vided the mortar is permeable to air. It has as yet no effect on the flame of this candle, because its velocity is not great enough. But if I fix a funnel on the other end of the cylinder, the air which has passed through the mortar can only escape through its narrow end, and there you see the flame sensibly deviating. You may even succeed in extinguishing it altogether. The velocity of the air in going through the tube must increase in proportion as the transverse section of the
tube is smaller than the mortar-surface, out of which the air escapes, exactly as with the water of the pond and its in and outflow. Now, when I dip the end of one tube into water, you see and hear the air which has passed through the mortar escape from the water. If you make a similar arrangement with a piece of wood, or a brick, you will see the same result.
Most kinds, also, of sandstone are so porous that water and air easily pass through them. Solid or quarried limestones are scarcely permeable to air, but, as they are mostly of irregular shapes, they require more mortar, and that is the reason why such walls are, after all, not so much more air-tight than walls made of regular bricks and thin layers of mortar. Observations have been taken of the average quantity of mortar used with different building-stones. We may suppose that, taking the wall as a whole, it is one-third with quarried lime, one-fourth with tufaceous lime, one-fifth to one-sixth with bricks, one-sixth to one-eighth with cubes of sandstone. In practice, then, the quantity of the mortar rises with the decrease of porosity in the building-stones, and assists in keeping the walls pervious to air to a certain degree.
It is self-evident that the quantity of air which passes through building-materials of a certain thickness must increase in proportion to the surface; two square feet must give passage to twice as much air as one square foot. I shall speak of ventilation in connection with this later on.