Mr Ti’fcdgold on the Principles and^Practic^~ 
over the same surfaces, with their temperatures under 800°, is 
pild and agreeable. The precise nature of the change which 
an excess of heat produces in air, is not, perhaps, thoroughly 
understood, but it is supposed to consist in a partial combustion 
of the particles of animal and vegetable matter suspended in 
the air ; it is a change, however, which produces a very sensi- 
ble effect ort any persoir who lives a considerable portion of his 
time in air which has undergone it. 
Hence, in selecting those methods which are adapted to give 
warmth to the air of an apartment, it will be desirable to avoid 
those where the air must be in contact with surfaces of a higher 
temperature than 800°, and even that should be considered the 
extreme limit of the heat of a surface to warm air. To confine 
the temperature of the heating surface within this limit, excludes 
so many of the usual methods of warming, that we have only 
few left to consider. 
The most useful for small liouses is that where the fuel, &c. 
is confined by such a thickness of matter that the external sur- 
face cannot be heated beyond 800°. A stove of this kind should 
be as completely insulated as possible ; so that the heat of the 
fire, and of the smoke and hot air passing through the flues, may 
be given out to the air it is intended to warm. The flues will 
be effective, with a good chimney, at a horizontal distance of 40 
feet from the fire, and of from 50 to 60 feet where the flue rises, 
either regularly or by steps. It is sometimes necessary to make 
flues descend again, before tlie smoke passes into the chimney ; 
but this renders them liable to explode, whenever the fire is so 
mismanaged as to fill the flues with gas. In hot-houses for 
plants, the flues are extended in one direction, so as to afford as> 
equable a heat as possible to a considerable length of house ;; 
but, in other cases, the flues may be made to wind backwards 
and forwards, so as to occupy only a small horizontal space, of 
which we have an example in Swedish stoves. The materials of 
these stoves should be of such a nature that the air may be 
warmed against the surface, without becoming loaded with dust. 
Indeed ail passages for air to circulate through, should be hard, 
smooth, clean, and durable. The wear of soft bricks, mortar, 
&c. by the friction of air, is much more considerable than those 
who have not observed it with care can have any idea of ; and. 
