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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 2 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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a distance equal to the difference in height between a column of air 10 ft. 6 in. high at 62 deg., in the case I have assumed, and the same air at 32 deg.

This difference will be equal to .643 of a foot, and the speed of the entering current will, by the rule for falling bodies, be 6.43 ft. per second.

Assuming that all the air enters through one opening, then about 8 sq. in. would be sufficient. It will be obvious, however, that it is not advisable to admit air at so high a velocity as this, nor would one inlet be so efficient as several.

I would propose, then, for existing rooms, that the inlets should be made in the side walls, near the floor, and in as many different sides of the rooom as its situation will allow. That these be made larger towards the room, and that they should be enclosed by metal gratings made ornamental on the outer and inner faces. The sketch marked A will show generally what I mean.

There is one point to which I would call your particular attention, and this is the

fine wire gauze screen shown just behind the ornamental front. This serves a very useful purpose, for it screens and breaks up the entering current of air, preventing all drafts, and, if the openings have sufficient area, allowing the incoming air to mingle so gently with that now in the room, that its presence is not felt save in a pleasant way.

FIG. 1.-SECTION.

I attach great importance to this screen, and to the size of the openings.

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