VARIATIONS OF THE AMOUNT OF C0 2 IN THE GROUND- AIR. 699 



Fodor's work extended over three years. The amount of carbonic acid in the air of a 

 uniform soil increases with the depth. This arises, not from increased chemical change 

 taking place, nor necessarily from more organic impurity being present, but simply because 

 the carbonic acid does not get so easy vent as in the more superficial layers. 



The soils on which these experiments were made were — (1) the grounds of the Edin- 

 burgh Eoyal Infirmary, and (2) the grounds of Heriot's Hospital. On the Infirmary site, 

 the part of the soil examined is for 3 to 4 feet made soil. Below this the soil is natural. 

 The upper made soil had a medium permeability, and contained 12 to 13 per cent, 

 of clay, and 73 per cent, sand, gravel, and small stones. It was part of a kitchen 

 garden, but had not been used for 1 year before the experiments. The organic matter 

 in it yielded 0"21 per cent, nitrogen. 



The ground of Heriot's Hospital was stiff and clayey, under grass, and had not been 

 disturbed for nearly 200 years. It may be taken as a typically pure soil. In the 

 Infirmary grounds the determinations were made daily, morning and evening, from June 

 1, 1887, to July 20, 1888. At the 3-feet tube there was no intermission except for 

 about three weeks in May 1888, and on rare occasions when, by the rise of ground-water, 

 the tube was blocked. The tubes were sunk within a distance of 3 feet from each other, 

 at depths of 3 feet, 6 feet, and 1 2 feet. From the low-lying nature of the soil, and the 

 nearness of the clayey subsoil to the surface, the 12-feet and even the 6-feet tube were 

 liable to frequent blocking with ground- water. This is the reason of the fewness of the 

 determinations at these depths. In the Heriot's Hospital grounds the determinations 

 were made in the morning only, on an average, three times weekly. 



Plate 1 shows the comparison between the weekly averages of the morning and 

 of the evening 3-feet determinations at the Eoyal Infirmary, with the weekly averages 

 of the daily maximum and minimum temperatures. 



It will be noticed that from September to the middle of December the evening 

 determinations are lower than the morning. From the middle of December to the end 

 of March they are higher, while from April to the end of June they are again lower, with 

 a tendency to again become higher in July and August. In another paper on atmos- 

 pheric air, I hope to discuss the bearings of this in fuller detail. 



Plate 2 shows the weekly averages of the morning and evening 3-feet determina- 

 tions combined, with the mean daily temperature. 



Plate 3 shows the comparison between the monthly averages of the amount of 

 carbonic acid at a depth of 3 feet in the ground -air of the typically pure soil of Heriot's 

 Hospital grounds and the comparatively impure soil of the grounds of the Eoyal Infirmary. 

 The small proportion of carbonic acid in the former, as compared with the latter, is very 

 marked, especially as the former is less permeable. 



Tables 1 to 6 show the detailed statement of all the experiments. 



The amount of carbonic acid in the 3-feet determinations at the Infirmary ground 

 being greater than that of those at 6 feet and 12 feet, is an exception to the rule that the 

 amount of this gas in ground-air increases with the depth. But this is explained by the 



