152 



HEAT. 



" radiometer action," on exceedingly small surfaces, at ordinary 

 pressures, in the dust-free region surrounding a heated body. 



If a thick copper rod is placed so as to project into an enclosure filled 

 with dusty air or smoke, on heating the part outside so that the rod inside 

 the enclosure becomes heated by conduction, the rod looked at endwise, 

 when the light is properly directed, may be seen to be surrounded by a 

 dust free space as in Fig. 82, where the rod is supposed to come end-on 

 against a window in the side of the enclosure, the light being thrown 

 through the space towards the observer. 



Lodge explains this by supposing that the heated body heats the 

 nearer face of the dust particles, and that there is a backward movement 

 if the dimensions of the particles are comparable with the mean free 

 path. They retreat on all sides from the heated body, leaving a space 

 of clear air. This ascends by convection, as 

 illustrated in the figure, being replaced by fresh 

 air, which in turn is cleared of its dust. 



If the body be cooler than the surround- 

 ings, the converse happens, the dust being forced 

 on to the cooler surface. Many illustrations of 

 the deposit of dust and smoke on cooler surfaces 

 may be found. Plaster ceilings very frequently 

 show the course of the laths and rafters behind 

 the plaster. Where the plaster is backed by 

 wood, it is probably kept warmer, and the dust 

 is not so freely deposited as on the neighbour- 

 ing cooler parts. Walls above hot-water pipes 

 are often very soon blackened, the hot dusty air 

 depositing its dust against the cooler wall. 



The Gas Equation of Van der Waals. The 



gas equation, expressing the laws of Boyle and of Charles, viz. : 



pv = H8 



is only approximately in agreement with observation. In a celebrated 

 paper " On the Continuity of the Liquid and Gaseous States of Matter " 

 (English translation, Physical Memoirs of the London Physical Society, 

 vol. i. pt. 3), Van der Waals deduced from theory the equation 



FTG. 82. 



(jp + )(,- 6)- R0 



where a and b are certain constants. This equation, though still not in 

 agreement with observation, represents the relation between pv and 6 

 much more exactly than the original gas equation. The following method 

 of obtaining the equation may serve to show how the kinetic theory 

 accounts for the failure of Boyle's Law. 



Returning to the investigation of the pressure exerted by a gas on the 

 sides of the containing vessel, let us now take into account (1) the size of 

 the molecules as affecting their length of path ; (2) their cohesive forces at 

 collision as lengthening out the time of collision. These are really two 

 aspects of one transaction, but it is convenient to consider them separately. 



Let us take first the size of the molecules. Suppose a molecule to 

 start normally from one face of a unit cube vessel. If it meets another 

 molecule in direct collision, moving with equal and opposite velocity, there 



