“You must have noticed the strong, penetrating odor prevalent in ill-kept water-closets; and you have also perceived the same odor when soiled garments are cleaned with a certain liquid that looks like clear water. Well, this odor, so pungent that it almost produces the effect of fine needles thrust up into the nostrils and brings tears to the eyes, is the odor of ammonia. “Ammonia is an invisible gas capable of being taken up in large quantities by water, the mixture being known as aqua ammoniæ, or water of ammonia. Combined with other substances ammonia loses its pungent odor and forms compounds which are among the most effective fertilizers. These compounds furnish vegetation with one of its essential ingredients called nitrogen. By itself nitrogen is an odorless and colorless gas. In this state it forms four-fifths of the volume of ordinary air, the air we breathe. The other fifth is composed of a second gas called oxygen, also colorless and odorless. It is oxygen that our lungs demand when we breathe, and it is oxygen that is necessary when we wish to burn anything. It is this alone that plays its invaluable part in the combustion of certain substances in our blood and in the generation of natural heat; it is this that in the process of combustion releases carbon, phosphorus, sulphur, and other combustibles, to combine with them and produce a compound known as carbonic acid gas in the case of burnt carbon, phosphoric acid in the case of phosphorus. In fact, to it belong the properties that we have until now attributed to the atmosphere as a whole. As for nitrogen, it has no other purpose in the atmosphere than to moderate by its presence the too violent energies of oxygen; it plays there the part of the water that we put into too strong wine.