time. This will occur before all the wood is consumed. Another splinter of wood, placed in a jar with the cover off, will burn slowly but completely. A third piece of wood burned in the air will be quickly and completely consumed. If now a little limewater' is poured into the jar which was closed, and the contents shaken up, the limewater will be found to turn a milky color. This milky appearance is due to the formation within the jar Apparatus for separating water into the two elements hydrogen and oxygen. of a material known as calcium carbonate. This is thrown down in the liquid as a result of the union of carbon with lime. Evidently some of the carbon from the wood has passed in the form of a gas into the limewater and there united with the calcium in the lime. Remembering what we know about oxidation, we see that the carbon of the wood has passed off and united with oxygen of the air in the jar. Thus, by the uniting of the two chemical elements, a chemical compound has been formed. The presence of carbon dioxide is known by the fact that it puts out a flame and that it turns limewater milky. This compound is known to chemists as carbon dioxide.2 Nitrogen. There is another gaseous substance that will not support combustion; this is the element nitrogen. Its presence in the atmosphere is shown by the following experiment: Invert a bell jar in a large, deep dish of water, having previously placed within the jar on the surface of the water a piece of phosphorus supported on a flat bit of wood or cork. Leave the experiment for at least two days undisturbed (or, the phosphorus may be lighted and then the jar left for a few hours untouched). After that time the water will be found to have risen considerably in the jar. If you make a mark on the cover 1 Limewater can be made by shaking up a piece of quicklime the size of your fist in about two quarts of water. Filter or strain the limewater into bottles and it is ready for use. A 2 Chemists have shown that any given structure is made up of molecules. molecule is the smallest bit of matter that can exist separately and still retain its composition and properties. A molecule is composed of still smaller particles called atoms. Carbon dioxide is so called because its molecule is made up of one atom of carbon and two atoms of oxygen. It is customary to use certain letters or symbols to designate certain chemical elements, as C for carbon, H for hydrogen, N for nitrogen, P for phosphorus, Fe (Latin ferrum) for iron. The molecule of carbon dioxide is made of one part of carbon and two of oxygen; it is written CO2. This is called the chemical formula. If an electric current is passed through a jar of water, the contents will be broken down into the elements hydrogen and oxygen. If now the gases are carefully collected, there will be found to be exactly twice as much of the hydrogen gas as there is of the oxygen. The chemical formula for water is H2O. See figure. It would be well for the teacher at this place to bring up the subject of atmospheric pressure. Air presses down on the earth's surface at sea level with a weight of fifteen pounds to every square inch of surface. to show where the water stood, you may measure the space occupied by the water in the jar. This space will be found to be almost exactly one fifth of the cubic contents of the jar. It was occupied by the oxygen of the air, this having been used up by the oxidation of the phosphorus. The remaining space at the completion of the experiment is occupied by the nitrogen, which makes the remaining four fifths of the atmosphere. The physical properties of nitrogen are its lack of color, taste, and odor. Its chief chemical characteristics are its inability to support combustion and its slight affinity for other substances. Experiment to show the amount of nitrogen present in the air. MINERAL MATTER IN LIVING THINGS.. We saw in the experiment for the detection of carbon by burning, that the sand or gravel contained no carbon. If a piece of wood is burned in a very hot fire, the carbon in it will all be consumed, and eventually nothing will be left except a grayish ash. This ash is well seen after a wood fire in the fireplace, or after a bonfire of dry leaves. This ash consists entirely of mineral matter which the plant has taken up from the soil, dissolved in water, and which has been stored in the wood or leaves. If we were able by careful analysis to reduce a plant and an animal to the chemical compounds of which they were formed, we should discover that both contained mineral or inorganic material. We have just seen examples of this in plants. Mineral matter is found in bone, in the shells covering mollusks, and in many of the other parts of the bodies of animals. WATER IN LIVING THINGS. Water forms an important part of the substance of plants and animals. This can easily be proved by weighing a number of green leaves, placing them in a hot oven for a few moments, and then reweighing. How much weight of a given quantity of leaves is made up of water? Make the same experiment with some soft-bodied animal, as an oyster removed from the shell. Some jellyfish are composed of almost 99 per cent water. The human body contains 60 per cent water. GASES PRESENT. -Some gases are found in a free state in the bodies of plants or animals. Oxygen is of course present wherever oxidation is taking place. Some plants and animals form nitrogen. Many other examples might be given. Classification of Organic Matter. The organic or living part of a plant or animal is made up largely of the elements carbon, hydrogen, oxygen, and nitrogen, with a very minute amount of several other elements, which collectively we may call mineral matters. If we were to separate a plant or animal chemically into various organic compounds, we should find it composed of various groups of tissues, the chemical compositions of which are more or less alike. For example, the living part of a plant corresponds chemically with the living part of an animal. The starch found in grains or roots of plants has nearly the same chemical formula as the animal starch found in the liver of man; the oils of a nut or fruit are of composition closely allied to the fat in the body, or in a sheep or cow. These building materials. of a plant or animal may be placed in