Jan. II, 1877] 



NA TV RE 



225 



To show the action of dark heat on the radiometer, a 

 candle was placed at such a distance from the instrument 

 that the arms would make one revolution a minute. A 

 small glass flask of boiling water was then placed half- 

 an-inch from the bulb. The revolutions instantly stopped, 

 two of the arms setting equidistant from the hot-water 

 flask. The flask of water was removed. As the portion 

 of the bulb which had been heated by the hot water 

 cooled, the white surface gradually crept nearer and 

 nearer to it, the superior repulsion of the candle on the 

 black discs urging the arms round, and acting in opposi- 

 tion to the repulsion of the hot glass to the white disc. At 

 last the force of the light drove the white disc with 

 difficulty past the hot spot of glass. Rotation then com- 

 menced, but for some revolutions there appeared to be a 

 difficulty in the white discs passing the spot of glass 

 which had been warmed by the hot water ; and the flask of 

 boiling water being replaced in its position half-an-inch 

 from the bulb of the radiometer, the rotation immediately 

 stopped. 



The instrument having become cool, the candle was 

 again placed in position, so that it produced one revolu- 

 tion in a minute. The finger was then pressed against 

 the side of the bulb, and as the spot of glass got warm, 

 the white surface experienced more and more difficulty in 

 getting past it, until at last one refused to pass, and the 

 arms came to rest. 



The instrument was again allowed to cool, and the re- 

 volutions recommenced at the usual speed (the laboratory 

 in which this was tried was somewhat cold). I then came 

 from a warm room, and stood a foot from the radiometer, 

 watching it. In about a minute the radiant heat from my 

 body had warmed the side of the bulb nearest to me 

 sufficiently to cause an appreciable difficulty in the move- 

 ment, and soon the revolutions stopped. The same effect 

 has been observed if the radiometer is brought into a 

 very warm room and placed near a cold window. If the 

 daylight is feeble, the instrument not very sensitive, or 

 an observer stands near the instrument, an appreciable 

 sticking is observed as the white discs come near that 

 part of the bulb which is the warmest. 



These experiments show that many precautions are 

 necessary to guard against the interfering action of un- 

 equal heating of the radiometer when it is being used for 

 accurate measurements. 



Having found such an antagonistic action of dark heat, 

 I tried the action of ice. This is equivalent to warming 

 the opposite side of the instrument. A lump of ice was 

 brought within half an inch of the bulb on the opposite 

 side to the candle. The revolutions got slower, until at 

 last the movement stopped altogether, one arm pointing 

 direct to the ice, and being apparently held there 

 by a powerful attractive force. Bringing the candle 

 nearer caused the arms to oscillate a little, and when it 

 was almost close tc the bulb the force of the light over- 

 came the action of the ice, and the arms revolved again, 

 but irregularly, and with jerks, the discs moving quickly 

 to the ice and leaving it with difficulty. In this action of 

 ice no preference was noticed for either the black or white 

 surface. 



A very delicate radiometer, in 2-inch bulb, was placed 

 in a sufficient hght to allow it to be seen distinctly, but 

 not enough to cause it to move. I then came out of a 

 warm room and stood near it. In a few seconds it 

 began to move slowly round in the negative direction, i.e. 

 the black discs advanced instead of retreated. On moving 

 away from the instrument the rotation gradually stopped. 

 I again approached it, and held one hand an inch from 

 the bulb. Rotation soon commenced, but still in the 

 reverse way. These experiments were frequently repeated 

 and always with the same results. 



When the instrument was at rest I came quickly to it, 

 and gently breathed on the bulb. There was a slight 

 movement in the normal direction, but this stopped 



directly, and the arms then revolved the reverse way for 

 more than a minute, performing three or four complete 

 revolutions. 



A glass shade four inches in diameter was held over a 

 gas-flame till the air inside was warm, and the inner 

 surface dim with steam. It was then inverted over the 

 radiometer. Rotation commenced the reverse way, and 

 kept up for several minutes. The glass shade was then 

 dried inside, and heated uniformly before a fire until it 

 had a temperature of about 50° C. It was then inverted 

 over the radiometer. Reverse rotation instantly com- 

 menced, and kept up with some vigour for more than five 

 minutes, diminishing in speed until the shade had cooled 

 down to the temperature of the surrounding air. 



The same experiment was repeated, and whilst the arms 

 were in full negative rotation, a lighted candle was slowly 

 brought near it. "When three feet off the negative rotation 

 slackened. When the candle was about two feet off the arms 

 became still, and when nearer than two feet the instru- 

 ment rotated normally, the antagonism between the action 

 of the hot shade and the lighted candle was perfect ; by 

 moving the candle to and fro it was easy to cause the 

 radiometer to move in one direction or the other, or to 

 become still. 



I now tried the action of a radiometer the moving parts 

 of which were made of a good conductor of heat, such as 

 a metal, instead of pith, which is a bad conductor of heat. 

 I selected thin rolled brass as the material wherewith to 

 make the arms and discs of a radiometer. The parts 

 were all fastened together with hard solder, and no 

 cement or organic matter was used, so that if necessary 

 the instrument could be submitted to a high temperature 

 without injury. The moving portion weighed I3'i grains. 

 One side of the discs was silvered and polished, the other 

 side being coated with lampblack. The apparatus v/as 

 exhausted with a charcoal reservoir attached. A candle 

 i^ inch from the bulb caused it to revolve about once a 

 second, the black surface being repelled in the normal 

 manner. 



Standing in a rather dark cold room, it was covered 

 with a warm glass shade, and it immediately began to 

 revolve the negative way, but very slowly. A few drops of 

 ether poured on the bulb caused the arms to move rather 

 rapidly the normal way. A hot shade put over whilst it 

 was thus moving caused it to stop, and then begin moving 

 the reverse way. A small non-luminous gas flame was 

 held vertically beneath the apparatus, so that hot air 

 should ascend and wrap round the bulb on all sides. 

 The arms now revolved the reverse way. 



The brass radiometer being somewhat heavy, one was 

 made of aluminium, the moving parts being hard soldered 

 as before. A siphon-gauge was attached, and the appa- 

 ratus connected direct on to the pump by a spiral, no 

 charcoal tube being used. One side of the wings was 

 bright aluminium, and the other was lampblacked. When 

 exhausted the arms revolved very quickly to a candle a 

 few inches off, the black being repelled. On removing 

 the candle the arms stopped and immediately commenced 

 revolving the reverse way, keeping up rotation for more 

 than ten minutes, and being little inferior in speed to 

 what it was when the candle shone on it. The whole of 

 the bulb was heated with a Bunsen burner ; whilst it was 

 getting hot the aluminium arms revolved rapidly in the 

 normal direction, but as soon as the source of heat was 

 removed and cooling commenced, rotation set up in the 

 reverse way, and continued with great energy till the 

 whole thing was cold. It appeared as if the reverse 

 movement during the cooling was equal in energy to the 

 normal movement as it was being heated. 



A little ether was poured on the bulb of a very sensitive 

 pith radiometer as it was standing still in a faint light. 

 The evaporation of the ether caused a chilling of the in- 

 strument and a rapid abstraction of heat from the arms. 

 They commenced to move in the normal direction and 



