalent to the 
quantity communicated to air in that manner. It is one of the 
advantages of an open grate, when properly constructed, that 
it allows all the burnt air, and the noxious gaseous matter from 
the fuel, to escape up the chimney, as they are formed ; but 
this desirable property does not belong to all kinds of grates, 
even when their chimneys are good, and not liable to smoke. 
In order that the arrangement may produce the effect, the en- 
trance to the chimney should be immediately above the fire, and 
large enough to give passage to the smoke, burnt air, &c. from 
the fire ; it should not be larger, because, then, too much air 
will be abstracted from the room, and much heat will be lost. 
This leads me to notice a defect of a species of grate, lately 
much in use, in which the opening for the smoke is at the back 
of the grate, and very little above the level of the fire ; as shewn 
in Fig. 2. Plate II., where the smoke passes through a long 
narrow opening at AB. A chimney of this kind will not act, 
unless it has a powerful draught ; and the greater the draught 
is, the less effect will be obtained from the fire ; but, however 
powerful the draught may be, a quantity of sulphureous vapour 
and burnt air will be intercepted at A, by the thin edge of the 
plate in which the aperture is made, and rise into the room. 
Common iron -stoves, with open fires, and descending flues, have 
the same defect ; they are very commonly employed for warm- 
ing shops and counting-houses in London, but are only felt op- 
pressive where the doors are not opened with sufficient frequen- 
cy, to change the air of the place very often. 
Where air is not in any degree injured by fire, but merely 
heated, it is felt oppressive ; because, air being increased in bulk 
