A6S Dr. G. Owen and Mr. H. Pealing on Condensation 



A large number of observations have since been made 

 (a) when the iodine vapour was placed inside the cloud- 

 chamber itself ; (6) when iodine vapour was swept through 

 glass-wool before admission into the apparatus. It will bo 

 convenient to discuss the two cases separately, although, as 

 will be shown, they are probably intimately connected. 



Experiments with Iodine placed in the Cloud-Chamber itself. 



When studying the effect of iodine placed inside the cloud- 

 chamber, the apparatus shown in fig. 2 was found convenient. 



Fig. 2. 



The apparatus could be filled either by filtered laboratory 

 air drawn in through A or by air from boiling liquid air 

 stored in B. As a matter of fact, both these sources of dust- 

 free air gave practically the same results. The apparatus 

 was first calibrated with pure air and then iodine vapour was 

 introduced by placing a few crystals in the hollow tap C. 

 The taps D, E allowed fresh distilled water to be admitted 

 when desired. In order to study the effect of light, the 

 cloud-chamber and expansion apparatus were wrapped in 

 black cloth. The covering of the cloud-chamber was pro- 

 vided with two vertical slits, one on each side ; the one slit 



