CHANGE OF STATE LIQUID VAPOUR. 171 



cules collide, and with so little energy that they form practically a liquid 

 particle. The radius is so minute that the saturation pressure for the drop 

 is far above the pressure of the vapour round it, and the drop instantly 

 evaporates. But if dust nuclei are present, a liquid layer may form on a 

 nucleus with radius sensibly equal to that of the nucleus, which may be 

 so large that the saturation vapour pressure is only very slightly in 

 excess of the normal pressure, and a very slight supersaturation may 

 make the liquid increase. There is another way in which soluble dust 

 may aid condensation. We shall see later that the saturation vapour 

 pressure over a solution is less than that over a pure liquid, so that 

 a normally saturated space may condense on to a liquid surface if the 

 liquid contains salt in solution. If, for instance, a particle of common 

 salt is floating in vapour-laden air, any accidental formation of a liquid 

 particle on it may lead to solution and the formation of a drop with 

 lowered vapour pressure, which may tend to grow even if the space is 

 not quite saturated for a normal plane surface of pure water. Aitken 

 believes that such condensation frequently occurs and largely accounts 

 for the haze in air. 



The conditions of condensation of water-vapour in air and other gases 

 have been very carefully investigated by 0. T. R. Wilson (Phil. Trans., 

 A., 1897, p. 265, and A., 1899, p. 403). It is sufficient here to consider 

 the case of air. He found that if all dust be filtered out, then the 

 saturated air at any temperature about 20 may be suddenly expanded in 

 the ratio 1 : 1'25 without condensation. But that, if the expansion ex- 

 ceeds this, condensation on a few nuclei (not more than a few hundred per 

 c.c.) does occur up to an expansion of 1 : 1'375. After this a dense fog 

 appears, the denser the greater the expansion. Now this means that if 

 we start with air, say ab 20, the sudden adiabatic expansion 1 : 1-25 

 cools it to 6, when the density of the vapour saturating the air at 20 is 

 about 4*2 times the density of the vapour saturating the air at - 6. The 

 expansion 1 : T375 cools it to 16, when the density is about 7*9 times 

 the density of vapour saturating the air at - 16. Then Wilson's experi- 

 ments show that there are always nuclei present in small numbers 

 sufficient to condense the vapour when it has about four times its 

 normal saturation density, and in large numbers if it has about eight 

 times its normal saturation density. We can hardly suppose that the 

 nuclei are of foreign matter. Wilson shows that drops of radius of the 

 order 10~ 8 cm. would, by the formula (2) above, be in equilibrium at the 

 8-fold density, and such drops would contain but few molecules according 

 to the calculations of molecular dimensions in chap. ix. We may 

 suppose, then, that in the molecular collisions water particles of such 

 order are continually being formed, and if the vapour density is 8 times 

 the normal they will tend to grow instead of evaporating. As to the 

 formation at the 4-fold density, Wilson supposes that there is chemical 

 action occurring ; for instance, in some of the collisions the oxygen, 

 nitrogen, and water might combine to form nitric acid. The vapour 

 at 4-fold its normal saturating density might be saturated for even the 

 minutest particle of acid formed by collision, and so the particle would 

 grow. Such combination may always be occurring to some slight extent, 

 the compounds being made and unmade in successive collisions. 



Wilson also found that ultra-violet light is active in producing con- 



