at Very Low Pressures. 17 



in rarefied gases (Ann. de Chim. et de Phys. [5] xxviii.), 

 maintains that Mendeleeff and his co-workers claim for their 

 measurements of low pressures a degree of accuracy that is 

 beyond present experimental possibility. He claims that his 

 data show that with present appliances one cannot read the 

 position of the top of a mercury column to within less than 

 *01 cm., which implies that at 20 mm.joB cannot be free from 

 liability to an error of 5 parts in 10,000, which is of the order 

 of the departure given by Mendeleeff and Hemilian for S0 2 ; 

 and while they obtain slight diminution of joB for H 2 down to 

 120 mm., Amagat gets an apparent slight increase at 6 and 

 3 mm., which, however, he speaks of as being of the order of 

 the unavoidable experimental error. Similarly, in the case of 

 C0 2 Amagat's results show that at 4 and 2 mm. a departure 

 from Boyle's law greater than experimental error could not 

 be found by his measurements. Thus it appears that when 

 oxygen and air are set aside as exceptions traceable to a 

 special cause, there is no satisfactory evidence of a failure of 

 Boyle's law in rarefied gases. 



The latest experimental paper on the subject is that of Baly 

 and Eamsay (Phil. Mag. [5] xxxviii.), the chief interest of 

 which lies in some startling experiences which they had with 

 oxygen and air, wherein Boyle's law appears to be completely 

 wrecked : for example, air which at 4*1 mm. of mercury gave 

 pB = 100, at 8 mm. gave pB = 9*4. This will be traced in the 

 separate paper to a liberation of free ions of oxygen, whose 

 electrical charges are responsible for the mischief. But with 

 hydrogen Baly and Ramsay found in a somewhat indirect 

 manner that Boyle's law holds down to 2*5 mm. of mercury, 

 and probably to 7 G0/10 7 , and the law of Charles down to 

 •4 mm. 00 2 behaved differently in two different gauges, 

 showing probably a different state of the glass in them, and 

 indicating that apparent departure of rare C0 2 in glass vessels 

 from Boyle's law is due rather to the chemical causes already 

 mentioned than to physical ones. Indeed, some other expe- 

 riments carried out by Baly and Ramsay are suggestive of 

 the existence of a dissociation pressure for the combination 

 between C0 2 and glass ; for they found that when the 

 pressure of C0 2 was about 1/10 6 atmo, 188 strokes with a 

 mercury-pump, supposed to reduce the pressure to *55 of its 

 original value at each stroke, failed to lower the pressure 

 at all, just as if the glass gave out and reabsorbed C0 2 in the 

 same way as a vessel lined with silver chloride and filled with 

 NH 3 at a suitable pressure would liberate and absorb NH 3 if 

 an attempt were made to pump it out with apparatus all lined 

 with AgOl. 



Phil Mag. S. 5. Vol. 43. No. 260. Jan. 1897. C 



