1-48 JRoyal Society: — Mr. W. Crookes on 



The number of revolutions 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 surface equally, and the movement of the radiometer is there- 

 fore arrested if a flask of boiling water is brought near it. The 

 same effect is produced by ice. 



Prom some observations made by the author, it appears pro- 

 bable 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 con- 

 tinues 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 haA'ing 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. 



" On Repulsion resulting from Radiation." — Part IV. By 

 William Crookes, F.R.S. &c. 



In this paper the author describes experiments on the repulsion 

 produced by the different rays of the solar spectrum. The ap- 

 paratus employed is the horizontal beam suspended by a glass fibre 

 and having square pieces of pith at each end coated with lamp- 

 black, The whole is fitted up and hermetically sealed in glass, 

 and connected with an improved mercury pump. In front of the 

 square of pith at one end a quartz window is cemented to the 

 apparatus ; and the movements of the beam, when radiation falls 

 on the pith, are observed by a reflected ray of light on a millimetre- 

 scale. The apparatus was fitted up in a room specially devoted 

 to it, and was protected on all sides, except where the rays of light 

 had to pass, with cotton-wool and large bottles of water. A 

 heliostat reflected in a constant direction a beam of sunlight, which 

 was received on an appropriate arrangement of slit, lenses, and 

 prisms for projecting a pure spectrum. Results were obtained in 

 the months of July, August, and September ; and they are given 



