756 Messrs. Owen and Hughes on Molecular 
air. Now considering the fact that ethylene itself is in- 
effective in the production of nuclei and the total quantity of 
air in this mixture is small, this is somewhat surprising. This 
may possibly be accounted for by the fact, that when the tap 
between the reservoir and the evacuated tester in liquid air 
is opened, the instantaneous condensation of the ethylene that 
takes place carries the air so rapidly into the tester that the 
latter is subjected to an extremely sudden fall of temperature. 
Water- Vapour in Air and C0 2 . 
In the experiments described in the previous paper we had 
taken great care to obtain the gases quite dry. Since then 
ive have examined the effect in wet gases. 
An apparatus was devised in which the gas after having 
been thoroughly dried could be drawn through either of two 
paths before entering the reservoir and tester. In one path 
was an arrangement by which the gas bubbled through 
water, thus saturating it with water-vapour. With dry air 
the effects started at about —129°. With wet air we 
obtained a very small effect (never more than a "few 
drops ") at temperatures between — 122° and — 129°. At tem- 
peratures below the " nucleating temperatures " the effects 
in dry and wet air were identical. 
With wet C0 2 we obtained a small effect (a " very thin 
shower ") at some five or seven degrees above the condensing- 
point, whereas with dry C0 2 no effects were obtained unless 
condensation occurred, as has been described above. But 
below the condensing temperature the effects in wet and dry 
C0 2 were indistinguishable. 
IV. The Persistency of the Nuclei at Different 
Temperatures. 
Air Nuclei. 
It had often been noticed that when air containing nuclei 
produced by sudden cooling was left over night in the tester, 
no effect was obtained on passing it into the cloud-chamber 
on the following morning, showing that the nuclei had in 
some way or other disappeared. On making a similar expe- 
riment with dusty air from the room there was only a small 
diminution in the effect in the same interval of time. This 
led to a more thorough investigation of the persistency of 
the nuclei. Several forms of apparatus were tried, but the 
