126 THE PRESSURE DUE TO RADIATION. 



through tlic diagTam gives the ineaii value of the moment of radiation 

 pressure computed from the data in Tal)le I. Decrease of the deflec- 

 tion with time indicates gas repulsion on the warmed silver faces and 

 increase in deflection gas suction. It will be seen from the curves 

 that beginning at a gas pressure of 66 mm. of mercury, the gas action 

 was repulsion changing to suction in passing from 19.8 to 11.2 mm. 

 In the last two cases the total gas action is small. For lower pressures 

 the suction increases to 0.05 mm. At a gas pressure of 0.02 nun. the 

 gas action is again a strong repulsion. 



The curves indicate the existence of two gas pressures, at which the 

 gas action in our arrangement of apparatus should l)e zero, one 

 between iU.S and 11.2 mm. and the other between 0.05 and 0.02 nmi." 

 The former region was chosen for the l)allistic measurements and 

 nearly all of the ol)servations were made at a gas pressure of approxi- 

 mately 1<> nnn. Even for the two pressures where the decrease in the 

 static deflection was most rapid, i. e.. at gas pressures of 66 and 0.02 

 mm., the tirst throw was always in the direction of radiation pressure. 

 The gas action is strongh^ influenc(»d ])y very slight changes in the 

 inclination of th(^ plane of the A'anes to the vertical and also by an}' 

 object introduced under the bell jar anywhere near the vanes. For 

 instance, a very considerable efl^ect was observed when a small vessel 

 of phosphoric anhydride was placed under the jar behind the vanes, 

 though the nearest wall of the vessel was separated from the vanes by 

 a distance of at least 8 cm. 



During the ()l)servations. the polished silver coatings on the vanes 

 deteriorated rapidl}*; new coatings rarely lasted for more than two 

 evenings' work. As the balance had to >)e removed and the mirrors 

 taken fi-om the hooks, silvered, polished, and replaced a great numl)er 

 of time.s during the entire series of measurements, although great care 

 was taken in setting the plane of the vanes vertical, it is not likely 

 that pr(»cisely the same conditions for gas action were ever repeated. 

 The pi'incipal value of the static results was in indicating favora))le 

 gas pressures for work, rather than afli'ording quantitative estimates of 

 the gas action in short exposures. The dotted parts of the curves are 

 not based on results of observation and might perhaps have been 

 omitted without loss. 



It was plain, therefore, that further elimination of the gas action 

 must be sought in exposures so short that the gas action would not 

 have time to reach more than a small fraction of its stationary value. 

 This led to the method of ballistic observations. ^ , 



"Crookesjiu his Avork witli the radiometer discovered certain gas pressures for 

 which the combined gas and radiation forces neutralized, but as he did not discrimi- 

 nate between forces (hie to radiation and gas forces his results were apparently 

 capricious and his reasoning somewhat confused. See Phil. Trans., p. 519, 1875. 



