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XXIX. — On the Variations of the Amount of Carbonic Acid in the Ground-Air 

 (Grund-luft of Pettenkofer). By C. Hunter Stewart, B.Sc, M.B. (From the 

 Public Health Laboratory of the University of Edinburgh.) (With Three Plates.) 



(Read 6th June 1892.) 



The chemical examination of ground-air, i.e., the air which is contained in the pores 

 of the soil, was first made by Boussingault and Levy in 1853.* Their results, however, 

 attracted little attention till Pettenkofer, in 1857, pointed out that the determination of 

 the amount of carbonic acid in the air of a given soil mio-ht be used as a means of 

 estimating the organic decomposition going on there. In 1871 he first published his 

 results, and since that time the subject has been worked at by many investigators both 

 from the agricultural and hygienic point of view, including in the latter class Fleck at 

 Dresden, Fodor at Buda-Pesth, Hesse in Saxony, and Nicholls in America. As 

 researches of this nature have not attracted much attention in this country, a short 

 account of the modus operandi may be interesting as a preliminary. 



Iron tubes, with an internal diameter of 1 inch, and having lateral perforations for 

 4 inches from the lower end, are sunk into the ground. In sinking the tubes care must 

 be taken to disturb the ground as little as possible, and, further, that neither the sunk 

 open end nor the lateral perforations are tightly plugged. A solid pointed rod of the 

 same diameter as the tube is first driven into the soil to make a hole of the required 

 depth, and then withdrawn. The tube is now armed with a solid piece of iron, shaped 

 like a spear head, and driven into the hole for 3 or 4 inches deeper than is desired. 

 The tube is now withdrawn for 3 or 4 inches, which, separating the iron guard, leaves 

 the open end of the tube quite patent. The iron guard having a slightly greater diameter 

 than the tube, prevents the lateral perforations being plugged with the soil. The ground 

 is carefully stamped round the tube to prevent the direct entrance of atmospheric air. 

 To insure the complete settling of the soil round the tube, it is advisable to wait for a 

 week before beginning experiments. For convenience of working, the upper end of the 

 tube should project for about 3 feet above ground. This upper end, fitted with a 

 perforated rubber cork, is connected by glass and rubber tubing with a Pettenkofer 

 carbonic acid absorption tube, and this communicates with an ordinary water aspirator.t 

 A solution of barium hydrate, about 13 grammes to the litre, is used for absorbing the 

 carbonic acid. The pure barium hydrate of commerce often contains traces of alkaline 

 salts, the presence of which interferes with the accuracy of the titration by oxalic acid. 

 The addition of 0*2 grams neutral chloride of barium to each litre of baryta solution 



* Annales de Chemie etcle Physique, 1853. 



t Fig. A shows the apparatus and arrangement of sunk tube. 



VOL. XXXVII. PART. IIL (NO. 29). 5 M 



