24 en EM IS TR V FOk A GRICUL TURAL STUDENTS 



into each other independently of their weight. If an inverted jar of coal 

 gas, which is lighter than air, be placed over a jar of carbonic acid gas, 

 heavier than air, the gases will be found in a few minutes' time completely 

 mixed, as shown by the inflammability of the gas in each jar, and the 

 milkiness it produces in lime water. It is due to this diffusion of ^ases 

 that the carbonic acid gas does not settle on the floor of a room, and that, 

 if apertures are provided, complete admixture with the outer air gradually 

 takes place. 



(2) Diffusion takes place through porous materials ; the rate of diffusion 

 of two gases is inversely as the square roots of their relative weights. This 

 is best illustrated by the apparatus figured. A porous earthenware cell is 

 fitted with an india-rubber cork and glass U tube half 

 filled with a coloured liquid. A bell jar filled with 

 O coal gas is placed over the cell. The lighter coal gas 



cO /* <v enters the cell more quickly than the heavier air escapes, 



and, in consequence of increased pressure in the cell, the' 

 red liquid is depressed. On now removing the bell jar, 

 the coal gas escapes from the cell more quickly than the 

 air enters, and, in consequence of the diminished pressure 

 in the cell, the red liquid rises. Bricks, mortar, plaster, 

 CQ '^ II =^ etc. , are porous ; iron and wood are non-porous. Brick 



walls provide better ventilation than walls of wood or 

 corrugated iron, and whitewashed walls belter than 

 painted walls. The effect of diffusion is often well seen 

 on a kitchen ceiling, where blackened strips between 

 the beams show where air has diffused through, and 

 black particles of soot and dust filtered out. 



(3) Gases expand by heat, consequently become 

 lighter and rise ; air currents are thus produced. The 

 best illustration of the expansion of gases by heat is 

 the air thermometer, the movement of the liquid in the stem indicating 

 expansion or contraction of the air in the bulb, due to changes of tem- 

 perature. The air current thus 

 ^^ produced may be illustrated by an 

 aspirating bottle in which a candle 

 is burning. The candle will shortly 

 Fig. 10. ^^ extinguished unless the cork 



from the lower aperture is opened, 



•in^ v.»n»Jio.;«. Tu J ^^"^ ^''''"S rise to an air current 



and^cntl!at.on The modern contrivances for the ventilation of rooms 



^A 



Fig. 9. 



