302. =Air as a Non-Conductor.=--Heat is rapidly diffused in air by convection; but it is only when the air is free that this can be done. When the air is confined in spaces or pores, or among fibres, heat makes its way through it very slowly, for it can be diffused through it then only by conduction. The variety of ways in which air is of service to us as a non-conductor is almost endless. I will notice some of them. 303. =Double Windows.=--The efficacy of double windows depends upon the confined air between them. In the case of the single window a great deal of the heat inside is lost in this way: The warm air of the room which comes in contact with the window imparts to it some of its heat, and, being thus cooled and therefore condensed, passes downward. As this process goes on continually this downward current by the window is constant. The current outside is in the opposite direction. The heat imparted to the window is taken up by the cold air, and as it thus becomes warmer it passes upward. And this upward current outside is as constant as the downward current inside. Now nearly all this is prevented by the non-conducting quality of confined air in the case of double windows. If a pane were taken out from the upper part of the inner window, and another from its lower part, the inner window would be of little use, for then the heat of the air in the room would be continually diminished by convection, as when the window is single. The warm air would pass in at the upper opening, and, being cooled, pass down through the lower one.[5] 304. =Air as a Non-Conductor in the Walls of Buildings.=--The spaces included between the outer wall of a building and the plastering inside being filled with confined air, prevent the heat of the air in the apartments from passing off readily through the wall. A house built of brick or stone, with the plastering placed directly upon the inside of the wall, would be kept warm with difficulty in winter, because the solid wall would so readily conduct off the heat to the external air. So, also, such a house would be very warm in summer, because the heat of the sun and of the external air would be so rapidly communicated to the air of the house. In this connection I will mention a contrivance to prevent the spreading of fires in blocks of buildings, which, though very effectual, is seldom made use of, partly because it occasions some trouble and expense, and partly because it takes up a little room. It is this: A small space is left in the division wall between each two houses from top to bottom, containing, of course, a body of confined air, that is, if the space be entirely shut in, which is as essential here as in the case of the double windows. With such an arrangement the interior of one house may be entirely consumed without communicating sufficient heat through the confined air to set on fire the other.