ON THE COMPOSITION OF THE AIR OF THE GROUND-ATMOSPHERE. BY WM. RIPLEY NICHOLS, PROFESSOR OP GENERAL CHEMISTRY IN THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY. ON THE COMPOSITION OF THE AIR OF THE GROUNDATMOSPHERE. That the atmosphere of invisible gas which surrounds us on every side, and which we call the air, is a ponderable substance; that it exerts a definite pressure upon every surface upon which it rests, a pressure which may be measured in pounds, these are facts with which we are familiar from childhood, although we do not ordinarily take cognizance of this substance, except when it is in motion, nor even then, unless it is moving with a velocity greater than one hundred feet per minute. Less familiar to our minds than the above-mentioned facts, is the consideration that the ground beneath our feet is penetrated to an indefinite depth by the atmosphere; it is a fact which we, of course, know, but which we do not generally recognize. We observe that water spilled upon the ground soaks into it sooner or later, but when we speak of the porosity of a soil, we are more apt to think of the possibility of filling the pores or interstices of the soil with water than of the fact that, under ordinary circumstances, those pores or interstices are filled with air. In fact, we may determine the relative porosity of different soils by taking vessels of equal size, filling them completely with the soils in question, and then pouring in as much water as the vessel will hold in addition. The water takes the place of air previously confined in the soil, and the volume of air displaced is measured by the volume of water absorbed. Nor is this true simply of sand, gravel and of soils which are recognized as porous; even firm rocks in many cases will absorb a considerable amount of water or, in other words, contain a considerable amount of air. Some varieties of sandstone are thus made up of air to the extent of one-third of their bulk. If this conception of the presence of air throughout the ground on which we walk is unfamiliar to many, it is far from being generally known that this "ground-atmosphere "* differs in composition from the atmosphere of the air. It is made up of the same gases,-oxygen, nitrogen and carbonic acid, but the proportion of these gases in the atmosphere beneath our feet is subject to great variation, while in the atmosphere about us it is nearly constant; moreover, the proportion of carbonic acid is considerably greater in the ground-air than in the air of the atmosphere. It is, indeed, only a very short time since investigations into the character of the ground-air were begun, as having possible reference to hygienic questions. Boussingault, to be sure, in 1852, investigated the composition of the atmosphere confined in the upper portion of the ground. His experiments were made, however, on air taken at an average depth of only about fifteen inches, and the matter was considered simply from the stand-point of the agricultural chemist.† In the year 1854, Pettenkofer, ‡ in his investigations on the cholera, called attention to the fact that the air in the ground deserved attention as well as the ground-water. It was not, however, until the year 1870,§ that he began to carry on systematic investigations. These were directed principally to determining the amount of carbonic acid present at various depths. The experiments were conducted in the alluvial gravel of the plain in which Munich is situated, in a locality which had not been subjected to cultivation, and it was found that, as a rule, the amount of carbonic acid was very much greater than in the air of the atmosphere, that the amount of this gas increased with the depth, and that the amount varied with the season, being greatest in summer and least in winter. To illustrate the largest amount at a depth of 4 meters (13 feet) below the surface was 18.38 parts in 1,000 The term "ground-water" as a translation of the German "Grundwasser" has already obtained a foothold in English; there seems to be no reason why "Grundluft" should not be translated "ground-atmosphere" or "ground-air." + Boussingault et Léwy. Memoire sur la composition de l'air confiné dans la terre végétale. [Annales de chimie et de physique, [3], xxxvii (1853), pp. 5-50.] Hauptbericht über die Cholera, von 1854, in Bayern, p. 377. § Max von Pettenkofer. Ueber den Kohlensäuregehalt der Grundluft im Geröllboden von München in verschiedenen Tiefen und zu verschiedenen Zeiten. [Zeitschrift für Biologie, vii. (1871), p. 395.] |