Chemical and Geological History of the Atmosphere. 435 



It will be noticed, however, that the ratio of the change of 

 resistance to the change of length is different for different 

 loads, so that different factors have to be used in bringing- 

 them into agreement, showing that the effect of loading is 

 different in the two cases. This is also seen by comparing 

 the set of resistance-curves in fig. 1 with the length-curves 

 in fig. 2. 



As may be seen by a comparison of the curves given by 

 Nagaoka and Barlow, there is no similarity between the 

 curves of change of length and change of resistance in iron. 



© © © 



The change of length is in this case different in sign at 

 high and at low fields, being a contraction in the former and 

 an elongation in the latter case. There is no such change of 

 sign in the change of resistance. It may be conjectured that 

 this is in some way connected with the Yillari reversal which 

 occurs in iron, and this view is perhaps supported by the fact 

 that there is a similar reversal in nickel at very low fields*, 

 where, as mentioned above, the greater part of the discre- 

 pancy shown by the present experiments arises. 



The experiments were made in the Physical Laboratory 

 of the University College of North Wales, and I desire to 

 express my best thanks to Prof. Taylor Jones for placing the 

 necessary apparatus at my disposal, and also for many 

 suggestions made in the course of the work. 



Bangor, July 1902. 



XLIX. The Chemical and Geological History of 

 the Atmosphere. By John Stevenson, M.A., FJ.C.\ 



II. — The Composition and Extent of the Atmosphere in 

 very Primitive Times. 



IN my former paper (Phil. Mag., Sept. 1900, p. 312, and 

 Oct. 1900, p. 399) it was shown that there was good 

 reason to believe that there was no free oxygen in the 

 primitive atmosphere of the earth, and even that reasons 

 could be given in favour of the view that considerable 

 quantities of hydrogen or of hydrocarbon gases were present 

 in the primitive atmosphere. We estimated that if the crust 

 of the earth contained on the average yL- of .1 per cent, 

 of carbonaceous matter, there would be as much of such 

 oxidizable matter in each half-mile thickness of the crust of 

 the earth belonging to the continental and transitional areas as 



* Heydweiller, Wied. Ann. vol. Hi. p. 228 (1894). 

 t Communicated by the Author. 



