38 Mr. W. Crookes on the [Dec. 11, 



and more delicate, which enabled him to detect and then to render very 

 sensible an action exerted by heat on gravitating bodies, which is not 

 due to air-currents or to any other known form of force. 



The following experiment with a balance made of a straw beam with 

 pith-ball masses at the ends enclosed in a glass tube and connected with 

 a Sprengel pump, may be quoted from the paper : 



" The whole being fitted up as here shown, and the apparatus being 

 full of air to begin with, I passed a spirit-flame across the lower part of 

 the tube at b, observing the movement by a low-power micrometer ; the 

 pith ball (a, b) descended slightly, and then immediately rose to con- 

 siderably above its original position. It seemed as if the true action of 

 the heat was one of attraction, instantly overcome by ascending currents 

 of air 



" 31. In order to apply the heat in a more regular manner, a thermo- 

 meter was inserted in a glass tube, having at its extremity a glass bulb 

 about 1| inch in diameter ; it was filled with water and then sealed up. . . 

 The water was kept heated to 70 0., the temperature of the laboratory 

 being about 15 C. 



" 32. The barometer being at 767 millirns. and the gauge at zero, the 

 hot bulb was placed beneath the pith ball at b. The ball rose rapidly ; 

 as soon as equilibrium was restored, I placed the hot-water bulb above 

 the pith ball at a, when it rose again, more slowly, however, than when 

 the heat was applied beneath it. 



" 33. The pump was set to work ; and when the gauge was 147 millims. 

 below the barometer, the experiment was tried again ; the same result, 

 only more feeble, was obtained. The exhaustion was continued, stopping 

 the pump from time to time, to observe the effect of heat, when it was 

 seen that the effect of the hot body regularly diminished as the rarefac- 

 tion increased, until when the gauge was about 12 millims. below the 

 barometer the action of the hot body was scarcely noticeable. At 

 10 millims. below it was still less ; whilst when there was only a difference 

 of 7 millims. between the barometer and the gauge, neither the hot- 

 water bulb, the hot rod, nor the spirit-flame caused the ball to move in 

 an appreciable degree. The inference was almost irresistible that the 

 rising of the pith was only due to currents of air, and that at this near 

 approach to a vacuum the residual air was too highly rarefied to have 

 power in its rising to overcome the inertia of the straw beam and the 

 pith balls. A more delicate instrument would doubtless show traces of 

 movement at a still nearer approach to a vacuum ; but it seemed evident 

 that when the last trace of air had been removed from the tube surround- 

 ing the balance when the balance was suspended in empty space only 

 the pith-ball would remain motionless, wherever the hot body were 

 applied to it. 



" 34. I continued exhausting. On next applying heat, the result 

 showed that I was far from having discovered the law governing these 



