caused by Evaporation and Condensation. 147 



This experiment was repeated under a variety of circumstances, 

 in different flasks and with different balances, the stem being some- 

 times 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 generated within the flask. When there was enough 

 vapour, these currents carried the pith with them ; they were, in 

 fact, then sufficient to overcome 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 Avas 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 sensitive- 

 ness 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 con- 

 dense on the ball near the ice, while the other ball would become 

 dry (this would be seen to be the case, and was also shown by the 

 tipping of the balance, that ball against the ice gradually getting 

 lower). It was then found, when the ice was removed, that the 

 dry ball was insensible to the heat, or nearly so, while that ball 

 which had been opposite to the ice was more than ordinarily sen- 

 sitive. 



If the flask were dry and the tension of the vapour reduced 

 with the pump until the gauge showed | of an inch, then, although 

 purely steam, the vapour was not in a saturated condition, and 

 the pith-balls which were dry were no longer sensitive to the lamp, 

 although they would still approach the ice. 



From these last two facts it appears as though a certain amount 

 of moisture on the balls were necessary to render them sensitive to 

 the heat. 



In order that these results might be obtained, it was necessary 

 that the vapour should be free from air. If a small quantity 

 of air was present, although not enough to appear in the gauge, 

 the effects rapidly diminished, particularly that of the ice, until the 

 convection-currents had it all their own way. This agrees with the 

 fact that the presence of a small quantity of air in steam greatly 

 retards condensation and even evaporation. 



"With a dry flask and an air-vacuum, neither the lamp nor the 

 ice produced their effects ; the convection-currents reigned supreme 

 even when the gauge was as low as \ inch. Under these circum- 

 stances the lamp generally attracted the balls and the ice repelled 



L2 



