252 74 



dioxide naturally becomes greater. This has probably some relation to the fact 

 that the thermal activity at Reykjafoss is not so intense as at Hengill. 



The gas sample Reykjafoss No. 1 is quite different to the three others, and 

 shows clearly that the spring gas in question is mixed with atmospheric air. 

 Calculating how much air is mingled with the original spring gas, we find that 

 about 2'/3 volumes of air are mixed with one volume of the original spring gas. 

 In this calculation I have assumed that the spring gas has originally had the same 

 composition as the three other gas samples from Reykjafoss, and also that no 

 carbon dioxide has been lost by intermixture or on ils way to the surface of the 

 earth. One observes that the proportion between the nitrogen and the oxygen is 

 very different from that in the atmosphere, in that the greater part of the oxygen 

 in the spring gas has disappeared, i. e. entered into 'chemical combinations in the 

 interior of the earth. 



As the spring from which the gas sample Reykjafoss No. 1 comes is also a periodic 

 alkaline spring, it is very probable that the intermixture of atmospheric air with the 

 spring gases has some connection with the periodicity of the hot springs. The 

 spring gases from Hveravellir supply evidence to confirm this. For one is doubt- 

 less quite safe in concluding that the oxygen and the comparatively large amount 

 of nitrogen in these spring gases are of atmospheric origin, while the carbon dioxide 

 and the small quantities of sulphuretted hydrogen and hydrogen gas may be 

 regarded as remnants of the original spring gas, which has probably had a similar 

 composition to the spring gases from Kerlingarfjöll and Hengill. 



According to the analyses before us, the spring gases from Hveravellir may 

 be divided into two groups. 



One is represented by the two gas samples : Hveravellir Nos. 2 and 25, which 

 come from hot springs apparently not periodic, situated in the north-west part of 

 the spring district. The three other samples, i. e. Nos. 10, 27 and 16 belong to the 

 second group, all coming from periodic springs. 



These gas samples are distinguished from the two first mentioned by the 

 great amount of nitrogen they contain in comparison with carbon dioxide and by 

 a considerable quantity of oxygen. Taking it for granted that the oxygen and the 

 nitrogen are of atmospheric origin, this shows that the spring gas in periodic 

 springs has received a greater contribution of air than that in the non-periodic 

 springs. The connection between the periodicity of the springs and the amount 

 of atmospheric gases in the spring gas is here shown very clearly. It is most prob- 

 able that the periodicity of the springs is a direct cause of the intermixture of 

 air with spring gases. 



The hot spring No. 12 at Hveravellir will perhaps be the means of throwing 

 considerable light on this subject. When p. Thoroddsen visited Hveravellir in the 

 summer of 1884, this spring was found to be periodic. In the summer of 1906 the 

 basin was occasionally filled with tranquil water, but generally it was quite dry, 

 and showed no signs of activity. As far as I know, this spring did not once spout 



