Ammonia is written by the chemist NH₃, or one part of nitrogen gas to three parts of hydrogen. It is a colorless, transparent gas with a very penetrating, characteristic odor. In concentrated form it causes suffocation. It is but little more than half as heavy as air. It is easily converted into liquid form by pressure and cold. When pressure is removed from the liquefied ammonia, it passes back very rapidly into gaseous form, and in so doing it absorbs heat. Investigators have taken advantage of these facts and are employing liquid ammonia in the manufacture of artificial ice. [Sidenote: Importance of proportioning food] While air is merely a mixture of oxygen and nitrogen, this does not prove that these two elements cannot unite. In fact they do unite in five different proportions so as to form five different substances. These are given below to illustrate how different substances can be formed from the same things, by merely combining them in different proportions. This example is also given to impress upon the mind of the practitioner the great importance of proportioning nutritive elements in diet so that the patient will not be overfed on some elements while underfed on others. It is absolutely essential, in order to know what effect a substance will have in the laboratory, or in the body, to know not only of what it is composed, but with what substances and in what proportions it is combined. Nitrous oxid .....N₂O Nitric oxid ......NO or N₂O₂ Nitrogen trioxid .. N₂O₃ Nitrogen peroxid.. NO₂ or N₂O₄ Nitrogen pentoxid N₂O₅ To further illustrate the wonders of chemical combinations, we give the properties of two of these oxygen and nitrogen compounds: [Sidenote: Properties and uses of nitrous oxid] Nitrous oxid, N₂O, is colorless, transparent, and has a slightly sweetish taste. When inhaled it causes a kind of intoxication which manifests itself in the form of hysterical laughing, hence it is commonly called "laughing gas." Inhaled in larger quantities it causes unconsciousness and insensibility to pain. It is, therefore, used in many surgical operations, particularly by dentists in extracting teeth. Nitrogen peroxid, NO₂, is a reddish-brown gas. It has an extremely disagreeable odor and is very poisonous. [Sidenote: Composition of nitric acid] [Sidenote: Properties of nitric acid]