_Estimation of the Oxygen._—The oxygen is estimated in a separate sample of the gas by means of potassium pyrogallate. The chief objection to this simple process is the uncertainty of being able to obtain an average sample of the occluded gas. In digging the hole and refilling, there must evidently be a considerable disturbance of the original distribution of the gas or vapor. The methods of Pettenkofer[185] and Aubry[186] are essentially like that just described. Pettenkofer found the largest quantities of carbon dioxid in the earth gases in July, August, and September, and the smallest quantities in the winter months. No greater detail concerning these methods of the direct aspiration of the air is considered necessary inasmuch as the methods about to be described, while more elaborate, are superior in accuracy to the older methods mentioned. In general, in these experiments, it is deemed sufficient to determine the carbon dioxid only. [Illustration: FIGURE 62. SCHLOESING’S SOIL-TUBE FOR COLLECTING GASES. ] =285. Method of Schloesing.=—The apparatus used by Schloesing[187] in the collection of the soil gases consists of a steel tube (Fig. 62) a little over one meter in length, ten millimeters in external diameter, and one and one-half to two millimeters in internal diameter. The end which penetrates the soil is made slightly conical for a distance of twenty-five to thirty centimeters. By reason of the shape of the tube, when it is driven into the soil all connection between the orifice in the point of the tube and the external air is prevented. The obstruction of the internal canal of the tube is prevented by introducing a thread of steel which penetrates the whole length of the tube. This thread, represented by A, B, C, D, is flush with the interior extremity of the tube at D. It extends for about three centimeters above the upper end of the tube in order to be easily handled when it is to be removed.