16 Mr. W. Sutherland on Boyle s Law 



We have now to consider the experimental evidence, and 

 ■will begin with the clearest case — that of oxygen, for which 

 Bohr discovered (Wied. Ann. xxvii.) a decided departure from 

 Boyle's law, and investigated it between pressures of 15 and 

 •1 mm. of mercury. As this departure in the case of oxygen 

 is due to its spontaneous change into ozone at low pressures, 

 it will he investigated in a separate paper. But since air 

 consists of about one volume of oxygen to four of nitrogen, it 

 is to be expected that the departure of the oxygen from Boyle's 

 law w 7 ill produce a departure of the air from that law, but at a 

 region of pressures five times as large as those in which the 

 deviations are perceptible for oxygen. The case of air will 

 therefore better be discussed in connexion with that for 

 oxygen. But w T hen once the departure of air from Boyle's 

 law is assigned to a special cause, the bulk of the experimental 

 evidence in favour of the departure of rarefied gases in general 

 from that law has disappeared, for naturally most of the 

 experimental evidence has related to air. That which has 

 attracted most attention is due to Mendeleeff, who seems to 

 have devoted a large amount of w r ork to the subject, but 

 accessible to nomBussian readers only in two short accounts 

 in the Ann.de Chim.et de Phi/s. (Mendeleeff and Kirpitscheff 

 [5] ii., and Mendeleeff and Hemilian [5] ix.). The most 

 important result obtained in the first of these papers is that 

 between 600 and 20 mm. of mercury pB for air diminishes 

 with diminishing p, and a series of numbers is given to 

 illustrate the case ; but in the next paper it is mentioned that 

 while diminution of pB with p between these limits of pressure 

 is always found, the determinations of its amount are not 

 afways accordant. The reason for this variation in the expe- 

 rimental numbers will be given when we are considering the 

 theory of the departure in oxygen and air. Of course it is 

 to be remembered that the departures of most gases from 

 Boyle's law at a few atmos pressure are such as make pB 

 increase wdth diminishing p, but towards the limit for a perfect 

 gas ; if, therefore, any tendency in the opposite direction is to 

 set in it must first neutralise the small residual tendency of pB 

 to increase with diminishing p before it can make itself seen 

 as an actual diminution, and this is why Mendeleeff and his 

 co-workers find a pressure of about 600 mm. as the one 

 at which diminution of pB begins to be apparent with 

 increasing rarefaction. The same investigators give data 

 to show that with H 2 , C0 2 , and S0 2 a diminution of pB with 

 p sets in at a certain pressure for each, but the amount of 

 diminution obtained is so slight that it can hardly be relied on. 

 Indeed Amagat, after carrying out a research on Boyle's law 



