49 



on which allows a few bubbles to pass at a time. The heated chloride of calcium bath 

 is next raised, so that the bulb retort is almost submerged; the temperature, shown 

 by a thermometer which forms part of the apparatus, should be 130 to 140 C. By 

 boiling small quantities of water or hydrochloric acid in the bulb retort in a stream 

 of carbonic acid the air present is expelled ; the supply of carbonic acid must be stop- 

 ped before the boiling has ceased, so as to leave little of this gas in the retort. Pre- 

 vious to very delicate experiments it is advisable to introduce through the funnel 

 tube a small quantity of nitre, ferrous chloride, and hydrochloric acid, rinsing the 

 tube with the latter reagent ; any trace of oxygen remaining in the apparatus is then 

 consumed by the nitric oxide formed, and after boiling to dryness, and driving out 

 the nitric oxide with carbonic acid, the apparatus is in a perfect condition for a quan- 

 titative experiment. 



The mixture of the extract with ferrous chloride and hydrochloric acid is introduced 

 through the funnel tube, and rinsed in with three or four successive half cubic centi- 

 meters of hydrochloric acid. The contents of the retort is then boiled to dryness, a 

 little carbonic acid being from time to time admitted, and a more considerable quan- 

 tity used at the end to expel any remaining nitric oxide. 



The gas is collected in a small jar over mercury. The gas analysis is of a simple 

 character ; the gas is measured after absorption of the carbonic acid by potash, and 

 again after absorption of the nitric oxide, the difference giving the amount of this gas. 

 For the absorption of nitric oxide, a saturated solution of ferrous chloride was for some 

 time employed. This method is not, however, perfectly satisfactory when the highest 

 accuracy is requred, the nitric oxide being generally rather underestimated, except 

 the process of absorption is repeated with a fresh portion of ferrous chloride. The 

 error is greater in proportion to the quantity of un absorbed gas present. The use of 

 ferrous chloride as an absorbent for nitric oxide has now been given up, and the oxy- 

 gen method substituted. All the measurements of the gas are now made without 

 shifting the laboratory vessel : the conditions are thus favorable to extieme accuracy. 



The chief source of error attending the oxygen process lies in the small quantity 

 of carbonic acid produced during the absorption with pyrogallol ; this error becomes 

 negligible if the oxygen is only used in small excess. The difficulty of using the oxy- 

 gen in nicely regulated quantity may be removed by the use of Prof. G. Bischof's re- 

 cently invented "gas-delivery tube." This may be made of a test tube, having a 

 small perforation half an inch from the mouth. The tube is partly filled with oxy- 

 gem over mercury, and its mouth is then closed by a finely perforated stopper made 

 from a piece of wide tube, and fitted tightly into the test tube by means of a covering 

 of rubber. When this tube is inclined, the side perforation being downward, the 

 oxygen is discharged in small bubbles from the perforated stopper, while mercury 

 enters through the side opening. Using this tube, the supply of oxygen is perfectly 

 under control, and can be stopped as soon as a fresh bubble ceases to produce a red 

 tinge in the laboratory vessel. The trials made with this apparatus have been very 

 satisfactory. 



REMARKS. 



Where such a complete analysis of a soil is not required, as that for 

 which the directions are given in the preceding pages, the estimation 

 of potash, soda, phosphoric acid, nitrogen, and lime will give valuable 

 information for judging of its fertility. 



The following qualitative tests may be applied in case only a very 

 preliminary examination is required. 



Test the slightly moistened soil with litmus paper; if this should show 

 an acid reaction, the presence of an excess of humic acids, or small 

 quantities of sulphate of iron, may be suspected. All good and fertile 

 13735 No. 10 4 



