[40] The course of nitrification may be followed by means of chemical tests. 1. The disappearance of ammonia. 2. The appearance of nitrite. 3. Its disappearance. 4. Appearance of nitrate. [41] Professor Warington, in Report IV. (p. 526) of his admirable series of papers on the subject, draws attention to Müntz's criticism that the nitrifying organisms only oxidise from nitrogenous matter to nitrites, and not from nitrites to nitrates. Müntz held that the conversion of nitrite into nitrate is brought about by the joint action of carbonic acid and oxygen. Professor Warington's experiments, however, clearly illustrate that the production of nitrates from nitrites in an ammoniacal solution can be determined by the character of the bacterial culture with which the solution is seeded, and that in a solution of potassium nitrite conversion into nitrate can be determined by the introduction of the nitric organism. Professor Warington still adheres to the opinion, in favour of which he has produced so much evidence, that the formation of nitrates in the soil is due to the nitric organism which soil always contains. [42] British Association for the Advancement of Science, Bristol, 1898, Presidential Address. [43] British Association for the Advancement of Science, Bristol, 1898, Presidential Address. [44] Sir John Lawes and Sir Henry Gilbert (_Times_, December 2, 1898), have pointed out that the addition of nitrates only would be of no permanent use to the wheat crop. They rely upon thorough tillage and proper rotation of crops as the means of improving the nitrogen value of the soil. [45] Geddes, _Nature_, xxv., 1882. [46] Sir Henry Gilbert, F.R.S., _The Lawes Agricultural Trust Lectures_, 1893, p. 129. [47] _Ibid._, p. 140. [48] This has been denied recently in the official report by the chemist of the Experimental Farm to the Minister of Agriculture at Ottawa (_Report_, 1896, p. 200). [49] It has already been pointed out that the nitrifying bacteria, though able to live on organic matter, do not require such either for existence or for the performance of their function. [50] Lehmann and Neumann, p. 305. [51] The conditions requisite for an outbreak of enteric fever were, according to Pettenkofer, (_a_) a rapid fall (after a rise) in the ground water, (_b_) pollution of the soil with animal impurities, (_c_) a certain earth temperature, and lastly (_d_) a specific micro-organism in the soil. These four conditions have not, particularly in England, always been fulfilled preparatory to an epidemic of typhoid. Yet the observations necessary for these deductions were a definite step in advance of mere dampness of soil. [52] _Supplement to the Report of the Medical Officer of the Local Government Board_, 1887, p. 7. [53] _Report of Medical Officer to Local Government Board_, 1895-1896, Appendix.