1904.] Compressibilities and Atomic Weights of Oxygen, etc. 153 



" On the Compressibilities of Oxygen, Hydrogen, Mtrogeif] and 

 Carbonic Oxide between One Atmosphere and Half an 

 Atmosphere of Pressure, and on the Atomic Weights of 

 the Elements concerned. — Preliminary Notice." By Lord 

 Eayleigh, O.M., F.RS. Eeceived February 3, — Bead 

 February 11, 1904 



The observations now referred to were conducted with an apparatus 

 designed upon the same lines as that already described.* It must 

 suffice to mention that the only important modification lay in the fact 

 that the two single volumes, which, when employed toother, con- 

 stitute the double volume, were used separately and alternately, so as 

 to eliminate in each set of measurements any question as to what the 

 ratio of these volumes exactly is. It is hoped to give a full description 

 of the method when it has been extended to the examination of other 

 gases, such as nitrous oxide and carbonic anhydride. The tem- 

 peratures ranged from 10° — 15°, and care was taken that in each 

 measurement the mean temperatures should be almost exactly the 

 same for the single and for the double volume. 



The results were reduced much as previously explained, and give 

 for the values of B, which, according to Boyle's law, should be 

 unity, 



Oxygen 1*00040 



Hydrogen 0*99976 



Nitrogen 1-00017 



Carbonic oxide 1-00028 



B here denotes the quotient of the value of pv at the half atmosphere 

 by the corresponding value at the whole atmosphere. That it would 

 be less than unity in the ease of hydrogen, and exceed unity for the 

 other gases, is what would be anticipated from their behaviour at 

 higher pressures. 



If we measure p in atmospheres, and assume, as has usually 

 been done, e.g., by Regnault and Van der Waals, that at small 

 pressures the equation of an isothermal is 



pv = PV(l+ap), 



where PV is the value of the product in a state of infinite rarefaction, 

 then 



a = 2(1- B). 



Probably the chief interest of a knowledge of the coefficient a is 



* " On the Law of the Pressure of Gases between 75 and 150 Millimetre* of 

 Mercury," ' Phil. Trans.,' A, vol. 198, pp. 417—430, 1902. 



