Repulsion accompanying Radiation. 89 



than when the mass acted on is heavy like brass ; whereas in 

 practice the contrary obtains. Pith ceases to move under the 

 influence of radiation at a rarefaction of about 7 to 12 millims., 

 whilst brass only ceases to be affected when the gauge and ba- 

 rometer are appreciably level. 



But even could the phenomena up to the neutral point be 

 explained by air-currents, these are manifestly powerless to act 

 after this critical point is passed. If a current of air within 7 

 millims. of a vacuum cannot move a piece of pith, certainly the 

 residual air in a Sprengel vacuum should not have more power; 

 and a fortiori the residual gas in a perfect chemical vacuum 

 cannot possibly move a mass of platinum. 



It is, however, abundantly demonstrated that, in all cases after 

 this critical point is reached, the repulsion by radiation is most 

 apparent ; it increases in energy as the vacuum approaches per- 

 fection, and attains its maximum when there is no air whatever 

 present, or at all events not sufficient to permit the passage of 

 an induction-spark. 



I will now refer to the electrical explanation. Very early in 

 my investigation, phenomena were noticed which caused me to 

 think that electricity played a chief part in causing the move- 

 ments. When a hot glass rod is held motionless against the 

 side of an exhausted tube containing a pith index, repulsion 

 takes place in a perfectly regular manner; but if the glass rod 

 has been passed once or twice through the fingers, or is rubbed 

 a few times sideways against the exhausted bulb, the index im- 

 mediately moves about in a very irregular manner, sometimes 

 being repelled from, and at others attracted to, the side of the 

 glass, where it adheres until the electrical excitement subsides. 

 Friction with the finger produces the same results; and a small 

 spirit-flame causes similar, but much fainter, electrical effects. I 

 soon ascertained, however, that, although electricity is capable 

 of producing many movements similar to those caused by radia- 

 tion, they are never so alike as to be mistaken. Electricity fre- 

 quently interferes with, disturbs, or neutralizes the true action of 

 radiation ; but it acts in such a manner as to show that it is not 

 the primary cause of the movement. At the highest rarefactions, 

 and when special precautions have been taken to avoid the pre- 

 sence of aqueous vapour, slight friction with the finger against 

 the bulb, or a touch with the flame of a spirit-lamp, excites so 

 much electrical disturbance in the pith and other indexes that 

 accurate observations become impossible with them for several 

 hours. I have tried many means of neutralizing the electrical 

 disturbance ; but they are only partially successful, and at the 

 highest rarefactions interference through electrification is very 

 troublesome. 



