In this, the products of combustion of the gas pass along a pipe placed between the ceiling and the floor of the room above, into one of the flues. This pipe is surrounded by another opening through the ceiling of the room at one end, and into the flue at the other, and guarded at its entrance to the flue by a valve which can be easily shut when the gas is not burning. This double tube, as it passes under the floor of the room above, is covered with a fire-proof material, so that the floor is not affected by it. The joists, where they are notched, have iron bearers put across to support the floor boards above. Air is admitted by another pipe passing through the wall of the house into the external air, and ending also in the ceiling of the room by openings around those of the exit shaft. Thus warm air is introduced into the room at the same time that vitiated air from the upper part of the room, and also the products of combustion of the gas, are carried out of it into the chimney flue.

I may say a few words about some grates and stoves that have been devised with the view of combining ventilation and heating. The first of these is Captain Douglas Galton's grate, in which there is an air-chamber placed around the flue, and communicating on one side with the external air, and on the other with the atmosphere of the room by various apertures. The outer air which passes into this chamber is warmed by contact with the heated flue, and issues into the room, thus supplying the room with warmed air, and utilising a considerable quantity of the heat that would otherwise be lost. There are several

other grates, such as the Manchester school grate, made upon this principle, with variations in the arrangement of the inlet apertures, which are placed vertically like Tobin's FIG. 14

tubes. It is important in all these contrivances, where the outer air passes through a chamber in which the back of the grate and the commencement of the flue are placed, that the back of the grate and flue in that chamber should be cast in one piece of metal, so as to have no joint. If there are joints they will after a time become defective, and air from the flue is liable to escape into the chamber round it and be brought back into the room by the entering air. The back of the grate should also be lined with fire-clay. Some slow combustion stoves, as George's "Calorigen," and Dr. Bond's "Euthermic" Stoves, have pipes passing through them, to admit warmed external air into the room. Iron slow combustion stoves dry the air too much, and unless they are lined with fire-clay, are apt to become too hot,

and to cause an unpleasant smell in the room by the charring of the organic matter in the air. They are much more suitable for warming large buildings, where

economy of fuel is an important object, than they are for use in sitting-rooms or offices. It is usual to place a vessel of water on the top of these with the view of obviating, as far as possible, the dryness of the air they produce. It must be borne in mind that closed slow combustion stoves do not act as ventilators, as the air to supply the fuel -usually coke-is brought by a pipe from the outside, and this is another reason why they are not so advantageous as an open fire or a quick combustion stove in dwellingrooms. In the Thermhydric grate of Mr. Saxon Snell, a small boiler is placed behind the grate, and communicates

with a series of iron pipes alongside of it. These are filled with water, which is of course kept warm, and air is admitted to the room between these hot water pipes. Thus,

it is neither dried nor heated too much. The products of combustion are carried away by a flue, which may be placed under the floor; so that the grates, if required, may stand in the middle or in any other part of the room.

Gas stoves are gradually becoming largely used instead. of coal, and when proper provision is made for the escape of the products of combustion, they are certainly very con venient and cleanly contrivances. I have no doubt that this will, in the end, be found to be the proper use for gas,

and that we shall cease entirely, or almost entirely, to use coal in our houses. By using coal in the way that we do, we lose all the valuable bye-products--the ammonia, the tar, the carbolic acid, the aniline dyes, &c., which are derived

from the refuse of gas-works, and most of which are worse

than useless in our fires.

Gas may be burned either