468 
MR. T. H. LABY ON THE SUPERSATURATION AND 
With the substances used by the writer in every case a larger expansion (as is shown 
in the following table) was necessary to cause condensation in air not acted on by 
Hontgen rays than in air so acted on. 
Least Expansion for Condensation. 
Vapour. 
Without rays. 
With Rontgen rays. 
Difference. 
Ethyl acetate. 
1-528 
1-480 
0-048 
Methyl butyrate. 
1-364 
1-334 
0-030 
Methyl iso-butyrate. 
1-387 
1-347 
0-040 
Propyl acetate. 
1-334 
1-310 
0-024 
Ethyl propionate .. 
1-438 
1-410 
0-028 
Formic acid. 
1-865 
1-782 
0-083 
Acetic acid. 
1-508 
1-441 
0-067 
Propionic acid. 
1-369 
1-343 
0-026 
w-Butyric acid. 
1-463 
1-380 
0-083 
Iso-butyric acid. 
1-403 
1-360 
0-043 
Iso-valeric acid. 
1-256 
1-220 
0-036 
Iso-amyl alcohol. 
1-210 
1-182 
0-028 
Water. 
1-256 
1-236 
0-020 
Natural and Rontgen-Ray Nuclei. 
The differences between the expansions necessary to catch natural and Ttontgen-ray 
nuclei arises partly probably from the size of the expansion chamber. We have seen 
(p. 445) that, by using a large apparatus, Wilson found for air and water vapour that 
the expansion for the natural nuclei was the same (1*247) as for Rontgen-ray ionic 
nuclei. Thus the differences in the table above must arise partly from the size of the 
apparatus. In the experiments made by the writer to determine the natural nuclei 
point in aqueously saturated air only a few drops were seen with the smaller expansions 
(1*256 one drop, 1*258 five or six drops, 1*260 one). Further, it was found that a 
drop may be formed and not be seen if the observer’s eye has been exposed to anything 
but feeble light immediately before or during the experiment. Thus the small 
number* of natural nuclei (and consequently of drops) which are dealt with in these 
experiments lead to a value for the expansion required to catch the natural nuclei 
which is too large. This difficulty does not arise with the far more numerous Ilontgen- 
ray nuclei. While it is thought that the above factors may account for the differences 
of 0*02 in the last table, they can scarcely account for the larger differences for formic 
(0*08), acetic (0*07), and butyric acids (0*08). These differences are very probably 
* The natural ionisation in hydrogen in a closed vessel is only O' 18 of that of air [WILSON, ‘ Proc. Roy. 
Soc.,’ 69, p. 277 (1901)], and consequently Wilson did not observe condensation on the natural nuclei 
