1874.] On Surf ace -Evaporation and Condensation. 401 



VIII. " On the Forces caused by Evaporation from,, and Conden- 

 sation at, a Surface." By Prof. OSBORNE REYNOLDS, of 

 Owens College, Manchester. Communicated by B. STEWART, 

 F.R.S. Received May 16, 1874. 



It has been noticed by several philosophers, and particularly by Mr. 

 Crookes, that, under certain circumstances, hot bodies appear to repel and 

 cold ones to attract other bodies. It is my object in this paper to point 

 out, and to des3ribe experiments to prove, that these effects are the 

 results of evaporation and condensation, and that they are valuable 

 evidence of the truth of the kinetic theory of gas, viz. that gas consists 

 of separate molecules moving at great velocities. 



The experiments of which the explanation will be given were as 

 follows : 



A light stem of glass, with pith-balls on its ends, was suspended by a 

 silk thread in a glass flask, so that the balls were nearly at the same 

 level. Some water was then put in the flask and boiled until all the air 

 was driven out of the flask, which was then corked and allowed to cool. 

 When cold there was a partial vacuum in it, the gauge showing from 

 | to | of an inch pressure. 



It was now found that when the flame of a lamp was brought near 

 to the flask, the pith- ball which was nearest the flame was driven away, 

 and that with a piece of ice the pith was attracted. 



This experiment was repeated under a variety of circumstances, in 

 different flasks and with different balances, the stem being sometimes of 

 glass and sometimes of platinum; the results, however, were the same in 

 all cases, except such variations as I am about to describe. 



The pith-balls were more sensitive to the heat and cold when the flask 

 was cold and the tension within it low ; but the effect was perceptible 

 until the gauge showed about an inch, and even after that the ice would 

 attract the ball. 



The reason why the repulsion from heat was not apparent at greater 

 tensions, was clearly due to the convection-currents which the heat gene- 

 rated within the flask. When there was enough vapour, these currents 

 carried the pith with them ; they were, in fact, then sufficient to over- 

 come the forces which otherwise moved the pith. This was shown by 

 the fact that when the bar was not quite level, so that one ball was 

 higher than the other, the currents affected them in different degrees ; 

 also that a different effect could be produced by raising or lowering the 

 position of the flame. 



The condition of the pith also perceptibly affected the sensitiveness of 

 the balls. When a piece of ice was placed against the side of the glass, 

 the nearest of the pith-balls would be drawn towards the ice, and would 

 eventually stop opposite to it. If allowed to remain in this condition 

 for some time, the vapour would condense on the ball near the ice, 



