Nitrogen and Conservation use too much, since strong solutions of them are likely to corrode the roots and kill the plants. Although nitrogen is a very abundant element, forming as has been said, four-fifths of the air, yet it is com paratively rare in forms which are of use to plants. As a rule plants cannot take it from the air and therefore require soluble compounds of nitrogen for food. One of the most important of these is ammonia. This is formed when organic substances decay, its odor being very noticeable about stables. Its action with acids was described in the pages about cleaning and it was explained how it unites with acids to form salts, usually soluble. Sulphate of ammonia is the form used in agriculture. A very little ammonia in the water used on house plants is a good thing for them. It has been seen that plants by aid of sunlight breathe in carbon dioxide and breathe out oxygen gas. In addition to this, they also breathe as animals do, to a slight extent, taking in oxygen and breathing out carbon dioxide. This action is .nore pronounced in dark ness. The wonderful principle called conservation is illustrated by what we know of plant life. Plants in growing store up energy derived from the heat and light of the sun. When they decay, or are burned, or are eaten by animals, exactly the same amount of energy is set free and changed into a new form, and just as much carbon dioxide as the plant breathed in, is given back to the air. A plant which was many years in growing may be consumed in an hour or may decay slowly for years. In either case the same total amount of energy is set free, fast or slowly. This energy is most apparent as heat. In the growth and destruction of the plant both energy and matter have been transformed, but neither energy nor matter has been made or lost-it has merely taken on a new appearance. When animals feed on plants they transform the energy of sunlight which is stored up in the plant into energy of vitality. In this sense man and all animals are "children of the sun." CHEMICAL TERMS. To explain various chemical and physical phenomena the scientists consider that matter consists of certain small molecules and atoms. If a drop of water be divided and sub-divided indefinitely, it is conceivable that a point would come when it could not be divided further by physical means. This final bit of water is called a molecule. It would be far from visible by the most powerful microscope. From calculation which we will not go into, we learn that a few hundred million ordinary sized molecules would cover the space of a pin head. If the water is broken up by some powerful force as by the electric current, we have seen that two different substances are obtained-oxygen and hydrogen. Consequently the molecules of water must have been made up of other still smaller particles and these are called atoms. The atoms of a chemical element, then, Molecules Atoms are of the same kind, for from an elemental substance like oxygen, only oxygen can be obtained by any means now known. The atoms may be likened to the letters of our alphabet and the molecules to the words. From a few different kinds of atoms (letters) can be made a great variety of molecules (words). Chemical Signs The atoms of an element are all exactly alike. They weigh the same and act the same whatever their source. Two or more atoms of an element may combine to make a molecule of that element. The molecules of a chemical substance are always composed of the same number and kind of atoms. To express the composition of substances chemists have made use of certain abbreviations and signs. To indicate an atom of hydrogen the letter H is used and for oxygen, the letter O, for nitrogen, N, and so on as shown in the table. When expressing a compound the number of atoms is indicated by sub-script; for example, H2 means two atoms of hydrogen; H2O expresses two atoms of hydrogen and one atom of oxygen, and as we have found, this is the composition of water; so H2O is the chemist's short way of indicating water. These are called chemical formulas. The formula for sulphuric acid is H2SO,. This indicates that it is made up of two atoms of hydrogen, one atom of sulphur, and four atoms of oxygen. The following table gives the chemical formulas of many of the chemical substances found in the household. THE HOUSEKEEPER'S LABORATORY. All modern science is based upon experiment. Chemistry was hardly a science until experimental research began. It must be confessed that the average housewife seldom thinks of making experiments. She is apt to remain helpless before any new problem of the home without printed directions or advice from friends. Very often the easiest and surest way to find out a thing is to try it. Use your kitchen as a laboratory. It would, of course, be most unwise to make experiments on expensive materials. For example, if a stain was to be removed from colored goods, it would be best to find the effect of the chemicals to be used on some small piece of the fabric. To test the color of a sample of gingham for fastness in washing, try a part of the sample in soap and hot water and see if the color "runs" or stains the water. Dry and iron the piece treated and compare with the portion of the original sample kept. A sample can be Expressing Experiments Testing |