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The reason is this. The air cooled by contact with the cold glass is no longer able to sustain the moisture, and the latter is therefore precipitated in the form of dew.

(C.) The third cause I have mentioned is the presence of living beings in the room. Let us now consider the effect of this.

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Man's body is a furnace, a slow combustion furnace if you will, but still a furnace, and the waste from this human furnace is precisely the same as that from any other furnace, viz., carbonic acid and water. quote the words of Professor Tyndall: "In the animal body the carbon and hydrogen of the vegetable are again brought into contact with the oxygen from which they had been divorced, and which is now supplied by the lungs. Re-union takes place, and animal heat is the result. Save as regards intensity, there is no difference. between the combustion that goes on within us, and that of an ordinary fire." We see then of what vital consequence is the ence of oxygen in the atmosphere. Without it fires and lights will not burn, our food will not digest, and the blood

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remains unpurified, as is shown by the pale faces and purple lips of people living in close warm rooms. These are the forerunners of certain death to persons deprived of the life-sustaining oxygen.

Atmospheric air consists of a mixture of several gases, for though it is commonly said to be formed of oxygen and nitrogen in proportion of 21 volumes of the former to 79 of the latter, in each 100, yet several other gases are mixed with these. The composition of air varies with its situation. Thus, inland air is not of precisely the same composition as that near the sea coast, where there is said to be a greater proportion of ozone. On rocky mountains there is more carbonic acid than is found in the valleys.

The quantity of air required by a man varies with the state of his body. Thus a man at hard work or violent exercise may require even five times as much air as the same man when asleep.

This amount of air has been variously estimated, but I think we may safely take the following calculation by Box to be cor

rect. He states that an ordinary man takes 20 respirations per minute, of the capacity of 40 cubic in. each, thus vitiating 28 cubic ft. per hour. Added to this, there is the quantity of air saturated by the moisture evolved to be considered. According to Dumas this quantity in the form of vapor equals .0836 of a pound of water per hour, which is sufficient to half saturate, for air should not be more than half saturated to be pleasant-187 cubic ft. of air at 62 deg. Fahr.

We thus see that to be in good health a man must have 215 cubic ft. of air per hour for his own use. Sick persons require very much more than this.

(D.) There is one more source of contamination of the air of our houses, which often forces itself unpleasantly upon our notice. I mean the foul gases which escape

from drains and water-closets. It is not too much to say that if architect and builder always did their duty, no foul air from any drain or closet ought ever to enter a house, and that the negligence of one and the ignorance, or worse, of the other, must be

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, so that the inmates may be healthy, 1,694, or say 1,700 cubic ft. of fresh air at 60 deg. F.

But air expands o, 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. :

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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:—

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