THE AIR OF TOWNS. 357 



2,000 gallons of air in twenty-four hours, or breathing out 10 cubic feet 



an hour of air containing 5 per cent of carbonic acid. For the air to 



remain fairly fresh the amount of carbonic acid should not rise above 



0.00 per cent; that is to say, the amount of carbonic acid should not 



increase more than 0.02 per cent, supposing the air to contain originally 



0.04 per cent. How much fresh air will be needed per hour? This may 



5 

 be calculated as follows: __x 16=4,000 cubic feet. 



0.02 



Air can not be renewed more than three or four times per hour with- 

 out producing a perceptible current or, as we should say, causing a draft. 

 It therefore follows that each individual should be allotted at least 



_ = 1000 cubic feet of air space. This renewal of air in closed places 



4 



constitutes a branch of study termed ventilation. I have not time to 



discuss fully this important subject. A whole course of lectures might 



be delivered upon it. All that I can do in the short time at my disposal is 



to indicate the principles which underly it. The replacement of vitiated 



air by fresh air without creating draft is the basis of good ventilation. 



This necessitates a flow of air. This flow of air may be produced by 

 mechanical means — a fan or pump driving in air, exhausting the bad, 

 or doing both simultaneously — or, more frequently in dwelling houses, 

 by the natural currents produced by hot air. 



When air becomes warm it expands. A certain bulk of this air com- 

 pared with an equal bulk of the original air will be lighter. The warm 

 air therefore ascends, colder air replaces it, and a flow of air is thereby 

 produced. 



To show that warm air ascends, a large glass globe open at the top 

 and bottom is supported upon blocks (fig. 11). On introducing a Bun- 

 sen burner at the lower opening a strong upward current of air is pro- 

 duced, which causes a spiral of paper pivoted to the horizontal rod to 

 revolve rapidly. Strips of tissue paper gummed around the edge of 

 the top opening form vertical streamers, also indicating the presence 

 of an air current. Toy fire balloons of tissue paper illustrate this prop- 

 erty of heated air exceedingly well. 



It is for this reason that the warm air, which includes the expired 

 air, finds its way toward the top of a room. It is for this reason also 

 that an open fireplace with a good chimuey produces a current of air, 

 which rushes up the chimney to the extent of 150 to 300 cubic feet per 

 minute. These two effects may be combined to draw off the vitiated 

 air by introducing an opening into the chimney near the ceiling. But 

 although by this means bad air is withdrawn and fresh air enters, the 

 method of ventilation can not be considered wholly satisfactory. In 

 my dining room with a good fire burning, I have found that the air 

 passes up the chimney at the rate of 240 cubic feet a minute with the 

 door open, and 200 cubic feet a minute with the door closed. In 

 the first case the fresh air comes mainly through the open door; in the 