one of the three following groups of organic substances: carbohydrates, materials containing a certain proportion of carbon, hydrogen, and oxygen; organic fats and oils, which contain chiefly hydrogen and carbon; and nitrogenous, or proteid substances, which contain nitrogen in addition to the above-mentioned elements. The above three kinds of organic materials also form the organic foods of all animals and plants. Foods. What is a food? We know that if we eat a certain amount of proper foods at regular times, we shall go on doing a certain amount of work, both manual and mental. We know, too, that day by day, if our general health is good, we are adding weight to our body, and that added weight comes as the result of taking food into the body. What is true of a boy or girl is equally true of plants. If food is supplied in proper quantity and proportion, they will live and grow; if the food supply is cut off, or even greatly reduced, they will suffer and may die. From this, the definition which follows is evident. A food is a substance that forms the material for the growth or repair of the body of a plant or animal or that furnishes energy for it. Nutrients. Food substances may be classed into a number of groups, each of which may be detected by means of its chemical composition. Such food substances are known as nutrients. Let us now examine a few of the nutrients that we are likely to meet in our daily life, and see how we could test chemically for their presence. Carbohydrates. Starch and sugar1 are common examples of this group of substances. Starch Test. If the substance to be tested is a solid, break or crush it and add water to it. Pour over it a few drops of iodine solution diluted with water. Notice the color of the iodine, a dark brown; after it has touched the material supposed to contain starch, note any change in color. If starch is present, it will turn dark blue. Grape Sugar. There are several forms of sugar commonly used as food; for example, cane sugar, beet sugar, and grape sugar, the latter commonly known as glucose. Glucose, or grape sugar, is manufactured commercially by pouring sulphuric acid over starch. It is used as an adulterant for many kinds of foods, especially in sirups, honey, and candy. Test for Grape Sugar. The presence of grape sugar is determined by the following test: Place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar. Add to the fluid twice its bulk of Fehling's solution, which has been previously prepared. Heat the mixture, which should now have a blue color, in the test tube. If grape sugar is present in considerable quantity, the contents of the tube will turn first a greenish, then yellow, and finally a brick-red color. Smaller amounts will show less decided red. This change also appears if Fehling's solution is boiled with cane sugar. A more accurate test is obtained by placing the substance believed to contain grape sugar in a test tube containing Fehling's solution and allowing the mixture to remain over night in a moderately warm room. If grape sugar is present, a red deposit or precipitate (copper oxide) will be found in the tube the next morning. Organic Fats and Oils. Tests for fats: Rub the material believed to contain oil several times on paper and hold the paper to the light. If oil is present, the paper will show a translucent grease spot. Try this with several different nuts and decide which has the most oil. A second test for oil is as follows: Heat the substance to be tested in an oven on a piece of paper. If oil is present, the paper will show a grease spot. Third test: Reduce the substance to small pieces and pour benzine, ether, or other volatile oil over it. Allow the benzine or ether to evaporate; the oil that remains is the extracted oil from the substance tested. 1 The chemical formula for starch is C&H10O5; that of grape sugar, C6H12O6. 2 Iodine solution is made by simply adding a few crystals of the element iodine to 95 per cent alcohol; or, better, take by weight 1 gram of iodine crystals, gram of iodide of potassium, and dilute to a dark brown color in weak alcohol (35 per cent) or distilled water. 3 To make Fehling's solution (so called after its discoverer), add to 35 grams of copper sulphate (blue vitriol) 500 cubic centimeters of water. Put aside until it is completely dissolved. Call this solution No. 1. To 160 grams of caustic soda and 173 grams of Rochelle salt add 500 cubic centimeters of water. Dilute to 1 liter. Call this solution No. 2. For use mix equal parts of solution 1 and 2. The following formula is also convenient: I. Copper Sulphate: 9 grams in 250 c.c. water. II. Sodium Hydroxide: 30 grams in 250 c.c. water. III. Rochelle Salt: 43 grams in 250 c.c. water. For use add to equal parts I, II, and III, two parts of water. Proteids. Nitrogenous foods, or proteids, contain the element nitrogen in addition to carbon, hydrogen, and oxygen of the carbohydrates and hydrocarbons. They include some of the most complex substances known to the chemist, and as we shall see, have a chemical composition very near to that of living matter. Proteids occur in several different forms, but the following tests will cover most cases commonly met. White of egg, lean meat, beans, and peas are examples of substances composed in a large part of proteid. Place in a test tube the substance to be tested; for example, a bit of hardboiled egg. Pour over it a little strong (80 per cent) nitric acid. Note the color that appears - a lemon yellow. Now wash the egg in water and add a little ammonium hydrate. The color now changes to a deep orange, showing that a proteid is present. If the proteid is in a liquid state, its presence may be proved by heating; if it coagulates or thickens, as does the white of an egg, when boiled, then proteid in the form of an albumen is present. Another characteristic proteid test easily made at home is by burning the substance. If it burns with the odor of burning feathers or leather, then proteid forms part of its composition. BOOKS FOR REFERENCE FOR THE PUPIL Avery-Sinnott, First Lessons in Physical Science. American Book Company. FOR THE TEACHER Foster, A Text-book of Physiology. The Macmillan Company. Green, Vegetable Physiology. J. and A. Churchill. Sedgwick and Wilson, General Biology. Henry Holt and Company. |