
Tonisation of Air under Normal Conditions. 681 
pressures and corresponding dimensions of vessels for different 
gases (according 10.5 ae -—_), 
a 
Also the thermic variability of conductivity—not yet known 
with desirable precision—may be investigated in an unalogous 
manner, by application of similar motions. If we make use, 
for the higher temperatures, of vessels with dimensions 
increased in proportion of the first, and of pressure increased 
in proportion of the (e— )th power of temperature, the 
quantity of heat transferred must be proportional to 6, 
whence e may be determined. The method of heating wires 
by electric currents may be easily adapted to this way of 
experimenting. 
We confine ourselves to these few examples on this sort of 
similarity, since its range of applications is less extensive and 
since there is little experimental work hitherto done which 
could serve as a basis for further speculations. 


LXXV. The Condensation Method of Demonstrating the Ion- 
sation of Air under Normal Conditions. By C. T. R. 
Witson, J/.A4., F.R.S., Fellow of Sidney Sussex College, 
Cambridge*. 
OME years ago I described experiments + which proved 
that when air saturated with water-vapour has been freed 
from dust particles, it will still give condensation in the form 
of drops on sudden expansion provided the expansion exceeds 
a definite limit. If v, v. be the volume of the air before and 
after the sudden See then if v./v, be less than 1:25 no 
drops are produced on expansion, but if this critical expansion 
be exceeded a rainlike condensation results. The drops remain 
comparatively few if v/v, does not exceed a second limit 
about 1°38. [t was found that exposure of the air to Réntgen 
or other ionising rays increased enormously the number of 
drops produced by expansions between these limits, the least 
expansion required to cause the formation of drops remaining, 
however, the same. It was concluded that the nuclei giving 
the clouds in air exposed to Rontgen rays are to be identified 
with the ions to which its conducting power under the action 
of the rays is attributed, and that the few drops always pro- 
duced with expansions exceeding the critical value are due to 
ions of the same nature continually being produced even in 
the absence of the rays. 
* Communicated by the Author. 
+ Phil. Trans. vol. clxxxix. p. 265 (1897). 
