12 PROCEEDINGS OF THE AMERICAN ACADEMY. 



boiling had proceeded for some hours. The nitrogen is the more vola- 

 tile, and so the boiling will proceed more vigorously just after a fresh 

 supply of air has been added than at any other time. Consequently 

 the temperature of the boiling liquid will be lower at first than it is 

 later, and the charcoal will thus absorb better at each addition of 

 liquid air to the Dewar vessel. The charcoal is necessary for the phe- 

 nomenon, for when the tube E was substituting for the tube containing 

 the charcoal, the effect disappeared, or became inappreciable. 



It was suggested earlier in the paper that there would be adduced 

 evidence to show that the mercury driven out from the apparatus col- 

 lected in the tube E. After the measurement of pressure and decre- 

 ment had proceeded down to the least value given in the table, the 

 supply of liquid air in the Dewar vessel in which E was placed was al- 

 lowed to disappear gradually. As the evaporation proceeded, it was 

 found that the decrement increased much more rapidly than the 

 pressure as indicated by the McLeod gauge, showing that vapor was 

 finding its way into the apparatus. 



Results. 



In the first and second columns of Table I are contained the corre- 

 sponding values of the decrement and pressure. Not all the numbers 

 given in these columns were obtained by actual measurement. Only 

 those which are marked with an asterisk were obtained in this way. 

 The others were obtained as follows : A curve was plotted using the 

 values of the pressure which were measured as abscissas and the corre- 

 sponding values of the decrement as ordinates. This curve was drawn 

 on such a scale that the value of the decrement, corresponding to any 

 arbitrarily chosen pressure, could be obtained from the cui've as accu- 

 rately as it could be measured by the apparatus. The unmarked num- 

 bers in the first two columns were obtained by choosing arbitrarily a 

 pressure and reading off from the curve the corresponding value of the 

 decrement. In no case has this procedure involved an extrapolation. 



After failing to obtain an analytical expression for the relation 

 between the logarithmic decrement and the pressure which would be 

 applicable over the whole range extending from very small pressures 

 right up to atmospheric pressure, it was decided to find, if possible, an 

 expression which would be applicable up to a certain pressure within 

 the range for which it is known that the McLeod gauge measurements 

 are quite reliable. Rayleigh 7 has shown that Boyle's Law holds down 

 to 0.01mm. of mercury, and Baly and Ramsay 8 found the McLeod 



7 Phil. Trans., 196 (1901). 8 Phil. Mgg., 38 (1894). 



