I may give you an idea of the amount of damage done to the air of rooms by candles, even where perfect combustion takes place, by telling you that two sperm candles produce as much carbonic acid, and consume as much oxygen, as one man. A man in a room with two sperm candles burning requires twice the amount of fresh air that he would if he were by himself. The same is the case with a good lamp. A cubic foot of gas destroys the oxygen of about eight cubic feet of air. A poor burner will consume at least two cubic feet in an hour, and so destroy the oxygen of sixteen cubic feet of air, that is to say, will destroy as much air as four men; so that a man sitting in a room with a gas burner that only burns two cubic feet in an hour, requires at least five times as much change of air as he would if he were there by himself.

We now come to the substances used for warming purposes, and the apparatus. I will content myself with speaking about coal, and the apparatus in which it is burned. When burned in the ordinary fireplace it is burned at an immense disadvantage. About ninety per cent of the heat escapes up the chimney, but it has one advantage, and that is, that it changes the air of a room very quickly, as air must be supplied to the fire, or else the fire will go out. The only way by which air can enter to supply the fire, when a room is closely shut, is the chimney, and so air comes down the chimney, and that is one cause of smoky chimneys.

Now, a word or two about stoves. Stoves are made of various materials, as earthenware, wrought iron, or cast iron. Quick combustion stoves very much resemble ordinary fireplaces. In slow combustion stoves coke is generally burnt, and a limited supply of air is admitted to them by a pipe from outside, so that there is no fire to be seen in the room.

They are used for warming large rooms which require a great deal more heat than is given by an ordinary fireplace. The disadvantage is that, when an iron stove gets hot, it dries the air of the room, and this dryness is only partially obviated by placing a vessel of water on the top of the stove. Another thing is that the air in which a stove is burning always contains carbonic oxide gas. That has been proved by a series of careful experiments. Another disadvantage is, that they always make the air of a room smell You all know what is called the smell of cast iron. It is due to the partial charring of the organic matter in the air of the room, and the smell is worse if the air in the room is rather foul. So that stoves, even though they have a pipe to convey the results of combustion from a room, have great disadvantages, especially in small rooms.

The ordinary fireplace has been immensely improved in construction, and that known as the Galton stove, invented by Captain Galton, is a great improvement, though it has not been brought into sufficient use. There is an air chamber around the flue of this stove.

which communicates by a pipe with the external air, so that as the fire burns in the stove, external air comes into the chamber round the flue, is warmed, and gets into the room, partly to supply the fire, and partly to supply the room. By this contrivance as much as 35 per cent of the heat is saved, and fresh air is brought into the room, and that is of very considerable importance.

The ordinary stoves do not change the air of a room sufficiently, but there are certain kinds of stoves in which either coal or gas may be burned in which there is a contrivance for bringing fresh air already warmed into the room by a pipe running through the stove. These are called calorigen stoves.

There are a large number of trades in which solid particles and offensive gases are given out into the air.

For instance, in the air of mines there is a large quantity of finely divided solid particles which get into the lungs of the miners and set up irritation there, often resulting in consumption; and consequently among men who work in mines there is a large death-rate from that disease, because consumption, in manhood especially, attacks the lungs.

It has been ascertained that the death-rate from consumption among miners who work in mines in which the air is changed rapidly, is very much less than among miners who work in mines that are badly ventilated.

In potteries, china works, pearl button manufactories, glass polishing, and cement works, there is a large amount of foreign mineral matter in the air, and the workpeople in them suffer from a high death-rate; this high rate of mortality might be lessened if the solid particles were prevented from getting into the lungs of the workmen, and if these workmen could be persuaded to wear some kind of mask or respirator, by means of which they could breathe and, while breathing, filter the air and separate the solid particles from it, the deathrate from lung diseases and consumption would be considerably diminished. But there is much difficulty in inducing workmen to adopt any such measures; they cannot see that they are for their benefit.

Then, again, workers in iron, and especially in steel, are exposed to the influence of fine particles getting into the air. These might easily be separated by wearing a kind of magnetic mask which has been devised, through which they could breathe, but which would collect the minute particles and prevent them getting into the lungs.