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THE POPULAR SCIENCE MONTHLY 



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 ex- 

 tinguishing 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 ai*e scarcely 

 permeable to air, but, as they are mostly of irregular shapes, they re- 

 quire 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 sup- 

 pose 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. 



