Mr. W. Orookes on the Radiometer. 469 



of 250 millionths of an atmosphere ; at 100 millionths it is 0*105, 

 at 50 millionths it is 0*093, at 20 millionths it is 0*068, and at 2 

 millionths it is 0*02. The force of repulsion in oxygen increases 

 very steadily up to an exhaustion of about 40 millionths of au at- 

 mosphere ; it is at its maximum at about 30 millionths, and thence 

 declines very rapidly. 



Hydrogen gives a remarkable diagram. The viscosity at the 

 normal pressure is measured by a log. dec. of 0*063 ; at 250 

 millionths of an atmosphere it is 0*057, at 100 millionths it is 

 0*052, at 50 millionths it is 0*046, whence it rapidly sinks. The 

 force of repulsion increases slowly up to an exhaustion of 250 

 millionths, then quickly until it attains its maximum at about 50 

 millionths, and it then rapidly declines. The force of repulsion is 

 very great in a hydrogen vacuum, being in comparison with the 

 maximum in an air vacuum as 70 to 41. Neither is it necessary to 

 get so high an exhaustion with hydrogen as with other gases to 

 obtain considerable repulsion. This shows that in the construc- 

 tion of radiometers it is advantageous to fill them with hydrogen 

 before exhausting. 



Carbonic acid has a viscosity of about *01 at the normal pres- 

 sure, being between air and hydrogen, but nearer the former. On 

 approaching a vacuum, the force of repulsion does not rise very 

 high, and soon falls off. 



Before working with this apparatus I thought that monohy- 

 drated sulphuric acid evolved no vapour, and I therefore freely 

 used it for cleaning out the pump and for drying the gases. I can 

 even now detect no vapour-tension; but a comparison of the 

 curves, with and without sulphuric acid, shows that the presence 

 of this body modifies the results. One of my curves represents the 

 action of the residual sulphuric anhydride gas. The experience 

 thus gained has led me to adopt phosphoric anhydride for drying 

 the gases. I can detect no ill effects from the presence of this 

 agent ; and I have been able in consequence to push the rarefaction 

 to higher points than before. 



The McLeod gauge will not show the presence of mercury va- 

 pour. It is therefore possible that I have a greater pressure in 

 the apparatus than is here stated. I have, however, entirely failed 

 to detect the presence of mercury vapour at any great distance 

 from the mercury in the pump ; and the tube packed with gold- 

 leaf, which I frequently interpose between the pump and the 

 apparatus, shows no trace of bleaching, and exerts no appreciable 

 effect one way or the ether on the results. 



"With this pump, assisted sometimes by chemical absorption, it 

 is not difficult to exhaust a radiometer to such a point that it will 

 not move to a candle placed a few inches off ; but I have not 

 yet succeeded in stopping the movement of the beam in the torsion- 

 apparatus. 



A long series of observations have been taken, at different degrees 

 of exhaustion, on the conductivity of the residual gas to the spark 

 from an induction-coil. Working with air, I find that ata pressure 



