48 



at about 55 C. : the drying is usually completed in twenty-four hours. As the tem- 

 perature of the room is one at which nitrification by an organized ferment does not 

 occur, it is probable that very little production of nitric acid takes place during the 

 operation. After dry ing, stones and roots are removed, and the soil is finely powdered 

 and placed in bottles. Soil samples thus prepared are not absolutely dry, but the 

 small amount of water present is apparently insufficient to allow of organic change. 



PREPARATION OF THE WATERY EXTRACT. 



From 200 to 500 grams of the dry powdered soil are taken, according to the sup- 

 posed richness of the soil in nitrates, and introduced on a large filter fitted for filtra- 

 tion under pressure. (Fres., $ 53, a.) The filter is previously moistened, the dry 

 soil introduced, and if the latter be of a loose texture, it is shaken firmly together, 

 but with a clay soil this is better avoided. The flask is connected with the air-pump, 

 the soil is kept moistened with water, and when 100 c. c. have run through, it may be 

 concluded that all the nitrates are washed out. This operation lasts from ten to forty- 

 five minutes, depending upon the nature of the soil. 



The watery extract thus obtained is placed in a small porcelain dish, the flask 

 washed well with water, and evaporated nearly to dryness on the water-bath. As 

 soon as cool, it is mixed with 1 c. c. of a cold saturated solution of ferrous chloride 

 and 1 c. c. of hydrochloric acid, both reagents having been boiled and cooled imme- 

 diately before use. The mixture is then ready to be introduced into the retort. 



DESCRIPTION OF THE MODIFIED SCHLOESING'S APPARATUS. 



The apparatus consists of a bulb retort If inches in diameter, the tubular of which 

 has been bent near its extremity to make a convenient juncture with the delivery 

 tube, which dips into a trough of mercury on the left ; the long supply tube attached 

 to the receiver is of small bore, and is easily filled by a half c. c. of liquid. The short 

 tube in the cork of the retort is also of small bore, and is connected by a piece of rub- 

 ber tubing fitted with a clamp to an apparatus for the continuous production of pure 

 carbonic acid. A long funnel tube, likewise passing through the cork of the retort, 

 completes the apparatus. 



The apparatus for the generation of the carbonic acid used is so arranged that the. 

 marble, which must previously be well boiled in w r ater to remove as much of the 

 oxygen present as possible, is contained in a lower reservoir into which the hydro- 

 chloric acid used, previously boiled, is introduced from a higher 'reservoir; thus the 

 former is always under internal pressure, and leakage of air from without cannot 

 occur. The hydrochloric acid, after it has been well boiled, has dissolved in it a 

 moderate quantity of cuprous chloride, and is then introduced into the upper reser- 

 voir and covered with a layer of oil. 



The presence of the cuprous chloride insures the removal of any dissolved oxygen, 

 and gives an indication by its change of color when this condition is exceeded ; as 

 long as it remains of an olive-green tint oxygen will be absent, but should the acid 

 become of a clear blue-green, it is no longer certainly free from oxygen, and more 

 cuprous chloride must be added. 



The reagents used must be freshly boiled and employed in as small a quantity as 

 possible. In boiling the hydrochloric acid it is well to add a few drops of ferrous 

 chloride, to be sure of removing any dissolved oxygen. 



The mode of conducting the operation is as follows : 



The apparatus previously described is fitted together, the long funnel tube attached 

 to the bulb retort being filled with water. Connection is made with the glass stop- 

 cock of the carbonic acid generator by means of a short, stout rubber tube, provided 

 with a pinch-cock. The pinch-cock being opened, the stop-cock is turned till a mod- 

 erate stream of bubbles rises in the mercury trough ; the stop-cock is left in this po- 

 sition, and the admission of gas is afterwards controlled by the pinch-cock, pressure 



