PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 211 



which the mist drops tioated ou water in the experiment previously 

 described. This method is not very satisfactory, on account of the dif- 

 ficulty of seeing' the drops wlien the temperature is high, owing to the 

 amount of condensed steam hanging over the water. It is also difficult 

 to keep the surface of the water clean. The tests by this method gave 

 a temperature considerably higher tha:i that given by the water jet. 

 Neither of these methods however, promises to give satisfactory infor- 

 mation on this point; but, if it were desired, the effect of temperature 

 on the contact of films could be studied in a more accurate way. 



It is difficult to imagine any sudden chang-e in the action of the films 

 at or about the temperatures indicated. There is no corresijonding 

 change, so far as I am aware, in the surface tension. We might picture 

 to ourselves the change to be brought about by the alteration which 

 takes place in the intervening" gases. When the drops are cold the 

 bounding surfaces are water and air with very little vapor in it. And 

 perhaps we may be permitted to assume that the surface film has a 

 layer of air condensed on it, and it may be this condensed layer of air 

 which prevents contact when the drops come into collision. But when 

 the temperature is high the ^conditions are changed. The bounding 

 surfaces are now water and air with a large amount of vapor in it, and 

 this vapor may play an important part in bringing about the contact, 

 by the violent interchange of water molecules taking place at the sur- 

 faces of the films and weakening the condensed films of air. If this 

 explanation be correct, then there is really no sudden change in the 

 action of the films, and the repulsion is a gradually increasing one with 

 fall of temperature. Though a somewhat sudden change in the api»ear- 

 ance of this jet might seem to indicate a sudden change in the action 

 of the films, yet the change may be really a slowly increasing one, and 

 the sudden change in the appearance of the jet may be due to the 

 repulsion rising to such an amount that the very small particles are 

 prevented from coalescing. If the relative temperatures given for the 

 coalescence of water drops and mist drops be correct, then the grad- 

 ual rise in the repulsion with fall in temperature may be the exj^lana- 

 tion 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 helj)S to account 

 for the increased density produced by increasing the dust i)articles, 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 condensation 

 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 electri- 



