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 might 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, Eodor 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.