VENTILATION. 



269 



Expt. 109, when increased in volume by the passage inwards of 

 hydrogen). On allowing the flask to cool again, the atmospheric 

 pressure will force the water up the tube, where it will stand at a 

 height above that in the bottle, depending on the difference 

 between the temperature to which the flask was heated and that to 

 which it has subsequently cooled ; as the temperature of the room 

 alters, so will the level of the water rise or fall, somewhat like the 

 quicksilver in an ordinary mercurial thermometer. Fig. 124 

 represents a more neatly made form of air thermo- 

 meter, where the flask and quill tube are replaced by 

 a large bulb blown on a piece of stout glass tubing, 

 the whole being mounted in a flask containing some 

 coloured liquid by passing the tube through a hole in 

 a cork, taking care that this does not fit air tight into 

 the flask. A scale for reading off variations in the 

 level of the liquid in the stem is also provided. 



Expt. 318. Ascending Currents of Hot Air. An 

 important result follows from the fact that gases ex- 

 pand largely in bulk when heated ; viz., that if they 

 are not confined in an inexpansible envelope (such as 

 a bladder when fully distended) they become lighter, 

 bulk for bulk, than previously. A soap bubble blown 

 with warm air from the lungs consequently ascends, 

 notwithstanding that the soap water itself is heavier 

 than air, and that the expired breath is also heavier 

 than air would be at the same temperature, on 

 account of its containing more carbon dioxide from 

 the chemical changes going on in the lungs (Expt. 

 153) : this extra weight is more than compensated 

 for by the increased bulk, and consequently su- 

 perior lightness, communicated to the breath in virtue 

 of its warmth as compared w T ith the surrounding air ; consequently 

 the bubble ascends, just as one filled with hydrogen or coal gas 

 would do (Expt. 307), or as a cork held at the bottom of a tumbler 

 of water would rise to the surface if let go. Hence, over any 

 heated object an ascending current of warm air is necessarily set up, 

 because the air in contact with the hot body becomes warmed and 

 expands, and consequently rises, cooler air flowing in to supply its 

 place. 



Expt. 150 illustrates this effect; here the source of heat is 

 the chemical action taking place between a lighted candle and 

 the air in which it burns ; ventilating appliances, where foul air 

 is removed and fresh supplied, mostly work on this principle, the 

 hot vitiated air being removed in virtue of its tendency to ascend 



Fig. 124. 

 Air Ther- 

 mometer. 



