8 PROCEEDINGS OF THE AMERICAN ACADEMY. 



moved from C and the Dewar vessel containing liquid air 3 was substi- 

 tuted for it, and the other part of the apparatus was allowed to cool 

 down. The charcoal was allowed to absorb what it would at the tem- 

 perature of the liquid air. Altogether the liquid air was kept sur- 

 rounding the charcoal for about eighty hours, and from time to time 

 during this interval a measurement of the decrement. I, was made. At 

 first the diminution in the value of the decrement was fairly rapid, but 

 after the first day the change was very slow. This, no doubt, was due 

 in part to the slow passage of the gas towards the charcoal through 

 the somewhat extended form of the apparatus. It was, also, probably 

 due to the fact, which was noted later in the investigation, that at a 

 given stage of exhaustion the raising of the free surface of the liquid 

 air in the Dewar vessel surrounding C invariably produced a very 

 appreciable diminution in the gas pressure in the apparatus, and the 

 lowering of the free surface as the evaporation of the liquid air pro- 

 ceeded resulted in a distinct rise in the gas pressure. It is to be un- 

 derstood that the free surface was never allowed to fall as low as the 

 top of the tube C, so that all of the charcoal was always below the free 

 surface of the liquid air. 



The following results show how the decrement changed with the 

 time in the final forty-eight hours : 



May 29, 12 m. to 2: 53 \. M. Decrement 0.0O0O51 



7: 1") l". m. to 8:58 0.000037 



10: 53 i". M. to 1 : 36 a. m. (May 30) 0.00003] 



May 30, 1 : 36 A. M. to 4:48 0.000028 



11:11 \. M. to 2:06 P. M. 0.000087* 



2:06 P. M. to 5: 27 0.000024* 



5:27 p. m. to 8: 21 0.000028* 



8:21 p. M. to 11:49 0.000022 



The smallest value of the decrement obtained was 0.000022, and this 

 could be measured moderately well. Its error cannot, I think, be as 

 much as ten per cent. Of course, it is clear that the true value of the 

 decrement due to the friction in the fibre is somewhat less than this, 

 for there is still, doubtless, some gas left to offer resistance to the 

 moving disk, so that the number to be used for /x in the above equa- 

 tion should be somewhat smaller than 0.000022. I have ventured to 



3 The liquid air used in this investigation was obtained at the Chemical 

 Laboratory, Harvard University. 



* These were taken in the afternoon when there is considerable jarring of 

 the apparatus and are probably not so accurate as the others. 



