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nauseous that people have imagined the results are due to the want of moisture in the air. The fact is, however, that if hot water pipes are employed, and strongly heated iron surfaces are discarded, very little will be heard of the necessity to add moisture to the warmed air. In dwelling houses where there is furnace or other heating besides firegrates, the importance of adding moisture is more pronounced, especially if a person is suffering from throat or lung affections, but dwelling houses are not under consideration.

A considerable proportion of the moisture in the air of public buildings comes from the illuminants employed, especially from coal gas. About one half of the volume of coal gas consists of free hydrogen, whilst that in the combined state as marsh gas, etc., when set free, will occupy a space approximately equal to the total volume of the coal gas— in other words, there is sufficient hydrogen free and combined in coal gas, if it all was in the uncombined state, to yield a volume about one and a half times as great as the coal gas itself. When this one and a half volume combines with three-quarters of a volume of oxygen, it forms water vapour, and the 2\ volumes contract in bulk until they occupy \\ volume, i.e., the same space as the free hydrogen occupied. Every three jets of gas burning for one hour, and each consuming five cubic feet, will yield twice as much vapour to a thousand feet of air as was probably originally contained in it, and so may be the means of saturating the thousand cubic feet even at the increased temperature. It is this moisture which adds to the oppressive, close and clammy feelings experienced after the gas has been lit some time, and the carbon products of combustion are even less to blame.

In churches, chapels, public halls and theatres, where the gas jets are distributed over the interior of the buildings, and the heating and ventilation have received so little attention that no warm fresh air has been admitted near the floor level, the same conditions occur, and the products of gas combustion, and of breath, circulate and remix until coming in contact with the walls, windows and other cold surfaces, some of the moisture is precipitated in the form of very fine mist, and the decomposition of the organic matters exhaled is so rapid, and the activity and multiplication of the bacteria so stimulated, that a person coming in from the fresh air gets a sufficiently strong experience of what foul and foetid air is like.

It may be urged that owing to the increased temperature of the confined atmosphere the capacity of the air to absorb moisture would also be increased considerably; and, as this is true, it will be well to inquire and define the extent. The temperature of the air in a public hall just before the gas was lit and the audience assembled was 59o F., and it had increased to 72o F. one hour afterwards. The tension of aqueous vapour, or the capacity of the air to absorb aqueous vapour by being raised in temperature from 59° to 72o F. would only be increased by one-third, whilst the saturated breath at about 97o F., and the vapour from the gas consumed, would saturate a very much larger volume at the increased temperature of 72o F. For example, the hall holds 1,ooo adults, who exhale one-twelfth of a pound of" moisture per head per hour at nearly 97o F., eighty-three and one-third pounds of water are therefore evolved from the skin and in the breath by 1,ooo persons, and this water will saturate 1oo,ooo cubic feet of air at 72o F., calculating that the air as it entered the building contained moisture equivalent to half saturation at that temperature (72o F.).

One thousand cubic feet of gas are consumed every hour, and the moisture so formed would saturate nearly 1oo,ooo cubic feet of air, so that about 2oo,ooo cubic feet of air will be fully saturated from all sources. One thousand cubic feet of average coal-gas require 6,ooo cubic feet of air to yield enough oxygen for combustion, but as only a portion of the oxygen would be used by the flame, probably 3o,ooo cubic feet of air took part in the combustion of the gas, and this, together with 2o,ooo cubic feet of air exhaled from the lungs, make 5o,ooo cubic feet, which saturate about 2oo,ooo cubic feet, or approximately four times the volume.

These figures are not exact because the facts relating to breath and to coal-gas cannot be determined with certainty, but they are not overstated. The quantity of water exhaled by the skin and in the breath is said to vary from 25 to 4o ounces in the twenty-four hours, and there is much difference between the composition of gas made from cannel coal and that made from the ordinary bituminous varieties.

The figures show very plainly how inadvisable it is to use naked coal-gas flames to heat churches and public buildings in cold weather, as the moisture condenses on the walls and cooler surfaces, and when exhaled air is present, organic products are condensed also, and give a stale, musty, unhealthy odour, which hangs about for a long time. Committee rooms, mission halls, school and class rooms are frequently heated in this manner, and, after a far too numerous audience has assembled, every inlet for fresh air is carefully closed, and the stinking organic products reinhaled into the lungs must seriously affect weak constitutions, and, strange as the fact is, it is the weaker portion of the community who are the most devoted mission and school workers.

When air is supersaturated with moisture it becomes heavier and impedes ventilation most seriously, as condensation occurs in the cracks and interstices through which the air finds exit, and stagnation of the atmosphere results. This is frequently the state of .things in unventilated buildings when the temperature of the outside air is low, and its condition damp and foggy.

Nature has acted wisely, however, in the matter of moisture in the air. The fact that water vapour is so much lighter than air, tends towards ventilation, and as the vapour in breath is exhaled generally at about 97o F., the high temperature also assists in causing the expired air to rise upwards. The nearer the temperature of the outer atmosphere rises to that of the body, the more difficult it is to cause much upward movement of the air in a building during the summer time, and, were it not for the fact that moisture is lighter than air, and that the moisture in breath is evolved at a high temperature, there would be no ventilating force at hand to cause the foul air to be removed. The ventilating power due to breath, although appreciable and valuable in the summer, is, notwithstanding, small in amount, and every means should be used to cause the circulation of air within a building by taking advantage of wind currents and open windows. This matter, however, will be further mentioned in the next chapter.

CHAPTER IV.

AIR, AIR INLETS AND OUTLETS.

In this treatise the' composition of air, the nature of the impurities common to the atmosphere, and theoretical considerations of the sciences bearing upon ventilation, are omitted, because such information can be got without difficulty from various sources. Neither ventilating engineers nor the caretakers of present buildings need worry much over the impurities in the atmosphere outside the building, but direct their energies almost entirely to keeping that inside the structure in a breathable condition.

No pains should be spared, however, to see that the air which feeds a building is not contaminated by sewer gas or such like impurities, and, once this has been ascertained satisfactorily, the only thing which need concern the caretakers of present buildings is to see that all the air possible shall be introduced, even to the point of causing dissatisfaction to some who frequent the building. With churches and public halls, what the caretaker should ascertain is not how much air is required, theoretically, to keep the percentage of carbonic acid below a certain point, but how much it is possible, under the existing arrangements, to introduce without causing unpleasant and injurious draughts. He need not trouble whether Parkes was right that 3,ooo cubic feet per head per hour was the proper quantity to keep the atmosphere breathable, or whether Carnelly, Haldane, and Anderson were nearer the mark with 1,ooo cubic feet per head, because there

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