242 Condensation of Vapour in the presence of dust- free Air, fyc, 



like condensation results in all the gases, except hydrogen, in which 

 scarcely any trace of condensation is seen when the supersaturation 

 is even slightly below 7'9. 



A statement of the effect of the Rontgen rays upon condensation 

 in the presence of air was given in a note read before the Royal 

 Society, on March 3, 1896. The rays have the effect of greatly 

 increasing the number of the drops, the minimum expansion required 

 to cause condensation being- unaltered. 



Experiments have been made upon the effect of these rays when 

 hydrogen is substituted for air. Their effect on the moist hydrogen 

 is to introduce nuclei, which only require the supersaturation to 

 reach the same limit as is required for rain-like condensation in air 

 and the other gases, in order that condensation may take place upon 

 them. In all these experiments the X-rays had to pass through 

 glass to reach the gas, and must have been thereby very much 

 reduced in intensity, yet their effect in the case of hydrogen was 

 quite noticeable when the bulb producing them was 120 cm. from the 

 expansion apparatus.' 



The nuclei which bring about the rain-like condensation are equiva- 

 lent, in their power of helping condensation, to water drops of 

 8'6 X 10 -8 cm. in radius, that is, water drops of this size would just 

 be able to grow in vapour supersaturated to the extent actually 

 required to bring about condensation. This number is calculated on 

 the assumption that Boyle's law holds for the supersaturated vapour, 

 and that the surface tension retains its ordinary value even in such 

 small drops. It must, therefore, only be considered as a rough 

 approximation. 



On the same assumptions, the nuclei which are able to act as centres 

 of condensation when the supersaturation is sufficient to cause the 

 cloud-like condensation, are equivalent to drops of 6'4 x 10 -8 cm., or 

 less, in radius. They are present, as we have seen, in all the gases 

 tried, when saturated with aqueous vapour, and are exceedingly 

 numerous. It is probable, therefore, that they are actually small 

 water particles, such as one would expect to come into existence 

 momentarily through encounters of the molecules. 



The nuclei which bring about the rain-like condensation are always 

 few in number, and they appear to be entirely absent in hydrogen ; 

 their number, therefore, depends on the nature of the gas. They are 

 probably, therefore, of a different natnre from those which come 

 into play when the supersaturation is great enough to cause the 

 cloud-like condensation. 



