CHANGE OF STATE LIQUID VAPOUR. 169 



ciably supersaturated with vapour without formation of cloud or fog, 

 that is without condensation. 



To show this a large clean glass globe may be arranged as in Fig. 94. 



The globe G is, to begin with, filled with the air of the room, and 

 contains some water which can be washed round the sides so as to ensure 

 complete saturation of the air. 



The taps ^ t% being turned off, the pump exhausts R. If now < 2 is 

 turned on for a moment, the air in G expands, cools, and becomes super- 

 saturated at the new temperature. The excess of moisture at once 

 condenses in the form of fog which, usually evident enough, is still 

 more evident if a light, such as a candle flame, be viewed through the 

 globe when beautiful diffraction rings may be seen round the flame, the 

 rings being larger the smaller the drops. Then turning on both ^ and 

 < 2 let clean dust-free air be slowly drawn through the cotton-wool filter. 

 After a short time < x and t z may be both turned off, and the air in G 

 be again expanded and cooled by turning on t 2 for a moment. There 

 will now be a much smaller number of dust nuclei, and as the vapour 

 condenses on these alone, the drops are much larger and the fog is less 

 dense. For though the same amount of water may be present as liquid 

 in the air its concentration into larger drops diminishes its surface, and 

 there is much less hindrance to the passage of light. The diffraction 

 rings round the flame at the same time grow less. 



If this process be repeated several times the drops soon become easily 

 visible, like those in a Scotch mist, and move at an appreciable speed 

 downwards in a shower of fine rain. When all the drops have fallen 

 down the air is clear and a fresh small expansion does not produce a 

 cloud or rain at all. 



Aitken (Nature, March 1, 1888, p. 429, and Feb. 27, 1890, p. 394), 

 has devised a " dust counter," an apparatus to count the number of 

 dust particles or, at any rate, condensing nuclei per c.c. in any specimen 

 of air. This consists essentially of a small chamber 1 cm. deep with 

 a glass floor ruled in square millimetres and a glass top through which 

 the glass floor can be seen. The chamber is connected to a pump, and 

 by an inlet either the surrounding air can be introduced or any known 

 proportion of filtered dust-free air. The air in the chamber is kept 

 saturated. In general it is necessary to dilute the air to be tested with 

 a large proportion of clean air to reduce the number of dust particles, 

 and so increase the size of the drops. Suppose, for instance, that nine- 

 tenths of the air is dust-free. If an expansion is suddenly made, drops 

 are formed on the dust particles, and these should be heavy enough to fall 

 at once on to the ruled floor. There they are evident as little specks. 

 The numbers on several square millimetres are counted by the aid of a lens 

 and averaged. Say that the average is 5. Then on a square centimetre 

 there would be 500. This is the total number falling down out of a 

 cubic centimetre. But the air has been diluted to one-tenth its original 

 dustiness. Therefore, in its original condition it contained 5000 dust 

 particles. 



Aitken has examined the air in various localities, and finds that the 

 number of dust particles per c.c. is greatest in rooms, greater in dry 

 town air than in the country, greater at lower altitudes than on moun- 

 tains, and that there is a great diminution after rain. 



