340 Lord Kelvin and Messrs. Maclean and Gait. [Feb. 21, 



The air in the vat was the ordinary air of the laboratory, and the 

 curve shows the effect of the water-dropper alone in electrifying the 

 air negatively. For the next two days the water-dropper was kept 

 running continuously for about eight hours each day, to wash the dust 

 out of the air, and on the 18th of October curve (7) was obtained. It 

 shows a much less rate of negative electrification than curve (6). In 

 the experiments of summer 1890 an aspirator was used to draw the 

 air from the vat, and a tube full of cotton-wool was used to filter 

 the air drawn into the vat. 



Curves (1) to (5) are reproduced from the * Philosophical Maga- 

 zine,' and they show that the more the air becomes free from dust 

 the less is the rate at which the water-dropper electrifies. Thus 

 curve (1) was obtained from the ordinary air of the laboratory, in the 

 vat, and curve (2) after the aspirator was working for some time. In 

 this curve the water-dropper itself was running for some time before 

 the first observation was taken. The other curves were obtained 

 after further continuous working of the aspirator. 



CURVES 1 TO 5. 



