5^4' 



NATURE 



{April 10, 1879 



0'05 inch in thickness ; No. 4, of aluminium, o"oo2 inch 

 in thickness. These four radiometers were plain on each 

 side, no lampblack being applied. Their appearance is 

 shown in Fig. 6. No. 5 was made of aluminium, iden- 

 tical with No, 4, but the vanes were lampblacked on each 

 side instead of being bright. Had the vanes pointed 

 radially there could hare been no tendency for any one 

 of the flies to move either way, but being inclined, the 

 normal movement, on exposure to radiation, should be in 

 the direction of the arrows — a direction which I called 

 the positive direction. 



In Fig. 7 the candle is represented shining on the bulb 

 of the mica-vaned radiometer. The rays of light pass 

 through the first wall without action. They then meet 

 the mica, and that also being transparent, the rays pass 

 through it likewise, and then escape through the opposite 

 side of the bulb as is shown by dotted lines, without 

 absorption and consequently without doing work. But in 

 addition to light the candle is radiating ultra-red dark 

 heat-rays, which in great measure are arrested by the 

 glass, and raise its temperature. The inner surface of 

 the bulb then becomes the surface on which molecular 

 pressure is generated, which may be called the driving 

 surface ; this is shown by the shading next the candle. 

 This molecular disturbance presses on the mica-vane 

 which is in front of it, and drives it round in the direction 

 of the arrows as if it were subjected to a bombardment of 

 small shot. The vanes, in fact, may be said to be blown 

 round by what may be likened to a wind, which however 

 is not molar but molecular, inasmuch as there is no wind 



Fig. 8. 



in the sense of an actual transference of gas from one 

 part of the bulb to the other. 



In Fig. 8 I have endeavoured to represent part of the 

 action which takes place when the candle shines on the 

 aluminium radiometer. The light passing through the 

 bulb falls on the aluminium plate, and raising its tempera- 

 ture, causes pressure to be exerted on all sides. The 

 molecules rebounding from the face next the glass, cause 

 increased molecular pressure on that side, and produce 

 movement in the direction of the arrows, or positive rota- 

 tion. As each vane passes the candle it takes up heat, 

 and acquires extra driving energy. As it swings round, 

 the opposite side of the glass acts as a cooler, and by the 

 time the vane has completed the circle, and has radiated 

 away some of its extra heat, it is ready to recommence 

 the cycle of transformation— hght, heat, molecular 

 pressure, motion. 



Unlike mica, which generates very little pressure on its 

 surface, the aluminium fly carries sufficient driving power 

 to enable it easily to pass the dead centre opposite the 

 candle. Therefore, as soon as the candle has shown on 

 the aluminium radiometer long enough to warm the vanes 

 a little, rotation readily continues. 



The action of the pith radiometer is similar to the 

 aluminium, except that the dissipation of pressure from 

 the back surface of the pith will be almost nil. The pith, 

 moreover, being sensitive to the heat-rays, and being a 

 non-conductor, moves quicker than the aluminium, which 

 requires time to get warm throughout. 



The agreement between theory and observation, so far, 

 seemed exact. I now tried numerous experiments with 



dark heat applied in various ways to these five radio- 

 meters. The results I obtained led me to think that the 

 kind of dark heat might vary in refrangibility according 

 to its source, and that the rays from hot water, hot glass, 

 and hot metal, might affect the materials composing the 

 vanes in a different manner, and being absorbed by one 

 body and transmitted by another, might cause the posi- 

 tive or negative rotation which I obtained. I immersed 

 the five radiometers in boiling water, and after cooling 

 again immersed them in water only a few degrees above 

 the temperature of the room ; the results were similar to 

 those I had previously obtained with water of 70° C. 

 The radiometers were covered successively with hot 

 shades of English, French, and German glass of different 

 thicknesses, and at different degrees of temperature. The 

 bulbs were also heated with a gas or spirit flame, but no 

 uniform results were obtained. 



A funnel was then heated in boiling water, and allowed 

 to rest on the five radiometers in succession. They all 

 moved in the positive direction, except the bright alumi- 

 nium radiometer, which remained stationary. When the 

 funnel was removed, the two aluminium and the thick 

 mica radiometers rotated positively till they were cold. 

 The funnel was allowed to cool. It was then inverted 

 over a radiometer, and steam was passed through for a 

 second or two. The same experiment was repeated with 

 each radiometer. The results were now equally uniform 

 with those of the last experiment, but the rotation was 



Fig. 9. 



negative, the bright aluminium fly moving the best o 

 all, and the pith fly the least. 



I repeated the experiment with a thick brass ring, the 

 internal diameter of which was about half that of the 

 bulb (Fig. 9, a) and then with another brass ring a little 

 larger in diameter than the bulb, b. Fig. 9. These rings 

 were each heated to about 400°. With the first the rota- 

 tion was negative, while in the second all the flies 

 revolved in the positive direction. The two brass rings 

 were made red hot, and held in position till the flies were 

 in rapid movement, when the rings were removed and 

 the hot part of the bulb dipped into cold water, so as to 

 chill the glass quickly, and still keep the fly warm. These 

 experiments proved that when heat is applied round an 

 equatorial ring of the bulbs the rotation is always in the 

 positive direction. The hot ring of glass generates 

 molecular disturbance, which presses towards the centre 

 and strikes the sloping vanes, driving them round as if 

 the wind were blowing on them. In Fig. 10 I have tried 

 to represent this action. The positive movement is inde- 

 pendent of the material of which the fly is made, and is 

 only slightly increased or diminished according to the 

 conducting power of the fly for heat. The lighter the 

 weight of the fly to be driven round, the easier it moves, 

 and the heavier the fly the longer it keeps in motion after 

 it is once started. 



When heat is applied to either pole of the bulb negative 

 rotation takes place. The molecular pressure proceedmg 



