The importance of isolating and studying them microscopically was recognised at an early period in these researches. Messrs Schloesing and Muentz were the first to attempt this. They reported that they had successfully accomplished this, and described the organism as consisting of very small, round, or slightly elongated corpuscles, occurring either singly or two together. According, however, to the most recent researches of Warington, Winogradsky, and P. F. Frankland, nitrification is not effected by a _single_ micro-organism, but by _two_, both of which have been successfully isolated and studied.[106] The first of these to be discovered and isolated was the _nitrous_ organism, which effects the conversion of ammonia into nitrous acid; the second, which has only been lately isolated by Warington and Winogradsky, effects the conversion of nitrous acid into nitric acid. Each of these ferments thus has its distinctive function to perform in this most important process, the nitric ferment being unable to act on ammonia, as the nitrous ferment is unable to convert nitrites into nitrates. Both ferments occur in enormous quantities in the soil, and seem to be influenced, so far as is at present known, by the same conditions. Their action will thus proceed together. Nearly all we know as yet on the subject of their nature is with regard to the nitrous ferment. _Appearance of Nitrous Organism._ Mr Warington[107] thus describes the appearance of the nitrous organism: "As found in suspension in a freshly nitrified solution, it consists largely of nearly spherical corpuscles, varying extremely in size. The largest of these corpuscles barely reaches a diameter of 1/1000th of a millimeter; and some are so minute as to be hardly discernible in photographs, although shown there with a surface one million times greater than their own. The larger ones are frequently not strictly circular. These forms are universally present in nitrifying cultures. The larger organisms are sometimes seen in the act of dividing." _Nitric Organism._ So far as at present known, the nitric organism is very similar in appearance to the nitrous organism, so much so that it is difficult to distinguish the one from the other. As the same conditions influence their development, the process may be regarded as a whole. _Difficulty in isolating them._ A great difficulty has been experienced in the attempt to isolate these micro-organisms for the purpose of studying their nature. This arises from the fact that they refuse to grow on the ordinary solid cultivating media used by bacteriologists. Winogradsky, however, has recently succeeded in cultivating them in _a purely mineral_ medium--viz., _silica-jelly_.[108] _Nitrifying Organisms do not require Organic Matter._