PART I.] Carbonic Acid in Soil in Relation to Rain-fall and Water-level. 193 



of carbonic acid at another. The lines of temperature neither directly nor conversely 

 correspond with those of carbonic acid. There is, however, one curious phenomenon 

 which comes out very distinctly during the period over which the observations extend, 

 and this is that the periods of maxi'nium, difference in the quantities of carbonic acid 

 in the two layers of soil coincide with the periods of inaximiim difference of temperature 

 in these layers. The viinimu'tn difference in the quantities of carbonic acid occurred in 

 August and September, and during the same period the minimum difference of 

 temperature also occurred. The two periods of maximum difference between the amounts 

 of carbonic acid in the two layers of soil were first in December, January and February, 

 and second in May ; at both of these periods maximum differences in temperature were 

 also present. Whether this be a mere coincidence we do not feel prepared to say, but it 

 may be pointed out that if the conditions of temperature be in any way causatively related 

 to the differences between the quantities of carbonic acid present in the layers of soil, 

 the essential element is the difference of temperature, not the absolute temperature of 

 either layer individually. The coincidence of m^aximum differences of temperature and 

 carbonic acid occurred at one time when the temperature of the lower layer of soil 

 exceeded that of the upper one, and at another when the reverse relation prevailed. 



(e) — Comparison of the aTuount of Carbonic Acid present in the Soil tvith the 



AtTTiospheric Temperature. — {Chart III.) 

 No clear relation of any kind can be observed to exist between the atmospheric 

 temperature and the amount of carbonic acid present in the soil — periods of 

 extreme elevation and depression of the latter occurring coincidently with conditions 

 of temperature showing no corresponding changes. 



(/) — Gom,parison of the amount of Carbonic Acid in the Soil ivith the Rain-fall. — 



{Chart III.) 

 In this case a general coincidence of conditions appears very distinctly, the 

 principal periods of rain-fall coinciding with the principal periods of elevation in 

 amount of carbonic acid, and the main periods of depression in the latter coinciding 

 with periods of drought. This general coincidence is, however, much closer and more 

 marked in reference to the carbonic acid in the upper than to that in the lower 

 layer of soil, for the amount of carbonic acid in the latter continues high long after 

 the cessation of the rains, and shows no immediate rise corresponding with their 

 commencement in the following season. 



{g) — Comparison of the aTnount of Carbonic Acid in the Soil tuith Water-Level. — 



{Charts II— III.) 

 Here also a general coincidence appears, but in this case the coincidence is 

 closer in regard to the lower than to the upper layer, as was seen to be the case 

 with the rain-fall. The elevation of water-level begins later and lasts longer than 

 the period of extreme elevation in the carbonic acid of the upper layer of soil. 



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