Among all the furnaces you have examined, a certain one is doubtless better for you than any other; when I find out which one, you shall be informed.  Reliable testimony on the subject can only be given by some one who has tried different kinds in the same house under similar circumstances for a considerable time.  As we never have two seasons alike, and do have about three new first-class furnaces every year, it is difficult to find this valuable witness.  Printed testimonials are worth three or four cents per pound.  I do not know that cast-iron furnaces are more liable to be overheated than others, and you cannot “burn the air” with them if they are, unless you burn the furnace too.  You may fill a room with air, every mouthful of which has been passed between red-hot iron plates, not over half an inch apart, and I do not suppose the essential properties of the air will be perceptibly changed, or hurt for breathing when properly cooled.

The danger from cast-iron is in its weakness, not in its strength.

You speak of poison carbon.  Carbonic acid is not poison.  It is harmless as water,—­just.  It will choke you to death if you are immersed in it.  Trying to breathe it in large quantities will strangle you.  But we drink it with safety and pleasure, and may breathe a little of it, even as much as thirty per cent, for a short time, without serious harm.  But carbonic oxide, which is also liberated from burning anthracite, is an active poison, and one per cent of it in the air we breathe may prove instantly fatal.  Now it is fully proven that these gases laugh at cast-iron and pass through it freely whenever they choose.  Wrought-iron plates are supposed to be more impervious.  The popular notion that foul air must be drawn from the bottom of a room is based, I think, upon a superficial knowledge of the weight of carbonic acid, an ignorance of the law of the diffusion of gases,—­upon a realizing sense of the cost of coal, and an insensibility to the worth of fresh air.  Even such unreliable witnesses as our senses assure us that the air at the top of a high room—­say the upper gallery of an unventilated theatre—­is far less salubrious, though not overheated, than that below.  We know, too, how quickly the sulphurous gas that sometimes escapes from those warranted furnaces not only ascends through the tin pipes, but rises in the open stairway if it has a chance.  The hurtful carbonaceous gases doubtless go with it, and are then diffused through the room.  The most forcible objection to allowing the air to escape through the ceiling is that it is a wanton waste, not only of heat, but of the fresh air that has just come from the north pole by way of the furnace and cold-air box, and which, by virtue of its warmth, goes in all its purity straight to the ceiling.  Accordingly the heavy cold air lying near the floor and laden with poison must be drawn out through the ventilating flue, till the upper warmth and freshness fall gently on our heads, like heavenly blessings.