The ammonia is prepared by the action of finely powdered lime on ammonium chlorid, and is dried by passing over lime and sticks of potassium hydroxid. The air which is to be saturated with aqueous vapor, in order to purify it from dust, carbon dioxid, and ammonia, is passed through two flasks in which are contained respectively, diluted sulfuric acid and potash lye. It is afterwards thoroughly saturated with aqueous vapor at the temperature desired. Various kinds of soil material may be employed as follows: (1) Pure quartz sand.—Freed from all fine particles by subjection to silt analysis, afterwards boiled with hydrochloric acid and washed with water to free it from all clayey materials. The sand prepared in this way should be passed through different sieves in order to prepare it in different states of fineness. (2) Quartz powder.—Prepared from pure quartz crystals by grinding in an iron mortar. (3) Kaolin.—Material such as is used in the manufacture of the finest porcelain which, after being freed of all foreign matter, is rubbed to a fine powder in a porcelain mortar. (4) Humus.—Washed with ether and alcohol, boiled with hydrochloric acid, washed, dried and reduced to a state of fine powder. (5) Iron oxid. (6) Calcium carbonate.—Precipitated, washed, and dried. (7) Soil mixtures.—Prepared artificially by mixing the kaolin, quartz, and humus, above mentioned. The quantity of gas absorbed by each of these materials is determined by filling the tubes, as above mentioned, with the dried material. The content of each tube is previously determined by filling with mercury and weighing. Having determined the weight of the substance to the exclusion of the air contained within its pores, it is treated with the gas in the apparatus described above and weighed from time to time until no further increase of weight takes place. The method of calculating the results is shown in the following scheme: V = content of the absorption tube obtained by filling with mercury and weighing. P′ = weight of the empty tube filled with air at 100°. pl = weight of the air in the tube (pl = V × specific gravity of the air at 100°). pt′ = weight of the tube (pt′ = P′ − pl). P² (second weighing) = weight of the tube filled with the substance with the included air at 100°. v^s = volume of the substance calculated according to the formula v^s = (P² − P′)/(s^s − specific gravity of air). s^s = specific gravity of the substance. vl = volume of the air in the flask filled with the substance (vl = V − v^s). pl′ (weight of this included air) = vl × specific gravity. p^s = weight of the substance (pl = p² − pt′ − pl) P³ = weight of the apparatus at the end of the experiment. sg = specific gravity of the gas employed for saturation. pg (weight of the gas remaining over the substance) = vl × sg. pa (weight of the absorbed gas) = P³ − pt′ − p^s − pg.