Opportunity for adding moisture, in the desired quantity, to the air of the average dwelling is limited to the evaporation of water in the heating plant, from vessels attached to the radiators or that which goes on in the kitchen. Household humidifying plants are within the range of possibility but there is not yet sufficient demand for their use to make attractive their manufacture.

In the hot-air furnace a water reservoir is usually a part of the chamber in which the air supply is heated. The water in the reservoir is heated to a greater or lesser degree, depending on the temperature of the furnace and vaporized both by heat and by the constantly changing air.

In the use of a steam plant or hot-water heating plant the opportunity of humidifying the air is very limited. One method is that of suspending water tanks to the back of the radiators from which water is vaporized. While this method is fairly efficient as a humidifier it is inconvenient and therefore apt to be neglected. In houses heated by stoves there are sometimes water urns attached to the top of the frame which are intended for the evaporation of water but as a rule they are not of sufficient size to be of appreciable value.

The quantity of water required to humidify the air of a house will depend first, on the temperature and humidity of the outside air; second, on the cubic contents of the building; third, on the rate of change of air in the building. If the ventilation is good the rate of atmospheric change is rapid and the amount of water in consequence must be correspondingly increased.

The data included in the following table showing the relative humidity and amount of water required were taken from a seven-room frame dwelling in Fargo, N. D., during particularly severe winter weather. The relative humidity determinations were made with a hygrodeik each day at noon. The house was heated by a hot-air furnace arranged to take its air supply from the outside.

The air supply is recorded under Cold-air intake. The furnace was provided with a water pan for humidifying the air supply. The amount of water evaporated each day is recorded in the column headed Evap. in 24 hours. The outside temperature ranged from - 12°F. to - 21°F. The weather was clear and calm except the last day, Jan. 12, which was windy. The higher humidity on that day was no doubt due to the greater amount of heat required from the furnace and the consequent evaporation of the water from the water pan.

The humidity determinations made by a hydrodeik, as before explained, are only approximately correct but sufficiently exact for practical purposes. The temperature is given in degrees Fahrenheit.

In the table it will be noticed that the outside air was used only a part of the time because of the severity of the weather. Attention is called to the quantity of water required to keep the humidity at the amount shown. This averages 27 1/2 quarts per day. At the time these observations were made the physics lecture-room at the North Dakota Agricultural College averaged 18 to 20 per cent. saturation during class hours, with observations made from a similar instrument. This is a steam-heated room with only accidental means of adding water to the air. The result was an atmosphere 3 1/2 per cent. above that of Death Valley.

The amounts of water evaporated may seem large to those who are unaccustomed to quantitatively consider problems in ventilation but the small amount of water in the air at -21° must produce a very dry atmosphere when it is raised to 70° in temperature.

The amount of moisture in air at 20°F,. and at 80 per cent. humidity is only 1.58 grains to the cubic foot. If this air is now raised to 70° the moisture will still be 1.58 grains where there should be 4 grains of water to make 50 per cent. humidity. It therefore will require the addition of practically 2.42 grains of water for each cubic foot of entering air in order to bring it up to 50 per cent. humidity.

In a case with the above conditions of atmosphere, suppose it is desired to know the amount of water that would be taken up in humidifying the air for a school-room of size to accommodate 40 pupils. The prescribed quantity of air for this purpose is 30 cubic feet per minute for each pupil. The air is to be maintained at a humidity 50 per cent. saturated. The problem will be one of simple arithmetic. If each pupil is to receive 30 cubic feet of air per minute or 1800 cubic feet per hour, the 40 pupils receiving 1800 cubic feet per hour will require 40 X 1800 = 72,000 cubic feet of air per hour. To each cubic foot of the air is to be added 2.74 grains of water, 72,000 X 2.42 = 164,240 grains of water. Reducing this to pounds, 164,240 ÷ 7000 = 23.46 pounds or 2.77 gallons of water per hour.

In practice the room will show a higher amount than 50 per cent. humidity with this addition of the amount of water, because of the water vapor that is exhaled from the lungs of the pupils. That a considerable amount of water vapor is added to the atmosphere by breath exhalation is made evident from the moisture condensed by breathing on a cold pane of glass. In any unven-tilated room occupied by a considerable number of people the humidity is thus increased a very noticeable amount.

The change in humidity of the air in a closed room filled with people is very pronounced. The constant exhalation of moisture from the lungs is sufficient to saturate the air in a short time. The heavy atmosphere of overcrowded, unventilated rooms is due to moisture exhalation, body odors and increased carbonic acid gas. As the humidity of the atmosphere is increased a sensation of uncomfortable warmth is the result of the lesser evaporation.