748 
Messrs. Owen and Hughes on Molecular 
new method, any given experiment can be repeated time 
after time with the certainty of always dealing with the same 
temperature to within half a degree. Consequently the 
results obtained by this method are much more consistent 
than those obtained by the original " slow " method. 
The temperature of the cooled petroleum ether was given 
by a pentane thermometer reading down to the temperature 
of liquid air. The thermometer is correct at 0° and —190° 
.tut is incorrect at intermediate temperatures, having a 
maximum error of 12° at —100°. The temperatures there- 
fore given in the previous paper, especially those for C0 2 , 
require correction. In the present paper corrected tem- 
peratures are given. 
Comparison of tlie Effects obtained in Air by the " Sudden " 
and u Slow " Methods of Cooling. 
The air was drawn from outside through potassium per- 
manganate, soda lime, P 2 5 and a spiral six feet long 
immersed in liquid air and then through a plug of cotton 
wool. 
The following table shows the effects obtained at different 
temperatures by the two methods : — 
Table I. 
(Tester X. Vol. 3 o.c.) 
Temperature. 
"Sudden" Method. 
"Slow" Method. 
-126° C 

Few drops. 
Fair shower. 
Good shower. 
Heavy shower. 
Tinted rain-cloud. 




Fair shower. 
Rain cloud. 
-131° C 
-135° 
-140° 
-145° 
-190° 
The above table illustrates two facts : — The more sudden 
the cooling the higher is the "nucleating temperature" and 
the larger is the number of nuclei produced at any given 
temperature. 
In the course of the experiments it was noticed that the 
values of the " nucleating temperatures " obtained depended 
somewhat upon the shape and size of the tester used. In 
order to investigate this effect systematically, the double 
