PART I.] The Carbonic Acid in Upper and Lower Layers of Soil. 191 



the difference in amount being least during the period of maxima, and great when 

 the amount of carbonic acid is low. 



In Munich, the points of maxima and m^inima appear to be determined by 

 temperature, whereas in Calcutta, as we shall see further on, this is not the case — 

 moisture being the apparent determinant. 



(c) — The quantities of Carbonic Acid 'present at different times in the Upper and 

 Lower Layers of Soil in Calcutta. — {Charts II and IV.) 



The chart illustrating the proportion of carbonic acid present in the layers 

 of soil of the first locality selected for observation (Tubes No. 1) shows the weekly 

 averages of the gas in 1,000 volumes of soil-air. There is not much calling for 

 comment on this point; as the principal phenomena of the fluctuations in amount 

 of carbonic acid have been already pointed out. 



One curious phenomenon appears in regard to both layers of soil, namel}', a 

 sudden short rise in the amount of carbonic acid during the month of January. 

 The amount of carbonic acid present in the upper layer in July 1874 was almost 

 identical with that at the corresponding period of the previous year ; while that 

 in the lower layer was greater in the second than in the first year. In the upper 

 layer a rapid rise is visible in June 1874, whilst in the lower the amount continued 

 low until the close of the observations. So far as the evidence goes, it would appear 

 that the period of minimum begins later, and is continued to a later date in the 

 lower than in the upper layer. 



The sudden depression in the upper layer in May is very remarkable, and no 

 corresponding phenomenon occurred in the case of the lower layer. Various of these 

 special phenomena characterising the separate layers may, apparently, be explained, 

 as will appear further on, but in the meantime attention is merely directed to them. 

 The relations between the quantities of carbonic acid estimated in the upper 

 and lower layers of soil in the second locality selected for observation — the set of 

 tubes No. 2, Diagram IV — resembled those in the former locality, in so far that the 

 amount of gas present in the lower layer of soil continuously exceeded that in the 

 upper one. The absolute differences in the quantities present in the layers were, 

 however, less. 



The absolute minimum, in the upper layer occurred in January with 3'8 volumes 

 per 1,000, but second periods of extreme depression occurred in February and July. 

 The maximum for the period of observation occurred in January— 6 volumes per 

 1,000. 



There were two periods of maximum amounts of carbonic acid in the lower 

 layer, the first in January — 7 volumes per 1,000 ; the second in May, also with 7 

 volumes per 1,000. 



