536 Messrs. Owen and Hughes on Condensation Nuclei 



<-> 



(2) Due to traces of water-vapour in the gas. — The im- 

 portance o£ thoroughly drying the gas before admitting it 

 into the experimental tube is obvious. Any water-vapour 

 present would be deposited on the cooled walls of the tube 

 in the form of minute particles of ice. On the gas warming 

 up again, these might become detached and float about in the 

 gas in the form of small drops of water, and, consequently, 

 if they did not evaporate (which, as a matter of fact, we 

 might reasonably expect them to do, the gas in our experi- 

 ments being very dry) they would act as condensation nuclei 

 on admission into the cloud chamber. 



Against this view that the nuclei are due primarily to 

 traces of water-vapour in the gas, there are several pieces of 

 evidence both direct and indirect. 



(a) It has already been suggested that any drops of water 

 formed as described above would evaporate on the gas warming 

 up again. To help these supposed droplets to evaporate, 

 on several occasions we surrounded the U-tube with hot 

 glycerine at 120° C. This treatment was ineffectual in 

 removing them. 



(b) All methods designed to dry the gas before its admission 

 into the apparatus failed to prevent the formation of the 

 nuclei at the temperatures already given. In the case of 

 oxygen and air the gases were derived from boiling liquid 

 oxygen and liquid air. The oxygen was dried before it was 

 liquefied as has been described. The liquid air was poured 

 into the reservoir through two layers of filter-paper in 

 order to remove small particles of ice which necessarily fall 

 into it from the mouth of the Dewar flask in which it was 

 contained. Thus there could hardly have been any appre- 

 ciable amount of ice mixed with the liquid gases ; and even 

 if there were, its partial vapour-pressure at that temperature 

 would be exceedingly small. (This applies equally to any 

 traces of C0 2 snow in the liquid gases.) 



(c) The air from the room was on one occasion treated as 

 follows : — It was drawn through calcium chloride, solid KOH, 

 over a long layer of P 2 ^5> an( ^ finally through a spiral 

 immersed in liquid air containing 17 feet of copper tubing 

 of 1 millim. bore, the last foot being of wider bore packed 

 with cotton -wool ; yet after this drastic treatment the air 

 behaved no differently from what has been described above. 



(d) Indirect evidence against the " moisture " explanation 

 is afforded by the fact that the " critical temperature " at 

 which the production of the nuclei can be first detected 

 depends so markedly upon the nature of the gas. We 

 might reasonably suppose all the gases tried to contain more 



