306 Mr. W. Crookes on the Radiometer. [Nov. 16, 



they both would come to zero when the last traces o£ an atmosphere 

 had been taken out of the apparatus. 



The oxygen diagram differs from that of air. The log. dec. is 

 0*126 at the atmospheric pressure; it falls to 0*111 at a pressure of 250 

 millionths of an atmosphere ; at 100 millionths it is O'lOo, at 50 mil- 

 lionths 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 np to an 

 exhaustion of about 40 millionths of an atmosphere ; it is at its maximum 

 at about 30 millionth s, 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 nntil it 

 attains its maximum at about 50 milHonths, 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. ISTeither is 

 it necessary to get so high an exhaustion with hydrogen as with other 

 gases to obtain considerable repulsion. This shows that in the con- 

 struction of radiometers it is advantageous to fill them with hydrogen 

 before exhausting. 



Carbonic acid las a viscosity of about -01 at the normal pressure, 

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

 ing a vacuum, the force of repulsion does not rise very high, and soon 

 falls off. 



Before working with this apparatus I thought that monohydrated 

 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 an- 

 hydride gas. The experience thus gained has led me to adopt phosphoric 

 anhydride for dr}dng the gases. I can detect no 111 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 vapour. 

 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 other on the 

 results. 



"With this pump, assisted sometimes by chemical absorption, it is not 



