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LXVII. On Molecular Aggregations produced in Gases by 
Sudden Cooling. By Gwilym Owen, M.A., M.Sc, 
Assistant Lecturer and Demonstrator in Physics, and 
A. Ll. Hughes, B.Sc. } Oliver Lodge Fellow, University 
of Liverpool* '. 
IN the Phil. Mag. for Oct. 1907 we described some experi- 
ments which showed that certain gases after passing 
through a process of severe cooling contained large numbers 
of nuclei, the presence of which was shown by their ability 
to act as centres for the condensation of supersaturated water 
vapour. The following paper is an account of the continua- 
tion of those experiments. 
I. The Conditions Governing the Production 
or the Nuclei. 
In the paper referred to, it was shown that the gas had to 
be cooled below a certain temperature (which we called the 
" critical temperature ") before the nuclei were produced. 
Since the term " critical temperature " has another meaning 
in Physics, it may be advisable to avoid using this term in 
future ; we shall therefore throughout this paper employ the 
term " nucleating temperature" instead. 
As stated in the previous paper, the number of nuclei 
produced depends upon the pressure of the gas and the 
temperature to which it is cooled. We have since found that 
the effect depends also upon the rate at which the gas is cooled. 
In fact the predominating factor in the formation of the 
nuclei appears to be suddenness of coding. 
It may be well here to describe the modification of the 
original apparatus which enabled the cooling of the gas to 
take place very rapidly. Figure 1 represents the original 
type of apparatus used. In performing an experiment, the 
bend C (which we call the " tester ") was surrounded by a 
cold liquid and as the temperature of the gas fell, the pressure 
was kept constant at any desired value by running the 
mercury up in the reservoir B. Fig. 2 represents the new 
form of apparatus designed to allow the gas to be very 
suddenly cooled. As is seen from the diagram the tester X 
is made in the form of a spiral. A glass tap M is introduced 
between the tester and the reservoir B, while the tap D is 
replaced by a three-way tap D'. This new form of apparatus 
was used as follows : — With M closed, the tester is first 
* Communicated by the Authors. 
