1892.] 



connected with Cloudy Condensation, 



419 



gradual rise in the repulsion with fall in temperature may be the 

 explanation of why the drops in a water jet coalesce at a lower 

 temperature than the mist drops on the surface of water. The 

 water may require to be cooled to a lower temperature before the 

 repulsion is sufficient to prevent the heavier drops from coalescing, 

 while the less repulsion at the higher temperature may be sufficient 

 to prevent the lighter mist drops from coming into contact. The 

 same explanation helps to account for the increased density produced 

 by increasing the dust particles, a less repulsion being sufficient to 

 protect the excessively small drops. 



The explanations we have here offered of the action of electricity 

 and low temperature are in complete agreement. In ordinary con- 

 densation when the temperature of the air is high there is no surface 

 repulsion, owing to the high temperature in the jet, and many of the 

 particles coalesce on collision with each other ; but, when the drops 

 are electrified, their mutual repulsions prevent contact, and the 

 result is a large increase in the number of drops and a dense form of 

 condensation. On the other hand, when the temperature is lowered, 

 surface film repulsion comes into action, contact is prevented, and 

 the drops do not coalesce on collision, and the result is exactly the 

 same as if they were electrified. 



In these remarks no reference has been made to the effect of the 

 dryness of the air on the density of the condensation. It seems 

 probable that the relative humidity of the air will have a less 

 influence on the density than on the duration of the jet, that is, the 

 length of time the drops take to evaporate. 



4. High Pressure of the Steam. 



The fourth cause of the dense form of condensation is high pressure 

 of the steam. If the temperature be below 46° the condensation is 

 dense at all pressures, but as the temperature rises, the condensation 

 ceases to be dense if the pressure of the steam be low. But if we 

 now raise the pressure, the jet again becomes dense, and the higher 

 the temperature of the air the higher the pressure must be raised to 

 produce the dense form of condensation. The action of the high 

 pressure in producing the dense condensation is more complex than 

 any of the previous causes. It acts, first, by the more rapid move- 

 ments of the jet mixing a larger amount of air with the steam, by 

 which means a greater number of dust nuclei are taken into the jet ; 

 and, second, a lower temperature is also produced, which probably 

 brings the temperature of the drops low enough for the repulsive 

 action of the films to come into play. But in addition to the effects 

 of a greater amount of air being mixed with the steam, a third action 

 here comes into play. Owing to the violent rush of steam, the con- 



