38 SECTION- PHYSICS. 



of heat, given out by a properly arranged Bunsen's burner, the mill 

 will keep up exactly the same rate of speed whether the flame be 

 rendered luminous or not. 



There has been much difference of opinion as to the cause of the 

 rotation of the arms. To solve the difficulty I have made a great 

 number of experiments, of which I shall mention but a few. The 

 Radiometer is not affected by electrical sparks, produced outside and 

 quite close to the globe in which it stands, however intense or 

 often repeated they may be. But under a magnetic current its metallic 

 arms are influenced, and its motion is either partially or completely 

 arrested. If, by means of lenses of equal power (ouverture), the 

 radiation of exactly similar moderator lamps be concentrated on both 

 surfaces of the same paddle, a perfect equilibrium can be obtained by 

 placing the white surface always much closer than the blackened one. 

 This equilibrium is established when the intensity of the radiations on 

 each surface is in inverse ratio to their absorbing power. The slightest 

 change in the distance of, or in the degree ot power displayed by the 

 lamps, is sufficient to set the machine in motion. 



In a horizontal plane, and concentrically to the axle of the radio- 

 meter, let an iron ring, thirty centimeters in diameter, and heated red 

 hot, be presented to the mill. The arms will immediately revolve with 

 a velocity which becomes very great indeed, when the plane of the 

 branches which support them is mingled with the plane of the ring. 

 The speed produced is the same, at equal distances from the ring, cither 

 above or below this plane. 



The radiometer, placed either in the air or in water, does not move 

 when heat is applied equally in all directions. And this is the case 

 even at the most elevated temperatures. 



If strong solar heat be concentrated on the paddles of the mill, the 

 mica, of which they are made, is partly split, and the instrument 

 pervaded by a grayish smoke; from that moment it loses a very great 

 part of its delicacy. 



When the same radiometer is repeatedly exposed to the focus of 

 an optical instrument, which collects the rays of a powerful lamp, an 

 occasional deposit can be seen gradually forming inside, which is com- 

 posed of a material that has been vaporized, and which at times assumes 

 the appearance of sublimated crystals. I suppose it is produced by the 



