474 Dr. G. Owen and Mr. H. Pealing on Condensation 



o 



From a consideration of the above we are led to regard as 

 follows the disappearance of the clouds and their reappear- 

 ance on washing the apparatus. As will be seen later on, 

 the nuclei probably result from the production of some 

 substance which is deposited on the walls of the apparatus, and 

 it is also likely that the action ceases when a certain amount 

 of this substance has been formed. The flow of water over 

 the glass would remove this deposit and allow the action to 

 proceed as before. 



The view that the glass plays a part in the production of 

 the clouds is supported by the results of experiments made 

 with glass-wool. A passing reference to these experiments 

 has a'ready been made. We shall now consider them in 

 greater detail. 



Experiments ivith Glass-wool. 



It has already been stated that when the iodine has 

 been in the apparatus for a few days the coloured clouds 

 obtained for an expansion corresponding to a pressure-drop 

 of 18*5 cms. entirely disappear, and that the introduction of 

 a number of foreign substances into the apparatus failed to 

 bring the clouds back. We found, however, that the intro- 

 duction of a plug of plass-ivool gave rise to a cloud at a 

 pressure-fall of 18*5. But after the wool had been in the 

 apparatus for some twenty- four hours, the effect had again 

 disappeared. Evidently, then, something was introduced 

 with the glass-wool which for a time facilitated the production 

 of nuclei. In order to ascertain whether this was accidental 

 or whether it was a general property of glass-wool, we 

 obtained through Messrs. J. H. and S. Johnson, of Liverpool, 

 three different lots of pure glass-wool, guaranteed to have 

 been supplied to them by three different makers. We found, 

 however, that plugs of each specimen when placed in the 

 apparatus brought back the clouds. Clearly, then, the 

 property is general. 



But the most effective way of making evident this action 

 of the glass-wool is shown in fig. 3. 



If a slow stream of air be drawn into the cloud-chamber 

 through this arrangement, a dense fog is obtained for a 

 pressure-drop of 18*5. Further, when the glass-wool has 

 become coloured by the iodine along its whole length, clouds 

 are obtained for much smaller expansions. Referring to 

 Table III., it is seen, too, that illumination for so short an 

 interval as one second is sufficient to produce the nuclei 

 caught with a pressure-drop of 18"5. On the other hand, we 

 found that to produce the large nuclei caught by small 



