454 
MR. T. H. LABY ON THE SUPERSATURATION AND 
or to make a small expansion and so condense larger drops on these nuclei left over 
from the previous larger expansion. 
Most of the vapours examined, especially the esters, gave only a few drops with 
the least expansion producing condensation, and on slightly increasing the expansion 
the number of drops did not increase as rapidly as they do in the case of water vapour. 
This makes the least expansion for condensation a less precise point with these 
vapours than it is with water and substances which behave like it in this respect. 
Usually more than one series of experiments were made with each vapour. 
Examples of the nature of the results obtained are given below, where the different 
series of observations are combined together. 
Air and Ethyl Propionate. Rontgen Rays. 
No drops: R390, D409. 
Drops: U404, R413, 1'426, 1-436, 1-456, 1'477, D497. 
The apparatus was cleaned and refilled. 
No drops: D314, 1-400, 1-400, D407, 1-408, D413, 1-418. 
Drops: D416 (2 drops seen), 1-420, 1*422. 
The first gives the condensation expansion as 1"404, the second as 1"416. 
Mean 1-410. 
Air and Acetic Acid. Rontgen Rays. 
No drops: 1-414, 1'431, 1-439. 
With drops: 1-441, 1-446, 1*454, 1-454, 1*497 (fog), D507 (fog). 
Least expansion for condensation 1-441. 
The drops obtained in these expansions are very small. With feeble Rontgen rays 
a larger expansion than 1"441 was necessary. These feeble rays were from the same 
bulb, but the alternative spark gap was 5 cms. instead of 10 cms., and an aluminium 
screen 2 ‘7 mm. thick was placed between the bulb and the expansion chamber. 
Feeble Rontgen Rays. 
No drops : D446, D446, 1-454, 1*471. 
With drops: R460, 1 471, 1-474, 1 *479, l - 482 (many drops). 
Least expansion for condensation 1-471. 
This is distinctly larger than the expansion with the more intense rays. 
This matter is being further investigated by using y-rays from radium and from 
thorium, but the results so far obtained have often been contradictory. 
Of the substances tried, acetic acid showed this effect of the intensity of the 
Rontgen rays on the least expansion for condensation the most. 
