Without displacing the tube from the soil, several samples of gas can be taken from the same spot. A sufficient number of the bulbs V should be at hand to hold the required number of samples. Instead of submitting the sample to eudiometric analysis it is usually sufficient to determine the quantity of carbon dioxid which it contains, inasmuch as numerous experiments have shown that in 100 parts of soil gas the oxygen and carbon dioxid together constitute twenty-one parts. No appreciable trace of marsh gas, or other combustible gas, has yet been detected in ordinary arable soils. These gases have only been found in special soils from marshes, in the neighborhood of gas wells, etc., and not in arable soils. [Illustration: FIGURE 64. SCHLOESING’S APPARATUS FOR DETERMINATION OF CARBON DIOXID. ] =286. Apparatus for Estimating the Carbon Dioxid.=—The apparatus used for determining the carbon dioxid in Schloesing’s work consists of the apparatus shown in Fig. 64. A represents a glass vessel surrounded by a jacket of glass, full of water, and sealed on its lower part to the tube BC of about six millimeters internal diameter. On its upper part it is sealed to the capillary tube D. The tube BC is graduated from C in hundredths of the volume of DAC, which volume is about twelve cubic centimeters. On its lower part it is connected by a rubber tube with a reservoir F which is capable of being raised or lowered. GHK are capillary tubes connected together by the rubber tubes L and M, which are furnished with pinch-cocks. The tube G is connected to a vacuum by the rubber tube N. The rubber tube should be of very small internal diameter and from forty to fifty centimeters in length. To the tube H are sealed, at right angles, the branch D and another branch O. This last dips into a little mercury which the tube P contains. It serves as a valve, permitting the exit of the gases but not their entrance. The tube K carries some lines engraved on its inferior part and is sealed to the system of the two bulbs Q and R. The bulb Q contains a concentrated solution of potash. It carries a number of pieces of glass tubing for the purpose of increasing the surface of the potash solution. All the parts of the apparatus are fixed upon a rectangular board, nineteen centimeters broad by twenty centimeters long. This forms one of the faces of a wooden box to which it is hinged and which serves for the transportation of the apparatus in a vertical position. The graduation of the tube BC is recorded behind this tube upon a card fixed upon the board. By means of these two graduations, the height of the mercury in the tube BC is most easily read, even when the tube is not perfectly vertical. Each one of the pinch-cocks L and M, on its upper part is fixed in a sort of guard which prevents it from being displaced laterally during the processes of the manipulation, thus avoiding all danger of breakage.