18 On Boyle s Law at Very Low Pressures. 



Baly and Ramsay consider the coefficient of expansion of 

 hydrogen is proved by their experiments to remain at 1/273 

 down to a pressure of '4 mm., below which there are the three 

 observations of 1/276 at *25 mm., 1/297 at *096 mm., and 

 1/300 at *077 mm., which they consider to prove a diminution 

 of the coefficient with diminution of pressure ; but it seems to 

 me that the sudden onset of the diminution is suggestive of 

 some unrecognized source of experimental error, such, for 

 instance, as electrification of the glass of one of the gauges 

 with alteration of the capillaiy correction for the meniscus of 

 mercury in the capillary volume-tube which amounts to 

 7 mm. of mercury. An error due to such a cause would, of 

 course, produce more effect in altering the apparent coefficient 

 of expansion the lower the pressure. 



For nitrogen Baly and Bamsay found the following 

 reciprocals of the coefficients of expansion : — 



p 5-3 mm. 497 3 11 -8 -6 



304 309 302 304 331 from 301 to 377 



Mean 343 



Although these values all mean coefficients smaller than 

 1/273, it seems to me that the sudden change from 304 at 

 1*1 mm. to 331 at '8 mm., so analogous to the sudden change 

 with hydrogen near the same region of pressure, is suggestive 

 of a hidden source of error such as that already indicated, and 

 casting doubt on the measurements even at 5 mm. Baly and 

 Bamsay mention that at pressures below *6 mm. some un- 

 ascertained cause of irregularity came in and caused their 

 method to give worthless results — a fact which is again 

 suggestive of the irregular operation of a source of error such 

 as variable electrification. With oxygen, Baly and Bamsay 

 found coefficients of expansion larger than 1/273 at pressures 

 from 5 to *07 mm., which in the special paper on oxygen will 

 be accounted for by dissociation of ozone at the higher tem- 

 perature. If the coefficient for nitrogen is really smaller than 

 1/273 at low pressures, this would be most readily accounted 

 for by association of the nitrogen molecules at the higher 

 temperature with possible production of an allotropic form of 

 nitrogen ; therefore, on all accounts, a continuation of Baly 

 and Bamsay' s research is desirable. 



The contentions of the present paper may be summarized 

 thus : — Departures from Boyle's law found experimentally in 

 rarefied gases may be due either to a true failure of Boyle's 

 law in the highest sense, that is to say, to a change in the 

 kinetics of all molecules, or to only an apparent failure, which 

 may be due to actions at the walls of the containing vessel or 



