216 Prof. Silas W. Holman on the Effect of 



Effect of Pressure. 



In Series IV. for air, the total driving pressure used, p^ —Pb, 

 was varied within considerable ranges, to test whether the 

 value obtained of rj 2 : r) u or of K, was thereby affected except 

 through errors of observation. In 3, 4, and 5, p\—pz = 

 248 mm., and tj 2 : r] 1 = 1*267 ; in 7 and 8, under the same 

 conditions otherwise, p 1 — ^3 = 376 mm., and rj 2 : rj^ — 1*266, 

 showing no effect due to increase of the pressure by one half. 

 Measurements 9 to 13, made withjoj— p 3 ranging from 258 

 to 485 mm., give values of K with an average deviation of 

 about 0*2 per cent. Measurements 21 to 48 give values of 

 7} 2 : 7) i about one per cent, lower than those obtained in the 

 remainder of the series, made with greater values of p l — p 3 , 

 but the precision of these measurements is much less than of 

 the later ones. The value of K found from 50 to 54 and 60 

 to 6Q, with p 1 —p B = 530 mm. about, is within 0*5 per cent, of 

 that found in 9 to 12, under^— p 3 =258 mm., and agrees 

 precisely with 12, under p 1 —p B = 361 mm. And, finally, the 

 mean of 97 and 98 differs from 99 by less than 0*25 per cent., 

 though the pressures p\—p 3 are respectively 540 and 250 mm. 



In the second series for carbonic acid, the effect of pressure 

 was somewhat tested in measurements 26 to 38, where pi—p s 

 varied from 370 to 111 mm. without producing any trace- 

 able effect in K. This is the best test made, owing to the 

 greater precision of the measurements. Also in measurements 

 93 to 98 of the same series, the pressure was changed from 

 105 to 207 without materially affecting the results. 



In the first series for air, the average value of p l —p 3 was 

 about 740 mm., and the mean value of tj 2 : rj { at £ 2 =100° is 

 1*270, while that of the fourth series, under p 1 —p 3 = from 

 248 to 540, is about 1*267, and that of the fifth series, under 

 p 1 ^p 3 =225 mm., is 1*272, thus showing no effect traceable 

 to the difference of the driving pressures used. 



Deduction from Results. 



The results of my measurements seem to show conclu- 

 sively, that the variation of the viscosity with the temperature 

 of the gas, in the case at least of dry carbonic acid and of dry 

 air freed from carbonic acid, which may be taken as typical 

 gases, is not proportionate either to the square root or to 

 any numerical power of the absolute temperature reckoned 

 from — 274° C. They point thus to the inference that all 

 hypotheses yet advanced to account for the variation of the 

 viscosity of gases, and hence also for the viscosity itself, are 



