Nitrogen.—Nitrogen forms about four fifths of the atmosphere, where, like oxygen, it exists in a free state. It may be separated from the oxygen of an inclosed portion of air by causing that gas to unite with phosphorus. Place a piece of phosphorus the size of a pea in a depression in a flat piece of cork. (Handle phosphorus with wet fingers or with forceps.) Place the cork on water and have ready a glass fruit jar holding not more than a quart. Ignite the phosphorus with a hot wire and invert the jar over it, pushing the mouth below the surface of the water. The phosphorus uniting with the oxygen fills the jar with white fumes of phosphoric oxide. These soon dissolve in the water, leaving a clear gas above. This is nitrogen. Place a cardboard under the mouth of the jar and turn it right side up, leaving in the water and keeping the top covered. Light a splinter and, slipping the cover to one side, thrust the flame into the jar of nitrogen, noting the effect. (Flame is extinguished.) Compare nitrogen with oxygen in its relation to combustion. What purpose is served by each in the atmosphere?
[pg 135]Oxygen.—Review experiments (page 114) showing the properties of oxygen.
Phosphorus.—Examine a small piece of phosphorus, noting that it has to be kept under water. Lay a small piece on the table and observe the tiny stream of white smoke rising from it, formed by slow oxidation. Dissolve a piece as large as a pea in a teaspoonful of carbon disulphide in a test tube, pour this on a piece of porous paper, and lay the paper on an iron support. When the carbon disulphide evaporates the phosphorus takes fire spontaneously. (The heat from the slow oxidation is sufficient to ignite the phosphorus in the finely divided condition.) What is the most striking property of phosphorus? What purpose does it serve in the match?
Sulphur.—Examine some sulphur, noting its color and the absence of odor or taste. (Impure sulphur may have an odor and a taste.) Burn a little sulphur in an iron spoon, noting that the compound which it forms with oxygen by burning has a decided odor.
Other Elements.—Magnesium. Examine and burn a piece of magnesium ribbon, noting the white compound of magnesium oxide which is formed. Iron. Examine pieces of the metal and also some of its compounds, as ferrous sulphate, ferric chloride, and ferric oxide or iron rust. Sodium. Drop a piece of the metal on water and observe results. Sodium decomposes water. It has to be kept under some liquid, such as kerosene, which contains no oxygen. (It should not be touched except with the fingers wet with kerosene.) Chlorine. Pour strong hydrochloric acid on a little manganese dioxide in a test tube, and warm gently over a low flame. The escaping gas is chlorine. Avoid breathing much of it.
Composition of the Nutrients.—The simplest way of determining what elements make up the different nutrients is by heating them and studying the products of decomposition, as follows:
To show that Carbohydrates contain Carbon, Hydrogen, and Oxygen.—Place one half teaspoonful of powdered starch in a test tube and heat strongly. Observe that water condenses on the sides of the tube and that a black, charred mass remains behind. The black mass consists mainly of carbon. The water is composed of hydrogen and oxygen. These three elements are thus shown to be present in the starch. The experiment may be repeated, using sugar instead of starch.
To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, and Sulphur.—Place in a test tube some finely divided proteid[pg 136] which has been thoroughly dried (dried beef or the lean of hard cured bacon). Heat strongly in the hood of a chemical laboratory or some other place where the odors do not get into the room. First hold in the escaping gases a wet strip of red litmus paper. This will be turned blue, showing ammonia (NH3) to be escaping. Next hold in the mouth of the tube a strip of a paper wet with a solution of lead nitrate. This is turned black or brown on account of hydrogen sulphide(H2S) which is being driven off. Observe also that water condenses in the upper part of the tube and that a black, charred mass remains behind. Since the products of decomposition (H2O, NH3, H2S, and the charred mass) contain hydrogen, oxygen, nitrogen, sulphur, and carbon, these elements are of course present in the proteid tested.
To show the Presence of Mineral Matter.—Burn a piece of dry bread by holding it in a clear, hot flame, and observe the ash that is left behind. This is the mineral matter present in the bread.
Tests for Nutrients. Proteids.—Cover the substance to be tested with strong nitric acid and heat gradually to boiling. If proteid is present it turns yellow and partly dissolves in the acid, forming a yellow solution. Let cool and then add ammonia. The yellow solid and the solution are turned a deep orange color. Apply this test to foods containing proteid such as white of egg, cheese, lean meat, etc.