The reason of the change was simple. The stratum of air lying up at the ceiling was comparatively cold. The column of heated air from the bodies of the twenty guests, joined to the heat produced by the movements of themselves and the waiters, together with the steam from the viands and respiration, displaced the colder air at the ceiling, and notably that coldest air lying against the surface of the glass. This cold air simply dropped straight down, after the manner of a douche, on candles and heads below. The remedy I advised was the setting up of a current of hotter steam and air from the gas burners, which stopped the cooling effect of the glass, and created a stratum of heated steam and air in slow movement all over the ceiling. The effect was a comfortable sensation of warmth and entire absence of draught all round the table. Later on, to avoid the possibility of overheating the room, the gas was put out, and the electric lights left to themselves. But before we left, the chilliness and draughts began to be again felt.
The incident here narrated occurred at the end of the month of April last, when we might reasonably have hoped to have tolerably warm nights. It is therefore clear that in this instance neither electricity nor candles could effectually replace gas for lighting purposes. They both did the lighting, but they utterly failed to keep the currents of air steady. I have always remarked draughts whenever I have remained any length of time in rooms where the electric light is used. On a warm evening the electric light and candles would undoubtedly have kept the room cooler than gas, with the same kind of ventilation; I do not think they would have put an end to cold draughts. This the steam from the gas does in all fairly built rooms.
It is a well-known fact that dry air parts with its relatively small amount of specific heat, in an almost incredibly rapid manner, to anything against which it impinges. Steam, on the contrary, from its great specific heat, remains in a heated state for a much longer time than air. It is not so suddenly reduced to a low temperature, and in parting with its own heat it communicates a considerable amount of warmth to those bodies with which it comes in contact. Thus the products of the combustion of gas (which are principally steam) serve a useful purpose in lighting, by keeping at the ceiling level a certain stratum of heated vapor, which holds up, as it were, the carbonic acid and exhalation from the lungs given off by those using the room. The obvious inference, therefore, is that if we take off these products from the level of the ceiling, we shall take off at the same time the impure and vitiated air. On the other hand, if we make use of a system of artificial lighting, which does not produce any steam, then we shall have to adopt means to keep the air at the ceiling level warm, in order to prevent the heated impure air from descending in comparatively rapid currents, after having parted with its heat to the ceiling.
It may very frequently be observed on chilly days that a number of currents of cold air seem to travel about our rooms, although there may be no crevices in the doors and windows sufficient to account for them; and, further, that these currents of cold air are not noticed when the curtains are drawn and the gas is lighted. The reason is that there is generally not enough heat at the ceiling level in a room unlighted with gas to keep these currents steady. Hence the complaints of chilliness which we constantly hear when electric lights are used for the illumination of public buildings. For example, at the annual dinner of the Institution of Civil Engineers, held at the end of April last in the Conservatory of the Horticultural Gardens, the heat from the five hundred guests, and from an almost equal number of waiters and attendants, displaced the cold air from the dome of the roof, and literally poured down on the assembly (who were in evening dress) in a manner to compel many of them to put on overcoats. If the Conservatory had been lighted with gas suspended below the roof, this would not have been the case, because sufficient steam would have been generated to stop these cold douches, and keep them up in the roof.
In fact, if electric lights are to be used in such a building, it will be necessary to lay hot-water pipes in the roof, to keep warm the upper as well as the lower stratum of air, and thus steady the currents.
Having pointed out difficulties which arise under certain conditions of the atmosphere in rooms built with care, to make them comfortable when electric lighting is substituted for gas, I will lay before you some few particulars relative to the condition of small rooms of about 12 ft. by 15 ft. by 10 ft., or any ordinary room such as may be found in the usual run of houses in this country. The cubical contents of such a room equals 1,700 cubic feet. If the room is heated by means of a coal fire, we shall for the greatest part of the year have a quantity of air taken out of it at about 2 feet from the floor by the chimney draught, varying (according to atmospheric conditions and the state of the fire) from 600 to 2,000 or more cubic feet. This quantity of air must, therefore, be admitted by some means or other into the room, or the chimney will, in ordinary parlance, "smoke;" that is, the products of combustion, very largely diluted with fresh air, will not all find their way up the flue with sufficient velocity to overcome the pressure of the heavy cold air at the top of the chimney. If no proper inlets for air are made, this supply to the fire must be kept up from the crevices of the doors and windows.
In the line of these currents of cold air, or "draughts" as they are usually called, it is impossible to experience any comfort - quite the contrary; and colds, rheumatism, and many other serious maladies are brought on through this abundant supply of fresh air in the wrong way and place.
According to General Morin (one of the best authorities on ventilation), 300 cubic feet of air per hour are required for every adult person in ordinary living rooms. Peclet says 250 cubic feet are sufficient; less than this renders the atmosphere stuffy and unhealthy. It is generally admitted that an average adult breathes out from 20 to 30 cubic inches of steam and vitiated air per minute, or, as Dr. Arnott says, a quantity equal in bulk to that of a full-sized orange. This vitiated air and steam is respired at a temperature of 90° Fahr.; and therefore, by reason of this heat, it immediately ascends to the ceiling, together with the heat and carbonic acid given off from the pores of the skin. This fact, by the bye, can be clearly demonstrated by placing a person in the direct rays from a powerful limelight or electric lamp, and thus projecting his shadow sharply on a smooth white surface. It will be observed that from every hair of the head and beard, and every fiber of his clothing, a current of heated air in rapid movement is passing upward toward the ceiling.
These currents appear as white lines on the surface of the wall; the cause probably being that the extreme rarefaction of the air by the heat of the body enables the rays of light to pass through them with less refraction than through the denser and more moist surrounding cold air. An adult makes, on an average, about 15 respirations per minute, and therefore he in every hour renders to the atmosphere of the room in which he is staying from 10 to 15 cubic feet of poisonous air. This rises to the ceiling line, if it is not prevented; and thus vitiates from 100 to 150 cubic feet of air to the extent of 1 per cent, in an hour. General Morin thought that air was not good which contained more than ½ per cent, of air which had been exhaled from the lungs; and when we consider how dangerous to health these exhalations are, we must admit that he was right in his view. Therefore in one hour the 15 foot by 12 foot room is vitiated to more than 2 feet from the ceiling by one person to the extent of ½ per cent., and it will be vitiated by two persons to the extent of 1 per cent, in the same time.