Transpiration and Radiometer Motion. 491 



sides facing in opposite directions, the radiation from a candle 

 causes the flies to revolve in opposite directions, which proves 

 that the driving action is chiefly localised close to the flies ; 

 this result is of course involved in our theory according to 

 which the action of the fall of temperature through each vane 

 is to raise the pressure near its hot face and lower it near the 

 cool face, but the region of lower pressure of the upper fly 

 being just above the region of higher pressure of the lower 

 fly, and, with no obstruction between, ought to produce dissi- 

 pation of the driving power of both flies, so that although 

 they move in opposite directions they hinder one another in 

 this direct manner as well as through the viscosity of the gas. 

 Another radiometer contained only one pair of vanes at the 

 ends of a single arm, and each vane carried opposite to its 

 black face, at a distance of a millimetre, a large disk of thin 

 clear mica ; the action of a candle on this was to cause 

 rotation in a direction opposite to the usual, that is, the black 

 face moved towards the light. When another disk of thin 

 clear mica was attached opposite the other side of each vane 

 a candle ceased to have any effect. The theoretical reason for 

 these facts is clear; in the first case the region of high 

 pressure set up near the edge of the black face of the vane has 

 more effect on the clear plate than on the vane and in the 

 opposite direction, so that there is a resultant differential 

 pressure driving the vane and its attachments in the opposite 

 direction to the usual ; when the other clear plate is attached 

 there is an equal opposite resultant differential pressure due 

 to it and so there is equilibrium ; in short, when the two clear 

 plates are attached the whole action is confined to the space 

 between them, so that there can be no motion of the whole 

 system. 



In another radiometer the four vanes were left clear, but at 

 the side of the bulb a plate of mica blackened on one side was 

 fastened in a vertical plane passing through the centre of the 

 bulb, so that a vane in passing it would leave a clear space of 

 a millimetre : when light is thrown only on the clear vanes 

 there is no motion, but as soon as it is allowed to fall on the 

 fixed plate the fly revolves as though a wind were blowing 

 from the black surface. This follows from theory at once, as 

 the edge of the black face becomes a region of higher pressure 

 and therefore a source of wind. 



On replacing the pith or mica vanes by metallic ones 

 Crookes encountered some new phenomena ; perfectly flat 

 aluminium vanes were found to be much less sensitive to the 

 light of a candle than mica or pith ; they move in the same 

 direction, that is with the black surface away from the light, 

 but when the candle is replaced by a source of dark heat their 



