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charged with a greater destruction of human life and with causing more disease every year than has been produced by the bloodiest battle recorded in history.

It is impossible to estimate correctly the air contaminated from this source, but I hope to show how such contamination may be avoided.

As a summary of results we see, that in a room of the net cubic capacity of 3,800 ft., having a fire burning, inhabited, by say six persons, and lighted by three gas lights, there will be required every hour, 80 that the inmates may be healthy, 1,694, or say 1,700 cubic ft. of fresh air at 60 deg. F.

But air expands to, or 0.00204, of its volume for every degree F. it is heated; it is obvious from this that, assuming the temperature of the outer air to be 32 deg: F., we shall not require to admit so much cold air into the room by about 100 cubic ft., but exit must be provided for the full quantity.

Professor Tyndall gives the following table of expansion of gases for an increase of temperature equal to 1 deg. F.:

Air..

0.00204 Carbonic Oxide............

0.00204 Carbonic Acid..................

0.00206 Protoxide of Nitrogen.....

0.00207 Sulphurous Acid...............

: 0.00217 We see from this that gases obey the almost universal law, and expand with increased heat. Upon this fact is built the whole system of thermo-ventilation, and upon its due appreciation depends success in this particular branch; for it follows that as gases expand so are their densities decreased, and they are lifted upwards by the heavier air without; rising, not because they are light, but because the outer air is colder and consequently heavier.

The following is a table of specific gravities of gases taken from the experiments and researches of Regnault and others. Atmospheric air being 1:

. 1.00000
Hydrogen Gas...

O 06926
Oxygen Gas ....

1 10563

1.11100 Nitrogen Gas ...

0.97137 Carbonic Acid...........

1 52901 16 Oxide..........

0.9727 Protoxide of Nitrogen...

1.5252 Sulphurous Aci

2.2464 Sulphuretted Hydrogen........ 1.1710 Vapor of Water...

0.6235

Air.......:

us Acid ............

This table shows us how varied are the densities of different gases. Thus, while hydrogen is more than 14 times lighter than air, sulphurous acid is about 24 times heavier.

There is another property of gases which merits our consideration besides that of density, and this is the power they have of mixing with one another.

The gases which compose common air, for instance, are not chemically combined to form any new substance; they exist each one of them just as if the others were not present. When we consider this, and the fact that they are of different densities, it would fill us with astonishment that they do not settle, as it were, away from one another, the heavier oxygen falling to the ground, and the lighter nitrogen ascending to the clouds, did we not know of this law of combination, which is called the law of diffusion of gases. Professor Graham expresses this law by saying that the diffusive power varies inversely as the square root of the density of the gas itself.

A simple but beautiful experiment of the Professor's illustrates the law.

I have stated that in the room before mentioned we require at certain times—and all works should be calculated for the greatest possible duty they may have to perform-space for 1,600 cubic ft. of fresh air to enter, and far 1,700 ft. to leave the room every hour.

I am aware that any proposal to admit cold air into a room will meet with opposition, and were houses built upon principles worthy of our civilization it might easily be avoided. I think, however, that, with proper means, the possible evil may be so reduced as to be practically unfelt. I am also now. treating of existing houses which could not well be rebuilt or altered.

I would propose that in such houses where the want of ventilation is felt, the openings for admission of fresh air should be made as near the floor of each room as possible. For it will be seen from what has gone before, that the cold air will, on entering, be heavier than that in the room, which it will gradually force upwards and out through the foul air exit channels.

Many people will probably say, “Oh

what a draft there would be to one's legs and feet.” Doubtless if a good sized hole be cut through the wall, say directly opposite the fireplace, and you shut the door and stand between the opening and the fire, very likely you would feel rather cool about the inferior extremities. But I do not propose a thing of this kind.

The fire must have its own separate supply, which should be given by means of a pipe leading through the hearth-stone, as is sometimes done now, with the opening facing the fire, which latter, as it constantly acts the part of a pump, will take . care to get its own allowance from the nearest source.

We have therefore only the forcing power of the difference in weight, between columns of air, of equal height, but of the different densities due to difference of temperature between the external air and that in the room. This difference I have assumed to be 30 deg. .

This forcing power will cause the air to enter the room with a velocity equal to that which would be acquired by a body falling

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