1873.] Disk by rapid Rotation in vacuo. 315 



43. Our next experiments were made with the view of testing whether 

 or not the two effects, the residual and the gas-effect, were resident in the 

 same particles of the disk ; and for this purpose the experiments made 

 immediately after rotation were compared with those made one minute 

 afterwards. 



The experiments available for this purpose are so numerous that they 

 can bear splitting into two portions, in each of which the same result is 

 seen. 



Thus we have for an atmosphere of ^ dry hydrogen, and as the mean 

 of 30 individual comparisons, 



Effect at first : Effect one minute after : : 1-30 : 1 ; 



also, as the mean of 22 individual comparisons, we obtain the proportion 

 of 149 : 1, while as the mean of the whole we obtain 1-25 : 1. 



Treating in a similar manner the observations made with an atmosphere 

 of ^ hyd. + 2 6 <y air, we obtain 



As the mean of 25 comparisons 147 : 1, 



As the mean of 21 comparisons 141 : 1, 



while as the mean of the whole we obtain 144 : 1. 



We therefore conclude that the residual effect is less diminished during 

 the interval of one minute than the gas-effect. 



44. "We next made experiments with two aluminium disks -05 and 

 •025 of an inch in thickness respectively. These disks were covered on 

 both sides with a coating of lampblack applied by negative photographic 

 varnish. 



In both disks we found that with a vacuum of ^ hyd. the effect one 

 minute after rotation was greater than the effect immediately after rota- 

 tion. Thus we have : — 



after : : '85 : 1 ; 



For thick disk (mean 1 -r,™ , , n , -n^ i • > 



of 4 observations) j Effect at flrst : Ef£ect one mimlte 



Eor thin disk (mean 

 of 2 experiments, I do _ d >86 . J 



embracing 7 obser- f 

 vations) J 



But in cases where the gas-effect was considerable, this peculiarity dis- 

 appeared. Thus we have for an atmosphere of ^ hyd. -f 2 6 F air : — 



For thick dish (mean 1 Effeet at first : Effect one nnnnte after :: 1-05 : 1. 

 oi 2 observations) J 



Tor thin dish (mean 1 fe d :: 1-45:1. 



or 2 observations) J 



Also, for an air-vacuum of ^5-, we have 



Eor thin disk (mean \ Effectatfirst : Effect one minute after 149 : 1. 

 or 3 observations) J 



