266 
MK. C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
shower of drops even in filtered air, while a slow steady one had no such efiect. The 
increase of volume was always the same, and amounted to one-third of the initial 
volume. He attributes the difference to the shock which results from a rapid stroke 
of the pump. 
E. V. Helmholtz,"^' on the other hand, was unable to observe any trace of con¬ 
densation in saturated filtered air, even with a fall of pressure of half an atmosphere. 
Whether, however, the pressure was reduced from one atmosphere to one-half, or from 
one-and-a-half atmospheres to one, is not clear ; fi'om his description of the method 
one woidd naturally take the latter interpretation. He deduces, however, a 
theoretical lowering of 50° C., and a ten-fold supersaturation which correspond to 
the former alternative. 
BARUS,t who made an extensive series of observations on the colour phenomena of 
a steam jet under varying conditions as to boiler-pressure and the temperature and 
dust contents of the surrounding air, concluded that with sufficient supersaturation, 
condensation takes place independently of dust. He does not appear, however, to 
have been able to deduce from his measurements the degree of supersaturation which 
is required to bring about this condensation. 
None of the experiments referred to above are entirely free from objection. 
When steam is blown into filtered air, as was done by Aitken and Kiessling, it 
is likely to carry over with it small drops of spray from the boiler. Even if these 
drops be made to evaporate by superheating the steam, each will leave behind a 
nucleus consisting of the solid matter which it contained in solution or suspension. 
The condensation noticed by Kiessling with very slight expansion may have been 
due to a similar cause, for he appears to have brought the air in his apparatus into 
a saturated state by allowing it to bubble through water after it had been filtered. 
That such treatment does actually introduce nuclei requiring Oidy a slight expansion 
of the saturated air to ctiuse condensation upon them is proved by certain experi¬ 
ments described below. Aitken j; noticed, too, that if the water in his dust-counting 
apparatus was allowed to splash about, such nuclei were produced. 
In none of the experiments mentioned above was the expansion very rapid, the 
apparatus in no case having been specially designed for the puiqiose of investigating 
this particular question. Helmholtz’s failure to obtain condensation may easily be 
explained by the expansion not being sufficiently rapid to produce anything like the 
theoretical lowering of temperature as, indeed, he himself admits. 
The interpretation of steam-jet experiments, such as those of Barus, is very 
difficult, especially as the phenomena depend largely on the roughness or smoothness 
of the bore of the nozzle from which the steam escapes. They cannot be taken as 
* Helmholtz, loc . cit . 
t “ Report on tlie Condensation of AfraosiAeric Moisture; ” 
Weather Bureau, 189.5 ; also ‘ Phil. iMag.,’ vol. 38, ]). 19, I89t. 
i ‘ Edin. Trans.,’ vol. 35, ]). 17, 1890. 
U.S. Department of Agriculture, 
