Chemistry is the science that deals with the changes which matter undergoes. There are also physical changes which take place in connection with matter. The latter are, however, only changes in form, whilst the former are changes in composition. Acids, Bases, and Salts.Readers will find constant references to these elsewhere, so a few words explaining exactly what they are may be useful. An acid considered chemically contains hydrogen, but for our purposes we may say that it has the power of neutralising a base. A base, on the other hand, has properties opposite to those of an acid. If the two combine, a salt is formed. As an example of such changes spirits of salts (hydrochloric acid) may be taken. It has the characteristic properties of an acid, for it is sour to the taste, and it turns blue litmus paper red. If now caustic soda (a base which turns red litmus blue) be added, the properties of the latter disappear at a certam point, for the soda, which is a base, has combined with the acid to form common salt (sodium chloride). Chemical changes such as the abovs are of great importance to gardeners, and an example of a change where undesirable compounds are removed from the soil by this means, is found in liming the land to neutralise the poisonous vegetable acids it may contain after over-manuring. The chemist tells us there are two classes of matter organic and inorganic. By the former is usually understood those substances which have been produced by living organisms (animal or vegetable), whilst inorganic matter has been produced without the aid of a living plant or animal. The distinction between organic and inorganic material is useful, and if by organic is understood what we regard generally as animal or vegetable products, the term is not likely to be wrongly used. Strictly speaking, however, organic chemistry is the branch which deals with the compounds of the element carbon. A great many of the changes which give rise to new compounds take place in solution, and water is the solvent that most commonly comes into consideration. Some elements or compounds do not dissolve in water nearly so easily as others ; but nothing is really insoluble, although such materials as glass, metals, etc., only dissolve after they have been heated in water for lengthy periods. In the soil, as also in the plant, water plays a most important part as a solvent and as an aid to chemical changes. Generally speaking, hot water dissolves materials better than cold ; the hotter the water gets the greater its power is of dissolving. Also, if a little acid is added, many compounds otherwise insoluble dissolve readily. A good illustration of this last point is shown in the ease with which water containing carbon dioxide gas (which compound is a weak acid) acts upon many minerals in the soil and dissolves them, though pure water does not dissolve them at all. In view of the special importance of certain elements and their compounds in gardening, a few short notes on these are given below. For further particulars, reference must be made, however, to some book on systematic chemistry. Oxygen.This element (a gas existing in air) is of first importance, as it enters so easily into combination with other elements. When carbon bums, it combines with oxygen and yields carbon dioxide gas. When an animal breathes, it takes in oxygen and gives out carbon dioxide gas. A plant, however, takes in carbon dioxide gas and gives out oxygen ; and in the soil oxygen from the air produces or assists in the production of new compounds. Hence the wisdom of rough digging in the winter to expose as large an area of the soil to the air as possible. Nitrogen.This is the element which makes up practically four-fifths of the air. It differs very markedly from oxygen in that it is, whilst in the free state, an inactive gas. When combined with other elements nitrogen i86 gives rise to very diiierent substances, some of them most active, e.g. aqua fortis (nitric acid), ammonia, etc., whilst others, such as nitrate of soda and nitrate of potash, are of great importance to the plant and the animal. Both plants and animals require nitrogen, and as it is only the leguminous plants which have the power of using nitrogen gas, the use of nitrogenous manures, such as nitrate of soda and sulphate of ammonia, is widespread. In fact the whole group of nitrogenous manures (see article on Manuring, Principles of) is of fundamental interest and value. Caibon.This is another important element from the gardeners point of view. In a pure form carbon is seen in charcoal made from sugar. It is the compounds of carbon which are of more interest. As already mentioned, organic chemistry is nowadays regarded as dealing with the compounds of carbon. Carbon dioxide gas, which is formed when carbon unites with oxygen, is the gas produced when coal or wood bums ; also carbon dioxide is breathed out by plants and animals day and night. Also carbon dioxide gas is the source from which the plant, thanks to its green leaf and the power of sunshine, prepares sugars and starches, which are known as carbohydrates. Cellulose is another carbohydrate ; it makes up the woody or stem portion of the plant. The carbohydrates all contain carbon, oxygen, and hydrogen. Phosphorus.This important element is related chemically to nitrogen, and it is also a substance of great value to the plant. The element itself is of no use to the gardener, but it combines readily with oxygen, giving an oxide which dissolves in water and thus produces phosphoric acid. When a base is added, salts called phosphates are produced. There are many of these, some containing more acid than others. Those with the greatest amount of The last two elements are present in all carbohydrates in the proportions in which they form water. This is easily shown by adding strong sulphuric acid to one of them, as, for instance, sugar. Chemical action sets in, a mass of pure black carbon is formed, and dilute sulphuric acid (i.e. acid to which some water has been added) remains. Editor. acid dissolve in water most freely, and are thus of great value for plant life. They are known as acid manures. Insoluble phosphates may be made soluble by the addition of sulphuric acid, which robs the compound of part of its base, and leaves an acid phosphate (soluble) and usually an insoluble sulphate. Other Elements.The elements described above have all been nonmeiallic, and in the same group we include many others, such as chlorine, sulphur, silicon, and hydrogen, which are of importance, but which must be passed over here. The metallic elements, as the name implies, have the general properties of metals, and a few notes on some of them are given below. Calcium.This element occurs in limestone, which is a carbonate of calcium, and quicklime, which is calcium oxide. Liming consists of applying this element to the soil in a suitable form. See article on Lime. Calcium combines readily with acids and forms salts. Those of greatest importance are included in our article on L/ME Compounds. Iron.Plants require this element in order that the green colouring matter of their leaves may be properly formed. Iron salts are also used in spraying mixtures. Iron is more fully dealt with in our article Iron and Iron Compounds. Magnesium is closely allied to calcium. Potassium and Sodium.These are two elements which form salts of great economic irnportance. Both elements are widely distributed, but potassium (which is abundant as a double silicate in clay) is, of course, the most important. Plants absorb potash from the soil, and when they are burnt the potash remains in their ash. In addition to its being found in clay, potash also occurs in large deposits, as at Stasfurt, from which kainit is obtained. These deposits also supply some of the impure sulphate and muriate (or chloride) of potash used as artificial fertilisers. Sodium also occurs in nature very freely, chiefly as common salt. From this caustic soda (used in spraying), carbonate of soda (or washing soda), and indeed most other compounds, can be easUymade. The chief potash and soda compounds, with instructions for their use, are given in our articles on Potash and Potassium Compounds, and Soda and Sodium Compounds. Ammonia.This, though a compound of nitrogen and hydrogen, is regarded by chemists as if it were a metalUc element, and it certainly forms many compounds with acids (such as sulphate of ammonia), of great use to the gardener. It is dealt with more fully in our article Ammonia and Ammonium Compounds.W. Gdn, See also EDUCATION and Gardening.

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