446 
ME. T. H. LABY ON THE SUPERSATURATION AND 
With a large apparatus (with working parts of brass) Wilson* some time later 
showed, in the absence of (artificial) external ionising rays and of an electric field, 
that rain-like condensation took place in air saturated with water vapour at an 
expansion of 1*247, while with an electric field no drops were produced even by an 
expansion of 1‘27. Thus the natural ions act as condensation nuclei for expansions 
of 1 '25. The number of natural ions per cubic centimetre deduced from the number 
of drops is in agreement with the number obtained by other methods. 
The simplest interpretation of Wilson’s experiments is that there are three kinds 
of nuclei concerned in condensation. Nuclei of one kind are naturally present in dust- 
free air, oxygen, nitrogen (hydrogen ?), carbon dioxide, and chlorine; they are 
increased in numbers by Rontgen and Becquerel rays, ultra-violet light acting on a 
negatively charged zinc plate, and they are electrically charged and produce 
condensation in water vapour when it is supersaturated at least 4'3 times; in short, 
these nuclei are ions. Another type of nuclei are uncharged and are possibly 
aggregates of water molecules, as mentioned above. A third kind are present near the 
surface of zinc, lead, and amalgamated zinc plates, are uncharged, and act as nuclei 
when the supersaturation is not less than 4'3. 
The Comparative Efficiency of Positive and Negative Ions. 
WiLSONf showed in 1899, using air and water vapour, that the negative ions are 
more efficient in producing condensation than the positive, for he found that 
expansions greater than 1*247 caught the negative ions, while both the positive and 
negative were caught by expansions greater than 1-31. It is to be noticed that the 
second point (1*31) cannot be determined as readily or with the same accuracy as the 
first one (1*247). 
Previous Work on Organic Vapours. —F. G. Donnan j determined the expansion 
necessary to produce rain-like condensation, and a fog in dust-free air, saturated with 
the vapours of some carefully purified organic liquids. 
K. Przibram § has made observations on the condensation of organic vapours in 
ionised air. He has followed Wilson in carrying out his experiments. The piston of 
his expansion apparatus worked in paraffin oil, which would seem undesirable when 
using vapours soluble in that oil. 
Experimental: The Expansion Apparatus. 
Two modifications of the expansion apparatus have been tried. It was not easy to 
* ‘Phil. Mag.,’ June, 1904, p. 681. 
t ‘Phil. Trans.,’ A, 193, pp. 289-308 (1900). 
t Mag.,’ March, 1902, p. 305. 
§ ‘ Sitzungsber. cl. kais. Akacl. cl. Wissen. in Wien,’ Math.-naturw. KL, Band 115 (1906) 
