McClelland AND M'tlENUY— Z7??cto-^erf iW^efe'. 295 



of the nuclei in moist air. In one experiment room air was filtered and 



C 

 passed over a water surface and exposed to ultra-violet light. ~ was found to 



be nearly 4. On putting about | cc. of alcohol in the water, whose volume 



0- 

 was about 1,000 cos., ^ dropped to the value unity, so that no nuclei were 



formed. 



The action of the alcohol-vapour in preventing the formation of the 

 nuclei might be in either of two ways. It might prevent the formation of 

 the hygroscopic substance (e.g., H2O2) which is the parent of the nucleus, or 

 it might act on the minute water-drops after formation. 



To test this point, nuclei were produced in moist air and then passed over 

 an alcohol surface. After passing over the alcohol surface the nuclei could 

 no longer be detected by our methods, the action of the alcohol-vapour being 

 just the same as if it had been contained in the air before exposure to the 

 ultra-violet light. 



Air saturated with alcohol-vapour was now exposed to intense ultra- 

 violet light in a closed vessel for some time. A narrow beam of intense 

 arc light traversed the vessel. Its path was at first invisible, but when the 

 spark had acted for thirty seconds or so a thin cloud of large drops was seen 

 in the beam. Using water instead of alcohol, and exposing for the same 

 time, a cloud is also formed, but very different in appearance. It is now 

 blue, the arc light appearing as a solid blue beam through the vessel. The 

 drops in the case of water-vapour are evidently much smaller and more 

 numerous than when alcohol-vapour is used. 



The effect of adding traces of alcohol-vapour to the water-vapour in the 

 above experiment is to prevent the formation of the characteristic blue 

 cloud. A thin cloud of large drops is, however, formed similar to that 

 obtained with alcohol-vapour alone. In the light of these results the reason 

 why no nuclei were detected in our experiments by the electrical method is 

 that, although nuclei were formed, they grew so big and were so few that 

 their presence could not be demonstrated by our methods. 



Lenard and Ramsauer also investigated other vapours besides water. 

 Alcohol, methyl, ether, benzol, chloroform, atrachinon, and alizarin were 

 found to be ineffective. Benzine was slightly effective in forming nuclei. 

 Ammonia, hydrogen bi-sulphide, carbon-disulphide were effective. In these 

 experiments pure air was passed over a bulb containing watei', and another 

 containing the liquid to be examined. In addition, Lenard examined the 

 nuclei formation in moist gases other than air. Oxygen and carbon-dioxide 

 behaved like air ; but no nuclei were formed in moist hydrogen. No nuclei 

 were formed in water- vapour alone. 



