1876.] Repulsion resulting from Radiation. 279 



o£ no consequence, provided the distance is not altered ; thus two 

 candles one foot off give the same number of revolutions per second, 

 whether they are side by side or opposite to each other. From this it 

 follows that if the radiometer is brought into a uniformly lighted space 

 it will continue to revolve. This is proved to be the case by experi- 

 ment. 



The speed with which a sensitive radiometer will revolve in full sun- 

 shine is almost incredible. Nothing is apparent but an undefined nebulous 

 ring, which becomes at times almost invisible. The number of revolu- 

 tions per second cannot be counted, but it must be several hundreds, for 

 one candle will make the arms spin round forty times a second. 



The action of dark heat (i. e. from boiling water) is to repel each sur- 

 face equally, and the movement of the radiometer is therefore arrested if 

 a flask of boiling water is brought near it. The same effect is produced 

 by ice. 



Eroni some observations made by the author, it appears probable that 

 heat of a still lower refrangibility repels the white more than it does the 

 black surface. Many instances are given of the radiometer revolving the 

 reverse way. Thus breathing gently on the instrument will generally 

 cause this effect to be produced. 



An experiment is described with a radiometer the moving parts of 

 which are of aluminium, blacked on one side. When exposed to the 

 radiation from a candle the arms revolve the normal way. On removing 

 the candle they revolve the reverse way. Heated with a Bunsen burner 

 the arms revolve the normal way as they are getting hot ; but as soon as 

 the source of heat is removed and cooling commences, rotation sets up 

 in the reverse way, and continues with great energy till the whole is 

 cold. The reverse movement during the cooling is apparently equal 

 in energy to the normal movement as it is being heated. 



It is easy to get rotation in a radiometer without having the surfaces 

 of the disks differently coloured. An experiment is described with one 

 having the pith disks blacked on both sides. On bringing a candle near 

 it, and shading the light from one side, rapid rotation is produced, which 

 is at once altered in direction by moving the shade to the other side. 



The author describes many forms of radiometer, by means of which 

 the movements can be exhibited to a large audience, or can be made to 

 record themselves telegraphically on a self-recording instrument. 



II. "On Repulsion resulting from Radiation. 5 ' — Part IV. By 

 William Crookes, F.R.S. &c. Received February 5, 1876. 



(Abstract.) 



In this paper the author describes experiments on the repulsion pro- 

 duced by the different rays of the solar spectrum. The apparatus 

 employed is the horizontal beam suspended by a glass fibre and having 



