SCHOOL-ROOM VENTILATION, 537 



are derived, tlie means of effecting their removal therefrom will next 

 be discussed. 



The chief factors in carrying on ventilation are (a) the difference 

 in temperature between the outside air and that within the room, and 

 {h) the diffusibility of gases. 



It is the difference in temperature that produces a draught up a 

 flue or chimney when a fire is lighted below ; for the products of com- 

 bustion have a very much higher temj^erature (several hundred degrees 

 Fahr.) than the surrounding atmosphere. Being so much warmer, they 

 are lighter in consequence (as will be explained presently), and there- 

 fore have a constant tendency to ascend being compelled by the force 

 of gravity till, after cooling little by little, they reach a layer of their 

 own temperature. Upon the same principle an inflated balloon ascends 

 and a cork immersed in water constantly tends to rise to the surface. 

 As the coefiicient of expansion for gases equals about -g-l-g i. e., they 

 increase about -^^ of their bulk for every degree centigrade increase in 

 temperature, thus becoming lighter in iw op ortion to their volume., and,^ 

 becoming lighter (some being originally lighter) than the atmosphere, 

 are compelled by gravitation to ascend. It is important that the pipe 

 or flue, in rooms heated by stoves or grates, should be vertical or near- 

 ly so ; also that it be not too wide, otherwise downward currents will 

 be produced, and these interfere with the draught, and cause the gases 

 of combustion to escape into the room. In a stove-pipe the elbows 

 should be as few in number as possible, and rounded rather than acute; 

 for a sharp or abrupt bend materially diminishes the velocity of the 

 draught. Two or more pipes opening into the same chimney should 

 have separate flues ; when they open into the same flue, the pipe that 

 draws best will interfere with the draught in the others, and set up 

 downward currents. 



The air consumed by combustion escapes by the chimney, and 

 tends to create a vacuum in the room ; but it is steadily replaced by 

 the atmosphere which rushes in at every available opening. This rush 

 is* strongest at the lowest openings (those nearest the earth), and here 

 the whole amount enters if the space is suflicient. On the other hand, 

 and for the reasons before given, the warmer (lighter) and fouled air 

 within has a constant tendency to escape at the highest points ; and it 

 is here, therefore, that ventilators should be placed to allow its exit. 

 Thus it is that, when a door is opened, the warmer (foul) air escapes in 

 a current at the top, and the colder (fresh) from the outside rushes in 

 at the bottom. This may be shown by a lighted taper held in these 

 situations the flame in each case taking the direction of the current. 

 When the outside air is the warmer, and per consequence the lighter, 

 as on a very warm summer day, the direction of the currents, other 

 things being the same, will be reversed the fresh air coming in above, 

 and the cool air within escaping below. But, owing to the large 

 amount of heat radiated from the pupils the normal temperature of 



